Astronomy

Have auroras on Titan been observed yet?

Have auroras on Titan been observed yet?


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After reading the very insightful introduction to auroras on other planets I started digging and found various questions and some answers here on the same topic, see below.

What I did not figure out yet: Have auroras been observed on the moon Titan?

I am aware that Titan does not have a significant magnetic field, but to my understanding that would only mean that the aurora would occur right at the impact point of solar winds rather than in polar regions, given that the particle flow is enough intense.

References


Scientists detect an unexpected, beautiful aurora in the Solar System

It turns out many more objects in the universe may experience their own Northern Lights than scientists suspected.

Four years ago, comet-hunting spacecraft, Rosetta, took a final swan dive into the face of the comet 67P/Churyumov-Gerasimenko. It was the dramatic culmination of the European Space Agency's twelve-year mission to study the icy object.

While Rosetta may be gone, its memory lives on in data. Scientists continue to analyze these data, collected from its time around and on the icy dirt ball, transforming scientists' understanding of tiny interstellar objects like comet 67P — and confounding expectations

A new analysis of emissions from the comet's gassy "coma" published Monday in Nature Astronomy marks yet another unexpected discovery: the first-ever observed aurora on a celestial object aside from moons and planets.

The discovery could enable scientists to better observe such objects in the future, and protect spacecraft and satellites against rogue solar radiation.

Auroras on Earth take place when charged particles from the Sun take a ride on our the planet's magnetic field lines all the way up (or all the way down) to the north and south poles. As these solar particles bounce off molecules in the Earth's atmosphere, they jump between different parts of the visible light spectrum.

This activity creates billowing and beautiful curtains of reds, greens, and blues in the upper atmosphere. Similar auroras have also been observed on Mars, and Jupiter's moons Ganymede and Europa, but never before on a stellar object like a comet.

Previous analysis of emissions coming from 67P's coma, the gassy envelope around the comet, had seemingly ruled out such phenomena. Initially, scientists dismissed the observed excitation of atoms as simply the effect of "day glow," a process caused when solar photons interact with the comet's coma.

But co-author on the study and lead scientist on one of Rosetta's instruments studying 67P, Joel Parker, said in a statement that a closer look revealed there was something else going on.

“We were amazed to discover that the UV emissions are aurora, driven not by photons, but by electrons in the solar wind that break apart water and other molecules in the coma and have been accelerated in the comet’s nearby environment," said Parker. "The resulting excited atoms make this distinctive light.”

A combined approach — Instead of relying on a single dataset to study emission behavior, the researchers instead worked with data from six of Rosetta's instruments at once, including the Alice far-ultraviolet (FUV) spectrograph, the Ion and Electron Sensor (IES,) and the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS.)

“By doing this, we didn’t have to rely upon just a single dataset from one instrument,” study author and professor of planetary science at Imperial College London, Marina Galand, said in a statement.

“Instead, we could draw together a large, multi-instrument dataset to get a better picture of what was going on. This enabled us to unambiguously identify how 67P/C-G’s ultraviolet atomic emissions form, and to reveal their auroral nature.”

Glow up — The scientists expected to see evidence of the comet's day glow, but their analysis showed that charged electron particles from the Sun interacting with the comet's cool gaseous envelope resulted in auroras in the far ultraviolet.

These auroras aren't necessarily as beautiful as those on Earth, but that may be because our eyes aren't designed to see in the far ultraviolet. But the monumental nature of the discovery is undeniable, the researchers say — especially because comet 67P lacks a magnetic field, one of the key features we associate with auroras here on Earth.

CHASING SOLAR WIND — These curious aurorae might not be much for us to look at, but scientists say they could change how we study solar wind — and protect satellites and space probes from its radiation.

Now that scientists know these far ultraviolet auroras are (at least sometimes) related to the flow of solar wind, they can train instruments on Earth to look for their signature, detecting the presence of wind without needing another space probe. Understanding its behavior could in turn refine the defenses for new probes going out into the storm.


NASA Spacecraft Help Solve Saturn's Mysterious Auroras

Scientists studying data from NASA's Cassini spacecraft and Hubble Space Telescope have found that Saturn's auroras behave differently than scientists have believed for the last 25 years.

The researchers, led by John Clarke of Boston University, found the planet's auroras, long thought of as a cross between those of Earth and Jupiter, are fundamentally unlike those observed on either of the other two planets. The team analyzing Cassini data includes Dr. Frank Crary, a research scientist at Southwest Research Institute in San Antonio, Texas, and Dr. William Kurth, a research scientist at the University of Iowa, Iowa City.

Hubble snapped ultraviolet pictures of Saturn's auroras over several weeks, while Cassini's radio and plasma wave science instrument recorded the boost in radio emissions from the same regions, and the Cassini plasma spectrometer and magnetometer instruments measured the intensity of the aurora with the pressure of the solar wind. These sets of measurements were combined to yield the most accurate glimpse yet of Saturn's auroras and the role of the solar wind in generating them. The results will be published in the February 17 issue of the journal Nature.

The findings show that Saturn's auroras vary from day to day, as they do on Earth, moving around on some days and remaining stationary on others. But compared to Earth, where dramatic brightening of the auroras lasts only about 10 minutes, Saturn's can last for days.

The observations also show that the Sun's magnetic field and solar wind may play a much larger role in Saturn's auroras than previously suspected. Hubble images show that auroras sometimes stay still as the planet rotates beneath, like on Earth, but also show that the auroras sometimes move along with Saturn as it spins on its axis, like on Jupiter. This difference suggests that Saturn's auroras are driven in an unexpected manner by the Sun's magnetic field and the solar wind, not by the direction of the solar wind's magnetic field.

"Both Earth's and Saturn's auroras are driven by shock waves in the solar wind and induced electric fields," said Crary. "One big surprise was that the magnetic field imbedded in the solar wind plays a smaller role at Saturn."

At Earth, when the solar wind's magnetic field points southward (opposite to the direction of the Earth's magnetic field), the magnetic fields partially cancel out, and the magnetosphere is "open". This lets the solar wind pressure and electric fields in, and allows them to have a strong effect on the aurora. If the solar wind's magnetic field isn't southward, the magnetosphere is "closed'' and solar wind pressure and electric fields can't get in. "Near Saturn, we saw a solar wind magnetic field that was never strongly north or south. The direction of the solar wind magnetic field didn't have much effect on the aurora. Despite this, the solar wind pressure and electric field were still strongly affecting auroral activity," added Crary. Seen from space, an aurora appears as a ring of energy circling a planet's polar region. Auroral displays are spurred when charged particles in space interact with a planet's magnetosphere and stream into the upper atmosphere. Collisions with atoms and molecules produce flashes of radiant energy in the form of light. Radio waves are generated by electrons as they fall toward the planet.

The team observed that even though Saturn's auroras do share characteristics with the other planets, they are fundamentally unlike those on either Earth or Jupiter. When Saturn's auroras become brighter and thus more powerful, the ring of energy encircling the pole shrinks in diameter. At Saturn, unlike either of the other two planets, auroras become brighter on the day-night boundary of the planet which is also where magnetic storms increase in intensity. At certain times, Saturn's auroral ring is more like a spiral, its ends not connected as the magnetic storm circles the pole.

The new results do show some similarities between Saturn's and Earth's auroras: Radio waves appear to be tied to the brightest auroral spots. "We know that at Earth, similar radio waves come from bright auroral arcs, and the same appears to be true at Saturn," said Kurth. "This similarity tells us that, on the smallest scales, the physics that generate these radio waves are just like what goes on at Earth, in spite of the differences in the location and behavior of the aurora."

Now with Cassini in orbit around Saturn, the team will be able to take a more direct look at the how the planet's auroras are generated. They will next probe how the Sun's magnetic field may fuel Saturn's auroras and learn more details about what role the solar wind may play. Understanding Saturn's magnetosphere is one of the major science goals of the Cassini mission.


Have auroras on Titan been observed yet? - Astronomy

View from the Space Station


From Space Weather Monday May 3, 2021: Red. Green. Purple. These are the colors we usually see during any display of auroras. On April 18th, Alan C. Tough of Hopeman, Moray, Scotland saw something else. "Black," he says. In the photo, "note the dark vertical strip above the green band, which is devoid of any normal auroral color."


For The First Time, Physicists Have Confirmed The Enigmatic Waves That Cause Auroras Science Alert - June 7, 2021
Now, for the first time, scientists have demonstrated and confirmed the mechanism whereby the particle acceleration occurs - by replicating the process in a laboratory. Just as scientists had thought, powerful electromagnetic waves known as Alfven waves accelerate electrons along the magnetic field lines.

Observations made by University of Helsinki researchers increased the validity of a speculative mechanism according to which a type of aurora borealis named 'dunes' is born. In the new study, photographs of the phenomenon taken by an international group of hobbyists in Finland, Norway and Scotland were compared to concurrent satellite data. The rare type of aurora borealis was seen in the sky on 20 January 2016 and recorded in photos taken by several hobbyists. The dunes were seen for almost four hours in a very extensive area, with the pattern extending roughly 1,500 kilometers from east to west and some 400 kilometers from north to south.


Scientists know a fair bit about these diffuse auroras, but an old video from 2002 revealing what seems to be an undocumented auroral phenomenon shows we definitely don't know everything. They appear as a section of diffuse aurora that rapidly brightens, then disappears and also erases the background aurora. Then, over the course of several tens of seconds, the diffuse aurora recovers to its original brightness.


An aurora (plural: aurorae or auroras) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere.

Aurora is classified as diffuse or discrete aurora. Most aurorae occur in a band known as the auroral zone which is typically 3° to 6° in latitudinal extent and at all local times or longitudes. The auroral zone is typically 10° to 20° from the magnetic pole defined by the axis of the Earth's magnetic dipole. During a geomagnetic storm, the auroral zone will expand to lower latitudes.

The diffuse aurora is a featureless glow in the sky which may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone. The discrete aurora are sharply defined features within the diffuse aurora which vary in brightness from just barely visible to the naked eye to bright enough to read a newspaper at night. Discrete aurorae are usually observed only in the night sky because they are not as bright as the sunlit sky. Aurorae occur occasionally poleward of the auroral zone as diffuse patches or arcs (polar cap arcs) which are generally invisible to the naked eye.

In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621. Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction.

Discrete aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near the equinoctes. The northern lights have had a number of names throughout history.

Its southern counterpart, the aurora australis (or the southern lights), has almost identical features to the aurora borealis and changes simultaneously with changes in the northern auroral zone and is visible from high southern latitudes in Antarctica, South America, New Zealand and Australia.

Historical Theories, Superstitions and Mythology

There is the claim from 1855 that in Norse mythology - The Valkyrior are warlike virgins, mounted upon horses and armed with helmets and spears. When they ride forth on their errand, their armor sheds a strange flickering light, which flashes up over the northern skies, making what Men call the "aurora borealis", or "Northern Lights".

While a striking notion, there is not a vast body of evidence in the Old Norse literature giving this interpretation, or even much reference to auroras. Although auroral activity is common over Scandinavia and Iceland today, it is possible that the Magnetic North Pole was considerably farther away from this region during the relevant period of Norse mythology. Today, the Northern Lights are visible in Iceland from September to April.

