Plasmas/Plasma objects/Auroras

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This dramatic panorama shows a colourful, shimmering auroral curtain reflected in a placid Icelandic lake. Credit: Carlos Gauna. {{fairuse}}

Auroras can be caused by electrons being absorbed into an atmosphere.

The "dramatic panorama [on the right shows a colorful], shimmering auroral curtain reflected in a placid Icelandic lake. The image was taken on 18 March 2015 by Carlos Gauna, near Jökulsárlón Glacier Lagoon in southern Iceland."[1]

"The celestial display was generated by a coronal mass ejection, or CME, on 15 March. Sweeping across the inner Solar System at some 3 million km per hour, the eruption reached Earth, 150 million kilometres away, in only two days. The gaseous cloud collided with Earth’s magnetic field at around 04:30 GMT on 17 March."[1]

"When the charged particles from the Sun penetrate Earth's magnetic shield, they are channelled downwards along the magnetic field lines until they strike atoms of gas high in the atmosphere. Like a giant fluorescent neon lamp, the interaction with excited oxygen atoms generates a green or, more rarely, red glow in the night sky, while excited nitrogen atoms yield blue and purple colours."[1]

"Auroral displays are not just decorative distractions. They are most frequent when the Sun's activity nears its peak roughly every 11 years. At such times, the inflow of high-energy particles and the buffeting of Earth’s magnetic field may sometimes cause power blackouts, disruption of radio communications, damage to satellites and even threaten astronaut safety."[1]

Astronomy[edit]

Main source: Astronomy
This aurora was viewed from the Space Shuttle. Credit: NASA.
A time-lapse animation of Aurorae over both Earth poles shows symmetries and simultaneous changes. Credit: Polar VIS, JPL, NASA.

On the upper right is an auroral ribbon viewed from one of the Space Shuttles.

On the lower right is a time-lapse animation of Aurorae over both Earth poles that shows symmetries and simultaneous changes.

"Auroras in the north and south can be nearly mirror images of each other. Such mirroring had been suspected for centuries but dramatically confirmed only last month by detailed images from NASA's orbiting Polar spacecraft. Pictured above, a time-lapse movie shows simultaneous changes in aurora borealis, at the top, and aurora australis, at the bottom. A cloud of electrons and ions moving out from the Sun on October 22 created the auroras. The solar explosion that released the particles occurred about three days earlier."[2]

Rays[edit]

Main sources: Radiation/Rays and Rays
This aurora suggests that the closer the observer is to being directly underneath the more it looks like radiation spraying down. Credit: Ole C. Salomonsen.
Aurora is seen from the island of Kvaløya in Norway during the 23rd of January 2011. Credit: Lars Tiede.
This form of an aurora is called an arc. Credit: Jan Curtis.
The image shows the bands form of auroras. Credit: Jan Curtis.
Some auroras occur locally to the observer as patches. Credit: Jan Curtis.
Auroras can occur as rays. Credit: Jan Curtis.
Auroras can also occur as veils. Credit: Jan Curtis.
This is another example of a black aurora. Credit: rgk.
Another form of overhead aurora is a corona. Credit: rgk.
This is a large general form called a curtain. Credit: rgk.
A more straightened form of the aurora is the drape. Credit: rgk.

The image on the right suggests that the closer the observer is to being directly underneath the aurora the more it looks like radiation spraying down. The more distant from being directly below the more the aurora looks like ribbons.

The image on the left is from a location within the auroral oval where it frequently appears directly overhead.

Auroras come in several different and sometimes overlapping forms:

  1. arcs (a slightly curving arc of light with smooth lower borders, second down on the left),
  2. bands (continuous but irregular lower borders with kinks and folds, third down on the left),
  3. blacks (a well-defined dark region within an extensive, otherwise uniform, auroral display, fourth down on the right),
  4. coronas (rays converging onto a point which indicates the overhead position of the magnetic field along which the observer is located, fifth down on the left),
  5. curls (a counter-clockwise (in northern hemisphere) rotation having a diameter of 0.5 to 3 miles (1 to 5 km); a ray viewed from the bottom rather than the side),
  6. curtains (similar to drape with large vertical extent having sharp lower edge and gradually fading upwards, fifth down on the right),
  7. drapes (long-rayed band with folds, sixth down on the left),
  8. folds (an S-shaped curve on a similar scale as a curl),
  9. patches (isolated small region of luminosity like a patch of cloud, second down on the right),
  10. rays (a streak or shaft of luminosity, third down on the right),
  11. spirals (a larger scale (50 to 1000 miles) curled pattern), or
  12. veils (uniform luminosity covering a large fraction of the sky, fourth down on the left).