The first Old Norse account of Noroljos is found in the Norwegian chronicle Konungs Skuggsja from AD 1230, (long after the Viking age). The chronicler has heard about this phenomenon from compatriots returning from Greenland, and he gives three possible explanations: that the ocean was surrounded by vast fires, that the sun flares could reach around the world to its night side, or that glaciers could store energy so that they eventually became fluorescent.

Magnetic control of the aurora was mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter, and Celsius described in 1741 evidence that large magnetic fluctuations occurred whenever the aurora was observed overhead. It was also later realized that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Multiple superstitions and obsolete theories explaining the aurora have emerged over the centuries.

Seneca speaks diffusely on auroras in the first book of his Naturales Quaestiones, drawing mainly from Aristotle he classifies them "putei" or wells when they are circular and "rim a large hole in the sky", "pithaei" when they look like casks, "chasmata" from the same root of the English chasm, "pogoniae" when they are bearded, "cyparissae" when they look like cypresses), describes their manifold colors and asks himself whether they are above or below the clouds. He recalls that under Tiberius, an aurora formed above Ostia, so intense and so red that a cohort of the army, stationed nearby for fireman duty, galloped to the city.

In 1619 A.D., Galileo Galilei coined the term "aurora borealis" after Aurora, the Roman goddess of morning renewing herself every morning to fly across the sky, announcing the arrival of the sun. He had the misconception that the auroras he saw were due to sunlight reflecting from the atmosphere. The persona of Aurora the goddess has been incorporated in the writings of Shakespeare, Lord Tennyson, and Thoreau. The name Aurora, however, simply comes from the Latin word for the dawn. The goddess was not associated with polar light phenomena, in Roman myth.

In the traditions of Aboriginal Australians, the Aurora Australis is commonly associated with fire. For example, the Gunditjmara people of western Victoria called auroras "Puae buae", meaning "ashes", while the Gunai people of eastern Victoria perceived auroras as bushfires in the spirit world. When the Dieri people of South Australia said that an auroral display was "Kootchee", an evil spirit creating a large fire. Similarly, the Ngarrindjeri people of South Australia referred to auroras seen over Kangaroo Island as the campfires of spirits in the 'Land of the Dead'. Aboriginal people in southwest Queensland believed the auroras to be the fires of the "Oola Pikka", ghostly spirits who spoke to the people through auroras. Sacred law forbade anyone except male elders from watching or interpreting the messages of ancestors they believed were transmitted through auroras.

After the Battle of Fredericksburg, the lights could be seen from the battlefield that night. The Confederate Army took it as a sign that God was on their side during the battle as it was very rare that one could see the lights in Virginia. The painting Aurora Borealis (1865) by American landscape painter Frederic Edwin Church is widely interpreted to represent the conflict of the American Civil War.

Native American Mythology

A variety of Native American myths surround the spectacle. Early European explorer Samuel Hearne traveled with Chipewyan Dene in 1771 and recorded their views on the aurora borealis, or the "ed-thin", as they called it, meaning caribou. Dene experience was that stroking caribou fur created sparks much like the aurora. They also believed that the lights were the spirits of their departed friends dancing in the sky, and when the lights shined the brightest it meant that their deceased friends were very happy. The Cree called the phenomenon the "Dance of the Spirits". In Medieval Europe, the auroras were commonly believed to be a sign from God.

Walter William Bryant wrote in his book Kepler (1920) that Tycho Brahe "seems to have been something of a homeopathist, for he recommends sulfur to cure infectious diseases "brought on by the sulphurous vapors of the Aurora Borealis."

In Europe, in the Middle Ages, the auroras were commonly believed a sign from God.

Benjamin Franklin theorized that the "mystery of the Northern Lights" was caused by a concentration of electrical charges in the polar regions intensified by the snow and other moisture.

Auroras occur on other planets. Similar to the Earth's aurora, they are visible close to the planet's magnetic poles.

Each of the gas giants (Jupiter, Saturn, Uranus, and Neptune) has a strong magnetic field, a dense atmosphere and, as a result, its own aurora. The exact nature of these auroras is slightly different from Earth's, since their atmospheres and magnetospheres are different. The colors, for example, depend on the gases in the planet's atmosphere. But the fundamental idea behind the auroras is the same.

For example, several of Jupiter's moons, including Io, Ganymede and Europa, affect the blue aurora created by the solar wind. Io, which is just a little larger than our own moon, is volcanic and spews out vast amounts of charged particles into Jupiter's magnetosphere, producing large electrical currents and bright ultraviolet (UV) aurora.

On Saturn, the strongest auroras are in the UV and infrared bands of the color spectrum and so would not be visible to the human eye. But weaker (and rarer) pink and purple auroras have also been spotted.

Auroras have also been observed on Venus and Mars. Because Venus has no intrinsic (planetary) magnetic field, Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc. Venusian auroras are produced by the impact of electrons originating from the solar wind and precipitating in the night-side atmosphere.

An aurora was also detected on Mars, on August 14, 2004, by the SPICAM instrument aboard Mars Express. The aurora was located at Terra Cimmeria, in the region of 177° East, 52° South. The total size of the emission region was about 30 km across, and possibly about 8 km high. By analyzing a map of crustal magnetic anomalies compiled with data from Mars Global Surveyor, scientists observed that the region of the emissions corresponded to an area where the strongest magnetic field is localized. This correlation indicates that the origin of the light emission was a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.

The brown dwarf star LSR J1835+3259 was discovered to have auroras in July 2015, the first extra-solar auroras discovered. The aurora is a million times brighter than the Northern Lights, mainly red in color, because the charged particles are interacting with hydrogen in its atmosphere. It is not known what the cause is. Some have speculated that material maybe being stripped off the surface of the brown dwarf via stellar winds to produce its own electrons. Another possible explanation is an as-yet-undetected planet or moon around the dwarf, which is throwing off material to light it up, as is the case with Jupiter and its moon Io. More about Auroras


Has life on Titan been discovered? No.

There has been a bit of an uproar the past day or so that scientists have found evidence of life on Saturn's giant moon Titan. As soon as I saw the press release I knew this was going to be a problem. So let's be clear: First, have we found life on Titan? No. Have we found evidence that there might be life on Titan? Sorta. The results are preliminary and not yet confirmed in fact, some of the evidence is from computer modeling and has not been directly observed. Bear in mind as well that evidence is not proof . Evidence just means an observation was made that is consistent with life on the moon, but doesn't say much else. There are non-biological explanations for the observations as well. Of course, speculation is running rampant, so much so that Chris McKay, an exobiologist who studies Titan, has released an article clearing things up . First, a little background. Titan is a monster, the second biggest moon in the solar system at 5150 km (3200 miles) in diameter. If it weren't orbiting Saturn, it would probably be considered a planet in its own right: it's bigger than Mercury and Pluto. It has a thick atmosphere, made up of nitrogen, methane, and other molecules. It's very cold, but it's known that lakes, probably of liquid methane , exist on the surface. Five years ago, McKay and other scientists pointed out that if methane-based life existed on Titan, it might be detectable through a surface depletion of ethane, hydrogen, and acetylene. New observations show that this is the case there are lower amounts of these substances than the chemistry of Titan would indicate. As McKay points out, "This is a still a long way from ɾvidence of life'. However, it is extremely interesting." Those are the basics. Go read McKay's article for details. The point he makes is that the results are preliminary, may yet turn out to be wrong, if they're right may have non-biological explanations, and we should not conclude biology is involved until we get a lot more evidence. As far as the media goes, headlines get eyeballs and sell advertisements, of course. But in cases where the news is like this, news outlets should be particularly careful how they phrase things. They know how the public will react to certain phrases, and the phrase "evidence of life" is substantially less accurate and more likely to incite chatter than "evidence for possible life" -- and the Telegraph's technically accurate but seriously misleading "evidence 'that alien life exists on Saturn's moon'" is just asking for trouble. The point is, when it comes to media outlets and big news like this, the phrase going through your head should be a variant of an old one, updated for this modern age: "Don't trust, and verify".


The origin of pulsating auroras

Allison N. Jaynes is in the Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242-1479, USA.

You can also search for this author in PubMed Google Scholar

The Northern and Southern lights, also known as auroras, are as varied as the colours they display in the night sky. Discrete auroras are the kind that typically grace our desktops and calendar covers, and that are produced a few thousand kilometres above Earth’s surface. By contrast, pulsating auroras that are created tens of thousands of kilometres away, in the equatorial region of the magnetosphere — the area around Earth that is dominated by the planet’s magnetic field. For decades, it has been suggested that pulsating auroras are the result of interactions between magnetospheric electrons and electromagnetic waves called chorus waves that send electrons careering towards Earth’s atmosphere along magnetic-field lines 1 – 3 . In a paper in Nature, Kasahara et al. 4 report direct evidence for this process using observations both from Earth’s surface and from a spacecraft positioned on a field line.

Read the paper: Pulsating aurora from electron scattering by chorus waves

Because magnetic fields are invisible to the human eye, the prediction of where a field line hits Earth and where that same field line exists out in space — a task known as magnetic-field-line mapping — is extremely difficult 5 . Luckily, electrons that move around Earth tend to follow these field lines closely. When these particles interact with chorus waves, they can be directed into a region of the upper atmosphere called the ionosphere, where they often generate auroral light. This allows us instantly to see the footprint of the associated field lines.

In addition, if we have an observation platform at a precise location out in space, we can detect the chorus waves that caused the electrons to head towards the atmosphere and see fluctuations in the electron population that arise from the oscillation of the waves. The trick is to get the ground-based and space-based observations to line up at the right time and place, and to have instruments sensitive enough to view both processes simultaneously. This feat has eluded observers ever since the theory of pulsating-aurora generation was developed 6 , 7 .

The first challenge is to have an instrument capable of making the in situ measurement of electrons in space at the required resolution. The Arase spacecraft 8 , launched by the Japan Aerospace Exploration Agency in late 2016, carries just such an electron detector, which was designed to observe electrons within a narrow window around a magnetic-field line. The spacecraft is also equipped with instruments to detect chorus waves. Kasahara and colleagues analysed data from the spacecraft to uncover fluctuations in the electron population and the associated chorus waves.

The next obstacle was to determine where the field line threading the position of the spacecraft would hit the ground. Magnetic-field models are now sophisticated enough to be able to inform researchers of the approximate location of a field-line footprint in Earth’s atmosphere, generally with higher accuracy when the level of geomagnetic activity (magnetic storms) is low. In the vicinity of this footprint, Kasahara et al. looked for a corresponding pulsating-aurora signal — namely, variations in auroral-light intensity that matched the fluctuations in the electron population. They identified such a signal in measurements from an all-sky imager based in central Canada 9 , which essentially records black-and-white video of the hemispherical view of the sky above (see Figure 2 of the paper 4 ).

Thanks to Kasahara and colleagues, we can see the complete process of pulsating-aurora generation for the first time: the fluctuations in an electron population out in space the chorus waves responsible for these fluctuations and the variations in auroral-light intensity from the ground (Fig. 1). The last part is somewhat analogous to watching an image on an old-fashioned television, where the ionosphere is the ‘screen’ onto which electrons are projected. Despite this simple picture, researchers are aware that the ionosphere probably changes the incoming signal — a detail that will no doubt be scrutinized in future studies.

Figure 1 | Pulsating-aurora generation. Kasahara et al. 4 report evidence for a mechanism that explains the occurrence of blinking patches of light in Earth’s atmosphere called pulsating auroras. In a region surrounding Earth known as the magnetosphere, electrons are trapped by the planet’s magnetic field and travel (red arrows) along magnetic-field lines. When these particles interact with electromagnetic waves called chorus waves, which are generated in the equatorial region of the magnetosphere, they can be directed towards Earth’s atmosphere, where they produce auroral light. The authors detected the interactions between the chorus waves and the electrons using the Arase spacecraft 8 , which was positioned on the relevant field line.

Kasahara et al. carried out an analysis in which they correlated the electron fluctuations and chorus waves in space with the pulsating-aurora signals seen by the all-sky imager on the ground. This step revealed the precise location in the atmosphere in which the field-line footprint resides. Such a technique has incredible potential to test and refine our current magnetic-field models by comparing the modelled footprint location to the observed location. In the future, magnetic-field-line mapping might well rely on a similar methodology to gain insight into magnetic topology — the structure and linkage of field lines.

There is one caveat, however: clear skies are required to see and measure the pulsating-aurora signals, so Earth’s terrestrial weather needs to cooperate. Furthermore, the chorus waves contain components of different frequency that interact with magnetospheric electrons in different ways depending on the energy of the particles. This affects which particles end up travelling down to Earth’s atmosphere. These details are directly related to geomagnetic activity and have not yet been fully quantified. There is still a rich body of research to be carried out regarding the mysterious pulsating auroras.

Nature 554, 302-303 (2018)


Contents

The word "aurora" is derived from the name of the Roman goddess of the dawn, Aurora, who travelled from east to west announcing the coming of the sun. [2] Ancient Greek poets used the name metaphorically to refer to dawn, often mentioning its play of colours across the otherwise dark sky (e.g., "rosy-fingered dawn"). [3]

Most auroras occur in a band known as the "auroral zone", [4] which is typically 3° to 6° wide in latitude and between 10° and 20° from the geomagnetic poles at all local times (or longitudes), most clearly seen at night against a dark sky. A region that currently displays an aurora is called the "auroral oval", a band displaced by the solar wind towards the night side of Earth. [5] Early evidence for a geomagnetic connection comes from the statistics of auroral observations. Elias Loomis (1860), [6] and later Hermann Fritz (1881) [7] and Sophus Tromholt (1881) [8] in more detail, established that the aurora appeared mainly in the auroral zone.

In northern latitudes, the effect is known as the aurora borealis or the northern lights. The former term was coined by Galileo in 1619, from the Roman goddess of the dawn and the Greek name for the north wind. [9] [10] The southern counterpart, the aurora australis or the southern lights, has features almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone. [11] The aurora australis is visible from high southern latitudes in Antarctica, Chile, Argentina, New Zealand, and Australia. The aurora borealis is visible from being close to the center of the Arctic Circle such as Alaska, Canada, Iceland, Greenland, Norway, Sweden and Finland.

A geomagnetic storm causes the auroral ovals (north and south) to expand, bringing the aurora to lower latitudes. The instantaneous distribution of auroras ("auroral oval") [4] is slightly different, being centered about 3–5° nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between the observer and the Sun. The aurora can be seen best at this time, which is called magnetic midnight.

Auroras seen within the auroral oval may be directly overhead, but from farther away, they illuminate the poleward horizon as a greenish glow, or sometimes a faint red, as if the Sun were rising from an unusual direction. Auroras also occur poleward of the auroral zone as either diffuse patches or arcs, [12] which can be subvisual.

Auroras are occasionally seen in latitudes below the auroral zone, when a geomagnetic storm temporarily enlarges the auroral oval. Large geomagnetic storms are most common during the peak of the 11-year sunspot cycle or during the three years after the peak. [13] [14] An electron spirals (gyrates) about a field line at an angle that is determined by its velocity vectors, parallel and perpendicular, respectively, to the local geomagnetic field vector B. This angle is known as the "pitch angle" of the particle. The distance, or radius, of the electron from the field line at any time is known as its Larmor radius. The pitch angle increases as the electron travels to a region of greater field strength nearer to the atmosphere. Thus, it is possible for some particles to return, or mirror, if the angle becomes 90° before entering the atmosphere to collide with the denser molecules there. Other particles that do not mirror enter the atmosphere and contribute to the auroral display over a range of altitudes. Other types of auroras have been observed from space for example, "poleward arcs" stretching sunward across the polar cap, the related "theta aurora", [15] and "dayside arcs" near noon. These are relatively infrequent and poorly understood. Other interesting effects occur such as flickering aurora, "black aurora" and subvisual red arcs. In addition to all these, a weak glow (often deep red) observed around the two polar cusps, the field lines separating the ones that close through Earth from those that are swept into the tail and close remotely.

Images Edit

The altitudes where auroral emissions occur were revealed by Carl Størmer and his colleagues, who used cameras to triangulate more than 12,000 auroras. [16] They discovered that most of the light is produced between 90 and 150 km above the ground, while extending at times to more than 1000 km. Images of auroras are significantly more common today [ when? ] than in the past due to the increase in the use of digital cameras that have high enough sensitivities. [17] Film and digital exposure to auroral displays is fraught with difficulties. Due to the different color spectra present, and the temporal changes occurring during the exposure, the results are somewhat unpredictable. Different layers of the film emulsion respond differently to lower light levels, and choice of a film can be very important. Longer exposures superimpose rapidly changing features, and often blanket the dynamic attribute of a display. Higher sensitivity creates issues with graininess.

David Malin pioneered multiple exposure using multiple filters for astronomical photography, recombining the images in the laboratory to recreate the visual display more accurately. [18] For scientific research, proxies are often used, such as ultraviolet, and color-correction to simulate the appearance to humans. Predictive techniques are also used, to indicate the extent of the display, a highly useful tool for aurora hunters. [19] Terrestrial features often find their way into aurora images, making them more accessible and more likely to be published by major websites. [20] Excellent images are possible with standard film (using ISO ratings between 100 and 400) and a single-lens reflex camera with full aperture, a fast lens (f1.4 50 mm, for example), and exposures between 10 and 30 seconds, depending on the aurora's brightness. [21]

Early work on the imaging of the auroras was done in 1949 by the University of Saskatchewan using the SCR-270 radar.

Aurora during a geomagnetic storm that was most likely caused by a coronal mass ejection from the Sun on 24 May 2010, taken from the ISS

Diffuse aurora observed by DE-1 satellite from high Earth orbit

Forms of auroras Edit

According to Clark (2007), there are four main forms that can be seen from the ground, from least to most visible: [22]

  • A mild glow, near the horizon. These can be close to the limit of visibility, [23] but can be distinguished from moonlit clouds because stars can be seen undiminished through the glow.
  • Patches or surfaces that look like clouds.
  • Arcs curve across the sky.
  • Rays are light and dark stripes across arcs, reaching upwards by various amounts.
  • Coronas cover much of the sky and diverge from one point on it.

Brekke (1994) also described some auroras as curtains. [24] The similarity to curtains is often enhanced by folds within the arcs. Arcs can fragment or break up into separate, at times rapidly changing, often rayed features that may fill the whole sky. These are also known as discrete auroras, which are at times bright enough to read a newspaper by at night. [25]

These forms are consistent with auroras being shaped by Earth's magnetic field. The appearances of arcs, rays, curtains, and coronas are determined by the shapes of the luminous parts of the atmosphere and a viewer's position. [26]

Colors and wavelengths of auroral light Edit

  • Red: At its highest altitudes, excited atomic oxygen emits at 630 nm (red) low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under more intense solar activity. The low number of oxygen atoms and their gradually diminishing concentration is responsible for the faint appearance of the top parts of the "curtains". Scarlet, crimson, and carmine are the most often-seen hues of red for the auroras.
  • Green: At lower altitudes, the more frequent collisions suppress the 630 nm (red) mode: rather the 557.7 nm emission (green) dominates. A fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to the high stability of the N2 molecule) plays a role here, as it can transfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to produce pink or yellow hues.) The rapid decrease of concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains. Both the 557.7 and 630.0 nm wavelengths correspond to forbidden transitions of atomic oxygen, a slow mechanism responsible for the graduality (0.7 s and 107 s respectively) of flaring and fading.
  • Blue: At yet lower altitudes, atomic oxygen is uncommon, and molecular nitrogen and ionized molecular nitrogen take over in producing visible light emission, radiating at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the lower edges of the "curtains", show up at the highest levels of solar activity. [27] The molecular nitrogen transitions are much faster than the atomic oxygen ones.
  • Ultraviolet: Ultraviolet radiation from auroras (within the optical window but not visible to virtually all [clarification needed] humans) has been observed with the requisite equipment. Ultraviolet auroras have also been seen on Mars, [28] Jupiter and Saturn.
  • Infrared: Infrared radiation, in wavelengths that are within the optical window, is also part of many auroras. [28][29]
  • Yellow and pink are a mix of red and green or blue. Other shades of red, as well as orange, may be seen on rare occasions yellow-green is moderately common. [clarification needed] As red, green, and blue are the primary colors of additive synthesis of colors, in theory, practically any color might be possible, but the ones mentioned in this article comprise a virtually exhaustive list.

Changes with time Edit

Auroras change with time. Over the night, they begin with glows and progress towards coronas, although they may not reach them. They tend to fade in the opposite order. [24]

At shorter time scales, auroras can change their appearances and intensity, sometimes so slowly as to be difficult to notice, and at other times rapidly down to the sub-second scale. [25] The phenomenon of pulsating auroras is an example of intensity variations over short timescales, typically with periods of 2–20 seconds. This type of aurora is generally accompanied by decreasing peak emission heights of about 8 km for blue and green emissions and above average solar wind speeds (

Other auroral radiation Edit

In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR), discovered in 1972. [31] Ionospheric absorption makes AKR only observable from space. X-ray emissions, originating from the particles associated with auroras, have also been detected. [32]

Aurora noise Edit

Aurora noise, similar to a crackling noise, begins about 70 m (230 ft) above Earth's surface and is caused by charged particles in an inversion layer of the atmosphere formed during a cold night. The charged particles discharge when particles from the Sun hit the inversion layer, creating the noise. [33] [34]

Atypical auroras Edit

STEVE Edit

In 2016, more than fifty citizen science observations described what was to them an unknown type of aurora which they named "STEVE", for "Strong Thermal Emission Velocity Enhancement". STEVE is not an aurora but is caused by a 25 km (16 mi) wide ribbon of hot plasma at an altitude of 450 km (280 mi), with a temperature of 6,000 K (5,730 °C 10,340 °F) and flowing at a speed of 6 km/s (3.7 mi/s) (compared to 10 m/s (33 ft/s) outside the ribbon). [35]

Picket-fence aurora Edit

The processes that cause STEVE also are associated with a picket-fence aurora, although the latter can be seen without STEVE. [36] [37] It is an aurora because it is caused by precipitation of electrons in the atmosphere but it appears outside the auroral oval, [38] closer to the equator than typical auroras. [39] When the picket-fence aurora appears with STEVE, it is below. [37]

Dune aurora Edit

First reported in 2020 [40] [41] and confirmed in 2021 [42] [43] the dune aurora phenomenon was discovered [44] by Finnish citizen scientists. It consists of regularly spaced, parallel stripes of brighter emission in the green diffuse aurora which give the impression of sand dunes. [45] The phenomenon is believed to be caused by the modulation of atomic oxygen density by a large-scale atmospheric wave travelling horizontally in a waveguide in the mesosphere in presence of electron precipitation. [42]

A full understanding of the physical processes which lead to different types of auroras is still incomplete, but the basic cause involves the interaction of the solar wind with Earth's magnetosphere. The varying intensity of the solar wind produces effects of different magnitudes but includes one or more of the following physical scenarios.

  1. A quiescent solar wind flowing past Earth's magnetosphere steadily interacts with it and can both inject solar wind particles directly onto the geomagnetic field lines that are 'open', as opposed to being 'closed' in the opposite hemisphere, and provide diffusion through the bow shock. It can also cause particles already trapped in the radiation belts to precipitate into the atmosphere. Once particles are lost to the atmosphere from the radiation belts, under quiet conditions, new ones replace them only slowly, and the loss-cone becomes depleted. In the magnetotail, however, particle trajectories seem constantly to reshuffle, probably when the particles cross the very weak magnetic field near the equator. As a result, the flow of electrons in that region is nearly the same in all directions ("isotropic") and assures a steady supply of leaking electrons. The leakage of electrons does not leave the tail positively charged, because each leaked electron lost to the atmosphere is replaced by a low energy electron drawn upward from the ionosphere. Such replacement of "hot" electrons by "cold" ones is in complete accord with the second law of thermodynamics. The complete process, which also generates an electric ring current around Earth, is uncertain.
  2. Geomagnetic disturbance from an enhanced solar wind causes distortions of the magnetotail ("magnetic substorms"). These 'substorms' tend to occur after prolonged spells (on the order of hours) during which the interplanetary magnetic field has had an appreciable southward component. This leads to a higher rate of interconnection between its field lines and those of Earth. As a result, the solar wind moves magnetic flux (tubes of magnetic field lines, 'locked' together with their resident plasma) from the day side of Earth to the magnetotail, widening the obstacle it presents to the solar wind flow and constricting the tail on the night-side. Ultimately some tail plasma can separate ("magnetic reconnection") some blobs ("plasmoids") are squeezed downstream and are carried away with the solar wind others are squeezed toward Earth where their motion feeds strong outbursts of auroras, mainly around midnight ("unloading process"). A geomagnetic storm resulting from greater interaction adds many more particles to the plasma trapped around Earth, also producing enhancement of the "ring current". Occasionally the resulting modification of Earth's magnetic field can be so strong that it produces auroras visible at middle latitudes, on field lines much closer to the equator than those of the auroral zone.

The details of these phenomena are not fully understood. However, it is clear that the prime source of auroral particles is the solar wind feeding the magnetosphere, the reservoir containing the radiation zones and temporarily magnetically-trapped particles confined by the geomagnetic field, coupled with particle acceleration processes. [46]

Auroral particles Edit

The immediate cause of the ionization and excitation of atmospheric constituents leading to auroral emissions was discovered in 1960, when a pioneering rocket flight from Fort Churchill in Canada revealed a flux of electrons entering the atmosphere from above. [47] Since then an extensive collection of measurements has been acquired painstakingly and with steadily improving resolution since the 1960s by many research teams using rockets and satellites to traverse the auroral zone. The main findings have been that auroral arcs and other bright forms are due to electrons that have been accelerated during the final few 10,000 km or so of their plunge into the atmosphere. [48] These electrons often, but not always, exhibit a peak in their energy distribution, and are preferentially aligned along the local direction of the magnetic field.

Electrons mainly responsible for diffuse and pulsating auroras have, in contrast, a smoothly falling energy distribution, and an angular (pitch-angle) distribution favouring directions perpendicular to the local magnetic field. Pulsations were discovered to originate at or close to the equatorial crossing point of auroral zone magnetic field lines. [49] Protons are also associated with auroras, both discrete and diffuse.

Auroras and the atmosphere Edit

Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen atoms and nitrogen based molecules returning from an excited state to ground state. [50] They are ionized or excited by the collision of particles precipitated into the atmosphere. Both incoming electrons and protons may be involved. Excitation energy is lost within the atmosphere by the emission of a photon, or by collision with another atom or molecule:

oxygen emissions green or orange-red, depending on the amount of energy absorbed. nitrogen emissions blue, purple or red blue and purple if the molecule regains an electron after it has been ionized, red if returning to ground state from an excited state.

Oxygen is unusual in terms of its return to ground state: it can take 0.7 seconds to emit the 557.7 nm green light and up to two minutes for the red 630.0 nm emission. Collisions with other atoms or molecules absorb the excitation energy and prevent emission, this process is called collisional quenching. Because the highest parts of the atmosphere contain a higher percentage of oxygen and lower particle densities, such collisions are rare enough to allow time for oxygen to emit red light. Collisions become more frequent progressing down into the atmosphere due to increasing density, so that red emissions do not have time to happen, and eventually, even green light emissions are prevented.

This is why there is a color differential with altitude at high altitudes oxygen red dominates, then oxygen green and nitrogen blue/purple/red, then finally nitrogen blue/purple/red when collisions prevent oxygen from emitting anything. Green is the most common color. Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue.

Precipitating protons generally produce optical emissions as incident hydrogen atoms after gaining electrons from the atmosphere. Proton auroras are usually observed at lower latitudes. [51]

Auroras and the ionosphere Edit

Bright auroras are generally associated with Birkeland currents (Schield et al., 1969 [52] Zmuda and Armstrong, 1973 [53] ), which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125 km) the rest ("region 2") detours, leaving again through field lines closer to the equator and closing through the "partial ring current" carried by magnetically trapped plasma. The ionosphere is an ohmic conductor, so some consider that such currents require a driving voltage, which an, as yet unspecified, dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms. In another interpretation, the currents are the direct result of electron acceleration into the atmosphere by wave/particle interactions.

Ionospheric resistance has a complex nature, and leads to a secondary Hall current flow. By a strange twist of physics, the magnetic disturbance on the ground due to the main current almost cancels out, so most of the observed effect of auroras is due to a secondary current, the auroral electrojet. An auroral electrojet index (measured in nanotesla) is regularly derived from ground data and serves as a general measure of auroral activity. Kristian Birkeland [54] deduced that the currents flowed in the east–west directions along the auroral arc, and such currents, flowing from the dayside toward (approximately) midnight were later named "auroral electrojets" (see also Birkeland currents).

Earth is constantly immersed in the solar wind, a rarefied flow of magnetized hot plasma (a gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the two-million-degree temperature of the Sun's outermost layer, the corona. The quiescent solar wind reaches Earth with a velocity typically around 400 km/s, a density of around 5 ions/cm 3 and a magnetic field intensity of around 2–5 nT (for comparison, Earth's surface field is typically 30,000–50,000 nT). During magnetic storms, in particular, flows can be several times faster the interplanetary magnetic field (IMF) may also be much stronger. Joan Feynman deduced in the 1970s that the long-term averages of solar wind speed correlated with geomagnetic activity. [55] Her work resulted from data collected by the Explorer 33 spacecraft.

The solar wind and magnetosphere consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday's work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts across (or is cut by), rather than along, the lines of the magnetic field, an electric current is induced within the conductor. The strength of the current depends on a) the rate of relative motion, b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the direction of flow is dependent upon the direction of relative motion. Dynamos make use of this basic process ("the dynamo effect"), any and all conductors, solid or otherwise are so affected, including plasmas and other fluids.

The IMF originates on the Sun, linked to the sunspots, and its field lines (lines of force) are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun angles them at Earth by about 45 degrees forming a spiral in the ecliptic plane, known as the Parker spiral. The field lines passing Earth are therefore usually linked to those near the western edge ("limb") of the visible Sun at any time. [56]

The solar wind and the magnetosphere, being two electrically conducting fluids in relative motion, should be able in principle to generate electric currents by dynamo action and impart energy from the flow of the solar wind. However, this process is hampered by the fact that plasmas conduct readily along magnetic field lines, but less readily perpendicular to them. Energy is more effectively transferred by the temporary magnetic connection between the field lines of the solar wind and those of the magnetosphere. Unsurprisingly this process is known as magnetic reconnection. As already mentioned, it happens most readily when the interplanetary field is directed southward, in a similar direction to the geomagnetic field in the inner regions of both the north magnetic pole and south magnetic pole.

Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind. [57]

Magnetosphere Edit

Earth's magnetosphere is shaped by the impact of the solar wind on Earth's magnetic field. This forms an obstacle to the flow, diverting it, at an average distance of about 70,000 km (11 Earth radii or Re), [58] producing a bow shock 12,000 km to 15,000 km (1.9 to 2.4 Re) further upstream. The width of the magnetosphere abreast of Earth is typically 190,000 km (30 Re), and on the night side a long "magnetotail" of stretched field lines extends to great distances (> 200 Re).

The high latitude magnetosphere is filled with plasma as the solar wind passes Earth. The flow of plasma into the magnetosphere increases with additional turbulence, density, and speed in the solar wind. This flow is favored by a southward component of the IMF, which can then directly connect to the high latitude geomagnetic field lines. [59] The flow pattern of magnetospheric plasma is mainly from the magnetotail toward Earth, around Earth and back into the solar wind through the magnetopause on the day-side. In addition to moving perpendicular to Earth's magnetic field, some magnetospheric plasma travels down along Earth's magnetic field lines, gains additional energy and loses it to the atmosphere in the auroral zones. The cusps of the magnetosphere, separating geomagnetic field lines that close through Earth from those that close remotely allow a small amount of solar wind to directly reach the top of the atmosphere, producing an auroral glow.

On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms. [60] Two of the five probes, positioned approximately one third the distance to the Moon, measured events suggesting a magnetic reconnection event 96 seconds prior to auroral intensification. [61]

Geomagnetic storms that ignite auroras may occur more often during the months around the equinoxes. It is not well understood, but geomagnetic storms may vary with Earth's seasons. Two factors to consider are the tilt of both the solar and Earth's axis to the ecliptic plane. As Earth orbits throughout a year, it experiences an interplanetary magnetic field (IMF) from different latitudes of the Sun, which is tilted at 8 degrees. Similarly, the 23-degree tilt of Earth's axis about which the geomagnetic pole rotates with a diurnal variation changes the daily average angle that the geomagnetic field presents to the incident IMF throughout a year. These factors combined can lead to minor cyclical changes in the detailed way that the IMF links to the magnetosphere. In turn, this affects the average probability of opening a door [ colloquialism ] through which energy from the solar wind can reach Earth's inner magnetosphere and thereby enhance auroras. Recent evidence in 2021 has shown that individual separate substorms may in fact be correlated networked communities. [62]

Just as there are many types of aurora, there are many different mechanisms that accelerate auroral particles into the atmosphere. Electron aurora in Earth's auroral zone (i.e. commonly visible aurora) can be split into two main categories with different immediate causes: diffuse and discrete aurora. Diffuse aurora appear relatively structureless to an observer on the ground, with indistinct edges and amorphous forms. Discrete aurora are structured into distinct features with well-defined edges such as arcs, rays and coronas they also tend to be much brighter than the diffuse aurora.

In both cases, the electrons that eventually cause the aurora start out as electrons trapped by the magnetic field in Earth's magnetosphere. These trapped particles bounce back and forth along magnetic field lines and are prevented from hitting the atmosphere by the magnetic mirror formed by the increasing magnetic field strength closer to Earth. The magnetic mirror's ability to trap a particle depends on the particle's pitch angle: the angle between its direction of motion and the local magnetic field. An aurora is created by processes that decrease the pitch angle of many individual electrons, freeing them from the magnetic trap and causing them to hit the atmosphere.

In the case of diffuse auroras, the electron pitch angles are altered by their interaction with various plasma waves. Each interaction is essentially wave-particle scattering the electron energy after interacting with the wave is similar to its energy before interaction, but the direction of motion is altered. If the final direction of motion after scattering is close to the field line (specifically, if it falls within the loss cone) then the electron will hit the atmosphere. Diffuse auroras are caused by the collective effect of many such scattered electrons hitting the atmosphere. The process is mediated by the plasma waves, which become stronger during periods of high geomagnetic activity, leading to increased diffuse aurora at those times.

In the case of discrete auroras, the trapped electrons are accelerated toward Earth by electric fields that form at an altitude of about 4000–12000 km in the "auroral acceleration region". The electric fields point away from Earth (i.e. upward) along the magnetic field line. [63] Electrons moving downward through these fields gain a substantial amount of energy (on the order of a few keV) in the direction along the magnetic field line toward Earth. This field-aligned acceleration decreases the pitch angle for all of the electrons passing through the region, causing many of them to hit the upper atmosphere. In contrast to the scattering process leading to diffuse auroras, the electric field increases the kinetic energy of all of the electrons transiting downward through the acceleration region by the same amount. This accelerates electrons starting from the magnetosphere with initially low energies (10s of eV or less) to energies required to create an aurora (100s of eV or greater), allowing that large source of particles to contribute to creating auroral light.

The accelerated electrons carry an electric current along the magnetic field lines (a Birkeland current). Since the electric field points in the same direction as the current, there is a net conversion of electromagnetic energy into particle energy in the auroral acceleration region (an electric load). The energy to power this load is eventually supplied by the magnetized solar wind flowing around the obstacle of Earth's magnetic field, although exactly how that power flows through the magnetosphere is still an active area of research. [64] While the energy to power the aurora is ultimately derived from the solar wind, the electrons themselves do not travel directly from the solar wind into Earth's auroral zone magnetic field lines from these regions do not connect to the solar wind, so there is no direct access for solar wind electrons.

Some auroral features are also created by electrons accelerated by Alfvén waves. At small wavelengths (comparable to the electron inertial length or ion gyroradius), Alfvén waves develop a significant electric field parallel to the background magnetic field this can accelerate electrons due to a process of Landau damping. If the electrons have a speed close to that of the wave's phase velocity, they are accelerated in a manner analogous to a surfer catching an ocean wave. [65] [66] This constantly-changing wave electric field can accelerate electrons along the field line, causing some of them to hit the atmosphere. Electrons accelerated by this mechanism tend to have a broad energy spectrum, in contrast to the sharply-peaked energy spectrum typical of electrons accelerated by quasi-static electric fields.

In addition to the discrete and diffuse electron aurora, proton aurora is caused when magnetospheric protons collide with the upper atmosphere. The proton gains an electron in the interaction, and the resulting neutral hydrogen atom emits photons. The resulting light is too dim to be seen with the naked eye. Other aurora not covered by the above discussion include transpolar arcs (formed poleward of the auroral zone), cusp aurora (formed in two small high-latitude areas on the dayside) and some non-terrestrial auroras.

The discovery of a 1770 Japanese diary in 2017 depicting auroras above the ancient Japanese capital of Kyoto suggested that the storm may have been 7% larger than the Carrington event, which affected telegraph networks. [67] [68]

The auroras that resulted from the "great geomagnetic storm" on both 28 August and 2 September 1859, however, are thought to be the most spectacular in recent recorded history. In a paper to the Royal Society on 21 November 1861, Balfour Stewart described both auroral events as documented by a self-recording magnetograph at the Kew Observatory and established the connection between the 2 September 1859 auroral storm and the Carrington–Hodgson flare event when he observed that "It is not impossible to suppose that in this case our luminary was taken in the act." [69] The second auroral event, which occurred on 2 September 1859, as a result of the exceptionally intense Carrington–Hodgson white light solar flare on 1 September 1859, produced auroras, so widespread and extraordinarily bright that they were seen and reported in published scientific measurements, ship logs, and newspapers throughout the United States, Europe, Japan, and Australia. It was reported by The New York Times that in Boston on Friday 2 September 1859 the aurora was "so brilliant that at about one o'clock ordinary print could be read by the light". [70] One o'clock EST time on Friday 2 September would have been 6:00 GMT the self-recording magnetograph at the Kew Observatory was recording the geomagnetic storm, which was then one hour old, at its full intensity. Between 1859 and 1862, Elias Loomis published a series of nine papers on the Great Auroral Exhibition of 1859 in the American Journal of Science where he collected worldwide reports of the auroral event. [6]

That aurora is thought to have been produced by one of the most intense coronal mass ejections in history. It is also notable for the fact that it is the first time where the phenomena of auroral activity and electricity were unambiguously linked. This insight was made possible not only due to scientific magnetometer measurements of the era, but also as a result of a significant portion of the 125,000 miles (201,000 km) of telegraph lines then in service being significantly disrupted for many hours throughout the storm. Some telegraph lines, however, seem to have been of the appropriate length and orientation to produce a sufficient geomagnetically induced current from the electromagnetic field to allow for continued communication with the telegraph operator power supplies switched off. [71] The following conversation occurred between two operators of the American Telegraph Line between Boston and Portland, Maine, on the night of 2 September 1859 and reported in the Boston Traveler:

Boston operator (to Portland operator): "Please cut off your battery [power source] entirely for fifteen minutes."
Portland operator: "Will do so. It is now disconnected."
Boston: "Mine is disconnected, and we are working with the auroral current. How do you receive my writing?"
Portland: "Better than with our batteries on. – Current comes and goes gradually."
Boston: "My current is very strong at times, and we can work better without the batteries, as the aurora seems to neutralize and augment our batteries alternately, making current too strong at times for our relay magnets. Suppose we work without batteries while we are affected by this trouble."
Portland: "Very well. Shall I go ahead with business?"
Boston: "Yes. Go ahead."

The conversation was carried on for around two hours using no battery power at all and working solely with the current induced by the aurora, and it was said that this was the first time on record that more than a word or two was transmitted in such manner. [70] Such events led to the general conclusion that

The effect of the aurorae on the electric telegraph is generally to increase or diminish the electric current generated in working the wires. Sometimes it entirely neutralizes them, so that, in effect, no fluid [current] is discoverable in them. The aurora borealis seems to be composed of a mass of electric matter, resembling in every respect, that generated by the electric galvanic battery. The currents from it change coming on the wires, and then disappear the mass of the aurora rolls from the horizon to the zenith. [72]

An aurora was described by the Greek explorer Pytheas in the 4th century BC. [73] Seneca wrote about auroras in the first book of his Naturales Quaestiones, classifying them, for instance as pithaei ('barrel-like') chasmata ('chasm') pogoniae ('bearded') cyparissae ('like cypress trees'), and describing their manifold colors. He wrote about whether they were above or below the clouds, and recalled that under Tiberius, an aurora formed above the port city of Ostia that was so intense and red that a cohort of the army, stationed nearby for fire duty, galloped to the rescue. [74] It has been suggested that Pliny the Elder depicted the aurora borealis in his Natural History, when he refers to trabes, chasma, 'falling red flames' and 'daylight in the night'. [75]

The history of China has rich, and possibly the oldest, records of the aurora borealis. On an autumn around 2000 BC, according to a legend, a young woman named Fubao was sitting alone in the wilderness by a bay, when suddenly a "magical band of light" appeared like "moving clouds and flowing water", turning into a bright halo around the Big Dipper, which cascaded a pale silver brilliance, illuminating the earth and making shapes and shadows seem alive. Moved by this sight, Fubao became pregnant and gave birth to a son, the Emperor Xuanyuan, known legendarily as the initiator of Chinese culture and the ancestor of all Chinese people. In the Shanhaijing, a creature named 'Shilong' is described to be like a red dragon shining in the night sky with a body a thousand miles long. In ancient times, the Chinese did not have a fixed word for the aurora, so it was named according to the different shapes of the aurora, such as "Sky Dog (“天狗”)", "Sword/Knife Star (“刀星”)", "Chiyou banner (“蚩尤旗”)", "Sky's Open Eyes (“天开眼”)", and "Stars like Rain (“星陨如雨”)".

In Japanese folklore, pheasants were considered messengers from heaven. However, researchers from Japan's Graduate University for Advanced Studies and National Institute of Polar Research claimed in March 2020 that red pheasant tails witnessed across the night sky over Japan in 620 A.D., might be a red aurora produced during a magnetic storm. [76]

In the traditions of Aboriginal Australians, the Aurora Australis is commonly associated with fire. For example, the Gunditjmara people of western Victoria called auroras puae buae ('ashes'), while the Gunai people of eastern Victoria perceived auroras as bushfires in the spirit world. The Dieri people of South Australia say that an auroral display is kootchee, an evil spirit creating a large fire. Similarly, the Ngarrindjeri people of South Australia refer to auroras seen over Kangaroo Island as the campfires of spirits in the 'Land of the Dead'. Aboriginal people in southwest Queensland believe the auroras to be the fires of the Oola Pikka, ghostly spirits who spoke to the people through auroras. Sacred law forbade anyone except male elders from watching or interpreting the messages of ancestors they believed were transmitted through an aurora. [77]

In Scandinavia, the first mention of norðrljós (the northern lights) is found in the Norwegian chronicle Konungs Skuggsjá from AD 1230. The chronicler has heard about this phenomenon from compatriots returning from Greenland, and he gives three possible explanations: that the ocean was surrounded by vast fires that the sun flares could reach around the world to its night side or that glaciers could store energy so that they eventually became fluorescent. [78]

Walter William Bryant wrote in his book Kepler (1920) that Tycho Brahe "seems to have been something of a homœopathist, for he recommends sulfur to cure infectious diseases 'brought on by the sulphurous vapours of the Aurora Borealis ' ". [79]

In 1778, Benjamin Franklin theorized in his paper Aurora Borealis, Suppositions and Conjectures towards forming an Hypothesis for its Explanation that an aurora was caused by a concentration of electrical charge in the polar regions intensified by the snow and moisture in the air: [80] [81] [82]

May not then the great quantity of electricity brought into the polar regions by the clouds, which are condens'd there, and fall in snow, which electricity would enter the earth, but cannot penetrate the ice may it not, I say (as a bottle overcharged) break thro' that low atmosphere and run along in the vacuum over the air towards the equator, diverging as the degrees of longitude enlarge, strongly visible where densest, and becoming less visible as it more diverges till it finds a passage to the earth in more temperate climates, or is mingled with the upper air?

Observations of the rhythmic movement of compass needles due to the influence of an aurora were confirmed in the Swedish city of Uppsala by Anders Celsius and Olof Hiorter. In 1741, Hiorter was able to link large magnetic fluctuations with an aurora being observed overhead. This evidence helped to support their theory that 'magnetic storms' are responsible for such compass fluctuations. [83]

A variety of Native American myths surround the spectacle. The European explorer Samuel Hearne traveled with Chipewyan Dene in 1771 and recorded their views on the ed-thin ('caribou'). According to Hearne, the Dene people saw the resemblance between an aurora and the sparks produced when caribou fur is stroked. They believed that the lights were the spirits of their departed friends dancing in the sky, and when they shone brightly it meant that their deceased friends were very happy. [84]

During the night after the Battle of Fredericksburg, an aurora was seen from the battlefield. The Confederate Army took this as a sign that God was on their side, as the lights were rarely seen so far south. The painting Aurora Borealis by Frederic Edwin Church is widely interpreted to represent the conflict of the American Civil War. [85]

A mid 19th-century British source says auroras were a rare occurrence before the 18th century. [86] It quotes Halley as saying that before the aurora of 1716, no such phenomenon had been recorded for more than 80 years, and none of any consequence since 1574. It says no appearance is recorded in the Transactions of the French Academy of Sciences between 1666 and 1716. And that one aurora recorded in Berlin Miscellany for 1797 was called a very rare event. One observed in 1723 at Bologna was stated to be the first ever seen there. Celsius (1733) states the oldest residents of Uppsala thought the phenomenon a great rarity before 1716. The period between approximately 1645 to 1715 corresponds to the Maunder minimum in sunspot activity.

In Robert W. Service's satirical poem "The Ballad of the Northern Lights" (1908) a Yukon prospector discovers that the aurora is the glow from a radium mine. He stakes his claim, then goes to town looking for investors.

In the early 1900s, the Norwegian scientist Kristian Birkeland laid the foundation [ colloquialism ] for current understanding of geomagnetism and polar auroras.

Both Jupiter and Saturn have magnetic fields that are stronger than Earth's (Jupiter's equatorial field strength is 4.3 Gauss, compared to 0.3 Gauss for Earth), and both have extensive radiation belts. Auroras have been observed on both gas planets, most clearly using the Hubble Space Telescope, and the Cassini and Galileo spacecraft, as well as on Uranus and Neptune. [87]

The aurorae on Saturn seem, like Earth's, to be powered by the solar wind. However, Jupiter's aurorae are more complex. Jupiter's main auroral oval is associated with the plasma produced by the volcanic moon Io, and the transport of this plasma within the planet's magnetosphere. An uncertain fraction of Jupiter's aurorae are powered by the solar wind. In addition, the moons, especially Io, are also powerful sources of aurora. These arise from electric currents along field lines ("field aligned currents"), generated by a dynamo mechanism due to the relative motion between the rotating planet and the moving moon. Io, which has active volcanism and an ionosphere, is a particularly strong source, and its currents also generate radio emissions, which have been studied since 1955. Using the Hubble Space Telescope, auroras over Io, Europa and Ganymede have all been observed.

Auroras have also been observed on Venus and Mars. Venus has no magnetic field and so Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed over the full disc of the planet. [88] A Venusian aurora originates when electrons from the solar wind collide with the night-side atmosphere.

An aurora was detected on Mars, on 14 August 2004, by the SPICAM instrument aboard Mars Express. The aurora was located at Terra Cimmeria, in the region of 177° East, 52° South. The total size of the emission region was about 30 km across, and possibly about 8 km high. By analyzing a map of crustal magnetic anomalies compiled with data from Mars Global Surveyor, scientists observed that the region of the emissions corresponded to an area where the strongest magnetic field is localized. This correlation indicated that the origin of the light emission was a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars. [87] [89]

Between 2014 and 2016, cometary auroras were observed on comet 67P/Churyumov–Gerasimenko by multiple instruments on the Rosetta spacecraft. [90] [91] The auroras were observed at far-ultraviolet wavelengths. Coma observations revealed atomic emissions of hydrogen and oxygen caused by the photodissociation (not photoionization, like in terrestrial auroras) of water molecules in the comet's coma. [91] The interaction of accelerated electrons from the solar wind with gas particles in the coma is responsible for the aurora. [91] Since comet 67P has no magnetic field, the aurora is diffusely spread around the comet. [91]

Exoplanets, such as hot Jupiters, have been suggested to experience ionization in their upper atmospheres and generate an aurora modified by weather in their turbulent tropospheres. [92] However, there is no current detection of an exoplanet aurora.

The first ever extra-solar auroras were discovered in July 2015 over the brown dwarf star LSR J1835+3259. [93] The mainly red aurora was found to be a million times brighter than the Northern Lights, a result of the charged particles interacting with hydrogen in the atmosphere. It has been speculated that stellar winds may be stripping off material from the surface of the brown dwarf to produce their own electrons. Another possible explanation for the auroras is that an as-yet-undetected body around the dwarf star is throwing off material, as is the case with Jupiter and its moon Io. [94]


Titan Outside the Protective Bubble

Titan spends about 95 percent of the time within Saturn's magnetosphere. But during a Cassini flyby on Dec. 1, 2013, the giant moon happened to be on the sunward side of Saturn when a powerful outburst of solar activity reached the planet. The strong surge in the solar wind so compressed the sun-facing side of Saturn's magnetosphere that the bubble's outer edge was pushed inside the orbit of Titan. This left the moon exposed to, and unprotected from, the raging stream of energetic solar particles.


Wiggly signal hints of an aurora on a planet far from the Solar System

Radio astronomers might have spotted the strongest hint yet that a planet outside the Solar System has auroras similar to Earth’s Northern Lights.

Joseph Callingham at the Leiden Observatory in the Netherlands and his collaborators sifted through data from a sky survey by the Low-Frequency Array (LOFAR), a gigantic array of antennas in northern Europe. To their surprise, the team found several sources of circularly polarized radiation, meaning that the radio signal’s electric fields spiralled like a corkscrew as it travelled across space. Astrophysical objects other than planets are not known to be sources of circularly polarized light with a steady twist, which is the type LOFAR observed.

The most intriguing signal was from CR Draconis, a pair of tightly orbiting red-dwarf stars that are 20 parsecs (65 light years) from Earth. LOFAR had observed the system for 21 periods of around 8 hours each, which revealed radiation in a pattern reminiscent of auroras observed on Jupiter. The astronomers suggest that the effect is produced in the atmosphere of a Jupiter-like planet, or by the rapid rotation of one of the stars.

Some medieval corpses were revisited and rearranged within the container in which they had been interred. Credit: Éveha—Études et valorisations archéologiques


Contents

Hesiod's genealogy Edit

According to Hesiod, the Titan offspring of Uranus and Gaia were Oceanus, Coeus, Crius, Hyperion, Iapetus, Theia, Rhea, Themis, Mnemosyne, Phoebe, Tethys, and Cronus. [2] Eight of the Titan brothers and sisters married each other: Oceanus and Tethys, Coeus and Phoebe, Hyperion and Theia, and Cronus and Rhea. The other two Titan brothers married outside their immediate family. Iapetus married his niece Clymene, the daughter of Oceanus and Tethys, while Crius married his half-sister Eurybia, the daughter of Gaia and Pontus. The two remaining Titan sisters, Themis and Mnemosyne, became wives of their nephew Zeus.

From Oceanus and Tethys came the three thousand river gods, and three thousand Oceanid nymphs. [3] From Coeus and Phoebe came Leto, another wife of Zeus, and Asteria. [4] From Crius and Eurybia came Astraeus, Pallas, and Perses. [5] From Hyperion and Theia came the celestial personifications Helios (Sun), Selene (Moon), and Eos (Dawn). [6] From Iapetus and Clymene came Atlas, Menoetius, Prometheus, and Epimetheus. [7] From Cronus and Rhea came the Olympians: Hestia, Demeter, Hera, Hades, Poseidon, and Zeus. [8] By Zeus, Themis bore the three Horae (Hours), and the three Moirai (Fates), [9] and Mnemosyne bore the nine Muses. [10]

While the descendants of the Titans Oceanus and Tethys, Cronus and Rhea, Themis, and Mnemosyne (i.e. the river gods, the Oceanids, the Olympians, the Horae, the Moirai, and the Muses) are not normally considered to be Titans, descendants of the other Titans, notably: Leto, Helios, Atlas, and Prometheus, are themselves sometimes referred to as Titans. [11]

Variations Edit

Passages in a section of the Iliad called the Deception of Zeus suggest the possibility that Homer knew a tradition in which Oceanus and Tethys (rather than Uranus and Gaia, as in Hesiod) were the parents of the Titans. [18] Twice Homer has Hera describe the pair as "Oceanus, from whom the gods are sprung, and mother Tethys", while in the same passage Hypnos describes Oceanus as "from whom they all are sprung". [19]

Plato, in his Timaeus, provides a genealogy (probably Orphic) which perhaps reflected an attempt to reconcile this apparent divergence between Homer and Hesiod, with Uranus and Gaia as the parents of Oceanus and Tethys, and Oceanus and Tethys as the parents of Cronus and Rhea and "and all that go with them", plus Phorcys. [20] In his Cratylus, Plato quotes Orpheus as saying that Oceanus and Tethys were "the first to marry", possibly also reflecting an Orphic theogony in which Oceanus and Tethys, rather than Uranus and Gaia, were the primeval parents. [21] To Hesiod's twelve Titans, the mythographer Apollodorus, adds a thirteenth Titan, Dione, the mother of Aphrodite by Zeus. [22] Plato's inclusion of Phorkys, apparently, as a Titan, and the mythographer Apollodorus's inclusion of Dione, suggests an Orphic tradition in which the canonical twelve Titans consisted of Hesiod's twelve with Phorkys and Dione taking the place of Oceanus and Tethys. [23]

The Roman mythographer Hyginus, in his somewhat confused genealogy, [24] after listing as offspring of Aether (Upper Sky) and Earth (Gaia): Ocean [Oceanus], Themis, Tartarus, and Pontus, next lists "the Titans", followed by two of Hesiod's Hundred-Handers: Briareus and Gyges, one of Hesiod's three Cyclopes: Steropes, then continues his list with Atlas, Hyperion and Polus [Coeus], Saturn [Cronus], Ops [Rhea], Moneta [Mnemosyne], Dione, and the three Furies: Alecto, Megaera, and Tisiphone. [25] The geographer Pausanias, mentions seeing the image of a man in armor, who was supposed to be the Titan Anytus, who was said to have raised the Arcadian Despoina. [26]

The Titans, as a group, represent a pre-Olympian order. [27] Hesiod uses the expression "the former gods" (theoi proteroi) in reference to the Titans. [28] They were the banished gods, who were no longer part of the upper world. [29] Rather they were the gods who dwelt underground in Tartarus, [30] and as such, they may have been thought of as "gods of the underworld", who were the antithesis of, and in opposition to, the Olympians, the gods of the heavens. [31] Hesiod called the Titans "earth-born" (chthonic), [32] and in the Homeric Hymn to Apollo, Hera prays to the Titans "who dwell beneath the earth", calling on them to aid her against Zeus, just as if they were chthonic spirits. [33] In a similar fashion, in the Iliad, Hera, upon swearing an oath by the underworld river Styx, "invoked by name all the gods below Tartarus, that are called Titans" as witnesses. [34]

They were the older gods, but not, apparently, as was once thought, the old gods of an indigenous group in Greece, historically displaced by the new gods of Greek invaders. Rather, they were a group of gods, whose mythology at least, seems to have been borrowed from the Near East. [35] These imported gods gave context and provided a backstory for the Olympian gods, explaining where these Greek Olympian gods had come from, and how they had come to occupy their position of supremacy in the cosmos. The Titans were the previous generation, and family of gods, whom the Olympians had to overthrow, and banish from the upper world, in order to become the ruling pantheon of Greek gods.

For Hesiod, possibly in order to match the twelve Olympian gods, there were twelve Titans: six males and six females, with some of Hesiod's names perhaps being mere poetic inventions, so as to arrive at the right number. [36] In Hesiod's Theogony, apart from Cronus, the Titans play no part at all in the overthrow of Uranus, and we only hear of their collective action in the Titanomachy, their war with the Olympians. [37] As a group, they have no further role in conventional Greek myth, nor do they play any part in Greek cult. [38]

As individuals, few of the Titans have any separate identity. [39] Aside from Cronus, the only other figure Homer mentions by name as being a Titan is Iapetus. [40] Some Titans seem only to serve a genealogical function, providing parents for more important offspring: Coeus and Phoebe as the parents of Leto, the mother, by Zeus, of the Olympians Apollo and Artemis Hyperion and Theia as the parents of Helios, Selene and Eos Iapetus as the father of Atlas and Prometheus and Crius as the father of three sons Astraeus, Pallas, and Perses, who themselves seem only to exist to provide fathers for more important figures such as the Anemoi (Winds), Nike (Victory), and Hecate.

The Titans play a key role in an important part of Greek mythology, the succession myth. [41] It told how the Titan Cronus, the youngest of the Titans, overthrew Uranus, and how in turn Zeus, by waging and winning a great ten-year war pitting the new gods against the old gods, called the Titanomachy ("Titan war"), overthrew Cronus and his fellow Titans, and was eventually established as the final and permanent ruler of the cosmos. [42]

Hesiod Edit

According to the standard version of the succession myth, given in Hesiod's Theogony, Uranus initially produced eighteen children with Gaia: the twelve Titans, the three Cyclopes, and the three Hecatoncheires (Hundred-Handers), [43] but hating them, [44] he hid them away somewhere inside Gaia. [45] Angry and in distress, Gaia fashioned a sickle made of adamant and urged her children to punish their father. Only her son Cronus was willing. [46] So Gaia hid Cronus in "ambush", gave him an adamantine sickle, and when Uranus came to lie with Gaia, Cronus reached out and castrated his father. [47] This enabled the Titans to be born and Cronus to assume supreme command of the cosmos, with the Titans as his subordinates. [48]

Cronus, having now taken over control of the cosmos from Uranus, wanted to ensure that he maintained control. Uranus and Gaia had prophesied to Cronus that one of Cronus' own children would overthrow him, so when Cronus married Rhea, he made sure to swallow each of the children she birthed: Hestia, Demeter, Hera, Hades, Poseidon, and Zeus (in that order), to Rhea's great sorrow. [49] However, when Rhea was pregnant with Zeus, Rhea begged her parents Gaia and Uranus to help her save Zeus. So they sent Rhea to Lyctus on Crete to bear Zeus, and Gaia took the newborn Zeus to raise, hiding him deep in a cave beneath Mount Aigaion. [50] Meanwhile, Rhea gave Cronus a huge stone wrapped in baby's clothes which he swallowed thinking that it was another of Rhea's children. [51]

Zeus, now grown, forced Cronus (using some unspecified trickery of Gaia) to disgorge his other five children. [52] Zeus then released his uncles the Cyclopes (apparently still imprisoned beneath the earth, along with the Hundred-Handers, where Uranus had originally confined them) who then provide Zeus with his great weapon, the thunderbolt, which had been hidden by Gaia. [53] A great war was begun, the Titanomachy, for control of the cosmos. The Titans fought from Mount Othrys, while the Olympians fought from Mount Olympus. [54] In the tenth year of that great war, following Gaia's counsel, Zeus released the Hundred-Handers, who joined the war against the Titans, helping Zeus to gain the upper hand. Zeus cast the fury of his thunderbolt at the Titans, defeating them and throwing them into Tartarus, [55] with the Hundred-Handers as their guards. [56]

Homer Edit

Only brief references to the Titans and the succession myth are found in Homer. [57] In the Iliad, Homer tells us that "the gods . that are called Titans" reside in Tartarus. [58] Specifically, Homer says that "Iapetus and Cronos . have joy neither in the rays of Helios Hyperion nor in any breeze, but deep Tartarus is round about them", [59] and further, that Zeus "thrust Cronos down to dwell beneath earth and the unresting sea." [60]

Other early sources Edit

Brief mentions of the Titanomachy and the imprisonment of the Titans in Tartarus also occur in the Homeric Hymn to Apollo and Aeschylus' Prometheus Bound. [61] In the Hymn, Hera, angry at Zeus, calls upon the "Titan gods who dwell beneath the earth about great Tartarus, and from whom are sprung both gods and men". [62]

In Prometheus Bound, Prometheus (the son of the Titan Iapetus) refers to the Titanomachy, and his part in it:

When first the heavenly powers were moved to wrath, and mutual dissension was stirred up among them—some bent on casting Cronus from his seat so Zeus, in truth, might reign others, eager for the contrary end, that Zeus might never win mastery over the gods—it was then that I, although advising them for the best, was unable to persuade the Titans, children of Heaven and Earth but they, disdaining counsels of craft, in the pride of their strength thought to gain the mastery without a struggle and by force. . That it was not by brute strength nor through violence, but by guile that those who should gain the upper hand were destined to prevail. And though I argued all this to them, they did not pay any attention to my words. With all that before me, it seemed best that, joining with my mother, I should place myself, a welcome volunteer, on the side of Zeus and it is by reason of my counsel that the cavernous gloom of Tartarus now hides ancient Cronus and his allies within it. [63]

Apollodorus Edit

The mythographer Apollodorus, gives a similar account of the succession myth to Hesiod's, but with a few significant differences. [64] According to Apollodorus, there were thirteen original Titans, adding the Titanide Dione to Hesiod's list. [65] The Titans (instead of being Uranus' firstborn as in Hesiod) were born after the three Hundred-Handers and the three Cyclopes, [66] and while Uranus imprisoned these first six of his offspring, he apparently left the Titans free. Not just Cronus, but all the Titans, except Oceanus, attacked Uranus. After Cronus castrated Uranus, the Titans freed the Hundred-Handers and Cyclopes (unlike in Hesiod, where they apparently remained imprisoned), and made Cronus their sovereign, [67] who then reimprisoned the Hundred-Handers and Cyclopes in Tartarus. [68]

Although Hesiod does not say how Zeus was eventually able to free his siblings, according to Apollodorus, Zeus was aided by Oceanus' daughter Metis, who gave Cronus an emetic which forced him to disgorge his children that he had swallowed. [69] According to Apollodorus, in the tenth year of the ensuing war, Zeus learned from Gaia, that he would be victorious if he had the Hundred-Handers and the Cyclopes as allies. So Zeus slew their warder Campe (a detail not found in Hesiod) and released them, and in addition to giving Zeus his thunderbolt (as in Hesiod), the Cyclopes also gave Poseidon his trident, and Hades a helmet, and "with these weapons the gods overcame the Titans, shut them up in Tartarus, and appointed the Hundred-handers their guards". [70]

Hyginus Edit

The Roman mythographer Hyginus, in his Fabulae, gives an unusual (and perhaps confused) account of the Titanomachy. [71] Jupiter's (Zeus') jealous wife Juno (Hera) was angry at her husband, on account of Jupiter's son Epaphus by Io (one of her husband's many lovers). Because of this Juno incited the Titans to rebel against Jupiter and restore Saturn (Cronus) to the kingship of the gods. Jupiter, with the help of Minerva (Athena), Apollo, and Diana (Artemis), put down the rebellion, and hurled the Titans (as in other accounts) down to Tartarus.

After being overthrown in the Titanomachy, Cronus and his fellow vanquished Titans were cast into Tartarus:

That is where the Titan gods are hidden under murky gloom by the plans of the cloud-gatherer Zeus, in a dank place, at the farthest part of huge earth. They cannot get out, for Poseidon has set bronze gates upon it, and a wall is extended on both sides. [72]

However, besides Cronus, exactly which of the other Titans were supposed to have been imprisoned in Tartarus is unclear. [73] The only original Titan, mentioned by name, as being confined with Cronus in Tartarus, is Iapetus. [74]

But, not all the Titans were imprisoned there. Certainly Oceanus, the great world encircling river, seems to have remained free, and in fact, seems not to have fought on the Titans' side at all. [75] In Hesiod, Oceanus sends his daughter Styx, with her children Zelus (Envy), Nike (Victory), Kratos (Power), and Bia (Force), to fight on Zeus' side against the Titans, [76] while in the Iliad, Hera says that, during the Titanomachy, she was cared for by Oceanus and his wife the Titaness Tethys. [77] Aeschylus' Prometheus Bound, has Oceanus free to visit his nephew Prometheus sometime after the war. [78] Like Oceanus, Helios, the Titan son of Hyperion, certainly remained free to drive his sun-chariot daily across the sky, taking an active part in events subsequent to the Titanomachy. [79] The freedom of Oceanus, along with Helios (Sun), and perhaps Hyperion (to the extent that he also represented the sun), would seem to be the result of cosmological necessity, for how could a world encircling river, or the sun, be confined in Tartarus? [80]

As for other male offspring of the Titans, some seem to have participated in the Titanomachy, and were punished as a result, and others did not, or at least (like Helios) remained free. Three of Iapetus' sons, Atlas, Menoetius, and Prometheus are specifically connected by ancient sources with the war. In the Theogony both Atlas and Menoetius received punishments from Zeus, but Hesiod does not say for what crime exactly they were punished. [81] Atlas was famously punished by Zeus, by being forced to hold up the sky on his shoulders, but none of the early sources for this story (Hesiod, Homer, Pindar, and Aeschylus) say that his punishment was as a result of the war. [82] According to Hyginus however, Atlas led the Titans in a revolt against Zeus (Jupiter). [83] The Theogony has Menoetius struck down by Zeus' thunderbolt and cast into Erebus "because of his mad presumption and exceeding pride". [84] Whether Hesiod was using Erebus as another name for Tartarus (as was sometimes done), or meant that Menoetius's punishment was because of his participation in the Titanomachy is unclear, and no other early source mentions this event, however Apollodorus says that it was. [85] Hesiod does not mention Prometheus in connection with the Titanomachy, but Prometheus does remain free, in the Theogony, for his deception of Zeus at Mecone and his subsequent theft of fire, for which transgressions Prometheus was famously punished by Zeus by being chained to a rock where an eagle came to eat his "immortal liver" every day, which then grew back every night. [86] However Aeschylus's Prometheus Bound (as mentioned above) does have Prometheus say that he was an ally of Zeus during the Titanomachy. [87]

The female Titans, to the extent that they are mentioned at all, appear also to have been allowed to remain free. [88] Three of these, according to the Theogony, become wives of Zeus: Themis, Mnemosyne, and Leto, the daughter of the Titans Coeus and Phoebe. [89] Themis gives birth to the three Horae (Hours), and the three Moirai (Fates), and Mnemosyne gives birth to the nine Muses. Leto, who gives birth to the Olympians Apollo and Artemis, takes an active part on the side of the Trojans in the Iliad, and is also involved in the story of the giant Tityos. [90] Tethys, presumably along with her husband Oceanus, took no part in the war, and, as mentioned above, provided safe refuge for Hera during the war. Rhea remains free and active after the war: [91] appearing at Leto's delivery of Apollo, [92] as Zeus' messenger to Demeter announcing the settlement concerning Persephone, [93] bringing Pelops back to life. [94]

Released? Edit

While in Hesiod's Theogony, and Homer's Iliad, Cronus and the other Titans are confined to Tartarus—apparently forever [95] —another tradition, as indicated by later sources, seems to have had Cronus, or other of the Titans, being eventually set free. [96] Pindar, in one of his poems (462 BC), says that, although Atlas still "strains against the weight of the sky . Zeus freed the Titans", [97] and in another poem (476 BC), Pindar has Cronus, in fact, ruling in the Isles of the Blessed, a land where the Greek heroes reside in the afterlife: [98]

Those who have persevered three times, on either side, to keep their souls free from all wrongdoing, follow Zeus' road to the end, to the tower of Cronus, where ocean breezes blow around the island of the blessed, and flowers of gold are blazing, some from splendid trees on land, while water nurtures others. With these wreaths and garlands of flowers they entwine their hands according to the righteous counsels of Rhadamanthys, whom the great father, the husband of Rhea whose throne is above all others, keeps close beside him as his partner. [99]

Prometheus Lyomenos, an undated lost play by Aeschylus (c. 525 – c. 455 BC), had a chorus composed of freed Titans. Possibly even earlier than Pindar and Aeschylus, two papyrus versions of a passage of Hesiods' Works and Days also mention Cronus being released by Zeus, and ruling over the heroes who go to the Isle of the Blessed but other versions of Hesiod's text do not, and most editors judge these lines of text to be later interpolations. [100]

It is generally accepted that the Greek succession myth was imported from the Near East, and that along with this imported myth came stories of a group of former ruling gods, who had been defeated and displaced, and who became identified, by the Greeks, as the Titans. [103] Features of Hesiod's account of the Titans can be seen in the stories of the Hurrians, the Hittites, the Babylonians, and other Near Eastern cultures. [104]

The Hurro-Hittite text Song of Kumarbi (also called Kingship in Heaven), written five hundred years before Hesiod, [105] tells of a succession of kings in heaven: Anu (Sky), Kumarbi, and the storm-god Teshub, with many striking parallels to Hesiod's account of the Greek succession myth. Like Cronus, Kumarbi castrates the sky-god Anu, and takes over his kingship. And like Cronus, Kumarbi swallows gods (and a stone?), one of whom is the storm-god Teshub, who like the storm-god Zeus, is apparently victorious against Kumarbi and others in a war of the gods. [106]

Other Hittite texts contain allusions to "former gods" (karuilies siunes), precisely what Hesiod called the Titans, theoi proteroi. Like the Titans, these Hittite karuilies siunes, were twelve (usually) in number and end up confined in the underworld by the storm-god Teshub, imprisoned by gates they cannot open. [107] In Hurrian, the Hittite's karuilies siunes were known as the "gods of down under" (enna durenna) and the Hittites identified these gods with the Anunnaki, the Babylonian gods of the underworld, [108] whose defeat and imprisonment by the storm-god Marduk, in the Babylonian poem Enûma Eliš (late second millennium BC or earlier), [109] parallels the defeat and imprisonment of the Titans. [110] Other collectivities of gods, perhaps associated with the Mesopotamian Anunnaki, include the Dead Gods (Dingiruggû), the Banished Gods (ilāni darsūti), and the Defeated (or Bound) Gods (ilāni kamûti). [111]

The sparagmos Edit

In Orphic literature, the Titans play an important role in what is often considered to be the central myth of Orphism, the sparagmos, that is the dismemberment of Dionysus, who in this context is often given the title Zagreus. [112] As pieced together from various ancient sources, the reconstructed story, usually given by modern scholars, goes as follows. [113] Zeus had intercourse with Persephone in the form of a serpent, producing Dionysus. He is taken to Mount Ida where (like the infant Zeus) he is guarded by the dancing Curetes. Zeus intended Dionysus to be his successor as ruler of the cosmos, but a jealous Hera incited the Titans—who apparently unlike in Hesiod and Homer, were not imprisoned in Tartarus—to kill the child. The Titans whiten their faces with gypsum, and distracting the infant Dionysus with various toys, including a mirror, they seized Dionysus and tore (or cut) [114] him to pieces. The pieces were then boiled, roasted and partially eaten, by the Titans. But Athena managed to save Dionysus' heart, by which Zeus was able to contrive his rebirth from Semele.

The anthropogony Edit

Commonly presented as a part of the myth of the dismembered Dionysus Zagreus, is an Orphic anthropogony, that is an Orphic account of the origin of human beings. According to this widely held view, as punishment for their crime, Zeus struck the Titans with his thunderbolt, and from the remains of the destroyed Titans humankind was born, which resulted in a human inheritance of ancestral guilt, for this original sin of the Titans, and by some accounts "formed the basis for an Orphic doctrine of the divinity of man." [115] However, when and to what extent there existed any Orphic tradition which included these elements is the subject of open debate. [116]

The 2nd century AD biographer and essayist Plutarch makes a connection between the sparagmos and the punishment of the Titans, but makes no mention of the anthropogony, or Orpheus, or Orphism. In his essay On the Eating of Flesh, Plutarch writes of "stories told about the sufferings and dismemberment of Dionysus and the outrageous assaults of the Titans upon him, and their punishment and blasting by thunderbolt after they had tasted his blood". [117] While, according to the early 4th century AD Christian apologist Arnobius, and the 5th century AD Greek epic poet Nonnus, it is as punishment for their murder of Dionysus that the Titans end up imprisoned by Zeus in Tartarus. [118]

The only ancient source to explicitly connect the sparagmos and the anthropogony is the 6th century AD Neoplatonist Olympiodorus, who writes that, according to Orpheus, after the Titans had dismembered and eaten Dionysus, "Zeus, angered by the deed, blasts them with his thunderbolts, and from the sublimate of the vapors that rise from them comes the matter from which men are created." Olympiodorus goes on to conclude that, because the Titans had eaten his flesh, we their descendants, are a part of Dionysus. [119]

Modern interpretations Edit

Some 19th- and 20th-century scholars, including Jane Ellen Harrison, have argued that an initiatory or shamanic ritual underlies the myth of the dismemberment and cannibalism of Dionysus by the Titans. [120] Martin Litchfield West also asserts this in relation to shamanistic initiatory rites of early Greek religious practices. [121]

The etymology of Τiτᾶνες (Titanes) is uncertain. [122] Hesiod in the Theogony gives a double etymology, deriving it from titaino [to strain] and tisis [vengeance], saying that Uranus gave them the name Titans: "in reproach, for he said that they strained and did presumptuously a fearful deed, and that vengeance for it would come afterwards". [123] But modern scholars doubt Hesiod's etymology. [124]

Jane Ellen Harrison asserts that the word "Titan" comes from the Greek τίτανος, signifying white "earth, clay, or gypsum," and that the Titans were "white clay men", or men covered by white clay or gypsum dust in their rituals. [125]

The planet Saturn is named for the Roman equivalent of the Titan Cronus. Saturn's largest moon, Titan, is named after the Titans generally, and the other moons of Saturn are named after individual Titans, specifically Tethys, Phoebe, Rhea, Hyperion, and Iapetus. Astronomer William Henry Pickering claimed to have discovered another moon of Saturn which he named Themis, but this discovery was never confirmed, and the name Themis was given to an asteroid, 24 Themis. Asteroid 57 Mnemosyne was also named for the Titan.

A proto-planet Theia is hypothesized to have been involved in a collision in the early solar system, forming the Earth's moon.


Triton and Its Volcanoes

Figure 4: Neptune’s Moon Triton. This mosaic of Voyager 2 images of Triton shows a wide range of surface features. The pinkish area at the bottom is Triton’s large southern polar cap. The south pole of Triton faces the Sun here, and the slight heating effect is driving some of the material northward, where it is colder. (credit: modification of work by NASA/JPL/USGS)

Neptune’s largest moon Triton (don’t get its name confused with Titan) has a diameter of 2720 kilometers and a density of 2.1 g/cm 3 , indicating that it’s probably composed of about 75% rock mixed with 25% water ice. Measurements indicate that Triton’s surface has the coldest temperature of any of the worlds our robot representatives have visited. Because its reflectivity is so high (about 80%), Triton reflects most of the solar energy that falls on it, resulting in a surface temperature between 35 and 40 K.

The surface material of Triton is made of frozen water, nitrogen, methane, and carbon monoxide. Methane and nitrogen exist as gas in most of the solar system, but they are frozen at Triton’s temperatures. Only a small quantity of nitrogen vapor persists to form an atmosphere. Although the surface pressure of this atmosphere is only 16 millionths of a bar, this is sufficient to support thin haze or cloud layers.

Triton’s surface, like that of many other moons in the outer solar system, reveals a long history of geological evolution (Figure 4). Although some impact craters are found, many regions have been flooded fairly recently by the local version of “lava” (perhaps water or water-ammonia mixtures). There are also mysterious regions of jumbled or mountainous terrain.

Figure 5: Triton’s Geysers. This close-up view shows some of the geysers on Neptune’s moon Triton, with the long trains of dust pointing to the lower right in this picture. (credit: modification of work by NASA/JPL)

The Voyager flyby of Triton took place at a time when the moon’s southern pole was tipped toward the Sun, allowing this part of the surface to enjoy a period of relative warmth. (Remember that “warm” on Triton is still outrageously colder than anything we experience on Earth.) A polar cap covers much of Triton’s southern hemisphere, apparently evaporating along the northern edge. This polar cap may consist of frozen nitrogen that was deposited during the previous winter.

Remarkably, the Voyager images showed that the evaporation of Triton’s polar cap generates geysers or volcanic plumes of nitrogen gas (see Figure 5). (Fountains of such gas rose about 10 kilometers high, visible in the thin atmosphere because dust from the surface rose with them and colored them dark.) These plumes differ from the volcanic plumes of Io in their composition and also in that they derive their energy from sunlight warming the surface rather than from internal heat.

Key Concepts and Summary

Saturn’s moon Titan has an atmosphere that is thicker than that of Earth. There are lakes and rivers of liquid hydrocarbons, and evidence of a cycle of evaporation, condensation, and return to the surface that is similar to the water cycle on Earth (but with liquid methane and ethane). The Cassini-Huygens lander set down on Titan and showed a scene with boulders, made of water ice, frozen harder than rock. Neptune’s cold moon Triton has a very thin atmosphere and nitrogen gas geysers.


Watch the video: What Huygens Saw On Titan - New Image Processing (July 2022).


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