"Those long arches that extend roughly east-west (actually magnetic east-west) from horizon to horizon are called arcs. If of nonuniform curvature, these forms are called bands. No really meaningful difference exists between arcs and bands, except that the more convoluted form, the band, is often brighter than the arc, and the appearance of bands usually signifies that the overall display is becoming more active."[3]

"Arcs and bands are thin ribbons set on edge parallel to the ground. The thickness of an arc or band may be as little as 100 meters (100 yds). The lower edge is typically 80 to 120 km (50 to 75 miles) above the earth and the upper edge is usually 10 to 100 km above that. Off out to the east or west, arcs and bands appear to meet the horizon. Still roughly 100 km above the surface, the aurora there is more than 600 miles distant. Hence, the aurora seen to meet the eastern horizon at Fairbanks is actually nearly directly overhead at Whitehorse, and visa versa. Similarly, an arc or band seen from Fairbanks to be 20 degrees above the north horizon is directly overhead Fort Yukon."[3]

Bands[edit]

This is a band aurora that occurred over Manitoba, Canada. Credit: Euan Mearns. {{fairuse}}
This image shows bands over Iceland. Credit: Theo Schacht. {{free media}}

An aurora occurring as bands can have several bands in close proximity following a similar path as with the bands in the right image.

The auroral oval is a permanent structure over the polar regions. Bands should occur around the polar regions as specific co-parallel contributors to the oval structure. In support of this, the image on the right shows bands over northern Canada. The second image down on the right shows bands over Iceland.

Drapes[edit]

This is an aurora over Svalbard on 27 November 2014. Credit: Martyn Smith from London, England. {{free media}}

The more straightened form of a curtain is a drape. This aurora over Svalbard on 27 November 2014 appears to be a slightly ragged edged drape.

Antimatter[edit]

The geomagnetically trapped antiproton spectrum measured by PAMELA is graphed. Credit: O. Adriani et al..

There is "an antiproton radiation belt around the Earth."[4]

"Some of the antiparticles produced in the innermost region of the magnetosphere are captured by the geomagnetic field allowing the formation of an antiproton radiation belt around the Earth."[4]

"During about 850 days of data acquisition (from 2006 July to 2008 December), 28 trapped antiprotons were identified within the kinetic energy range 60–750 MeV."[4]

In the graph on the right are shown "the [South Atlantic Anomaly] SAA region (red full circles). The error bars indicate statistical uncertainties. Trapped antiproton predictions by Selesnick et al. (2007) for the PAMELA satellite orbit (solid line), and by Gusev et al. (2008) at L = 1.2 (dotted line), are also reported. For comparison, the mean atmospheric under-cutoff antiproton spectrum outside SAA region (blue open circles) and the galactic [cosmic ray] CR antiproton spectrum (black squares) measured by PAMELA (Adriani et al. 2010a) are also shown."[4]

Planetary sciences[edit]

Split auroral ring is shown. Credit: Unknown, or unstated.
North and South Auroras Aren't Mirrored: This series of near-simultaneous auroras were observed between 11:24 am and 12:10 pm Universal Time. Credit: NASA.

Usually, planets exhibit auroral rings as seen from space around their magnetic poles.

In the example on the right, the auroral ring is actually split into two portions, perhaps with a black aurora between the rings.

"This series of near-simultaneous auroras [on the left] were observed between 11:24 am and 12:10 pm Universal Time (6:24am and 7:10am ET) on October 23, 2002. Observations were made of the northern (left) and southern (right) hemispheres by IMAGE and Polar satellites, respectively. White dots indicate the geographic poles. Analysis of the spacecraft images showed how the auroras shift depending on the "tilt" of the Earth's magnetic field toward the sun and conditions in the solar wind. The "12" at the top indicates noon (the direction toward the sun), and "0" at the bottom indicates midnight, (the direction away from the sun). Likewise, the "6" indicates dawn or morning side of the Earth, while "18" indicates dusk or evening side of the Earth, thus placing the auroras on a 24 hour clock face."[5]

"Looking at the auroras from space, they look like almost circular bands of light around the North and South Poles."[5]

"From spacecraft observations made in October, 2002, scientists noticed that these circular bands of aurora shift in opposite directions to each other depending on the orientation of the sun's magnetic field, which travels toward the Earth with the solar wind flow. They also noted that the auroras shift in opposite directions to each other depending on how far the Earth's northern magnetic pole is leaning toward the sun."[5]

"What was most surprising was that both the northern and southern auroral ovals were leaning toward the dawn (morning) side of the Earth for this event."[5]

"This is the first analysis to use simultaneous observations of the whole aurora in both the northern and southern hemispheres to track their locations."[6]

Colors[edit]

The image shows separate curtains of an aurora borealis. Credit: surangaw / Fotolia.

Usually auroras seen locally are arcs that are part of an auroral oval around or near the magnetic poles. In the image on the right are separate curtains apparently from one aurora borealis.

Ices[edit]

Main sources: Minerals/Ices and Ices
The image shows light pillars over Finland. Credit: Thomas Kast.

"Pictured [on the right] are not aurora but nearby light pillars, a local phenomenon that can appear as a distant one. In most places on Earth, a lucky viewer can see a Sun-pillar, a column of light appearing to extend up from the Sun caused by flat fluttering ice-crystals reflecting sunlight from the upper atmosphere. Usually these ice crystals evaporate before reaching the ground. During freezing temperatures, however, flat fluttering ice crystals may form near the ground in a form of light snow, sometimes known as a crystal fog. These ice crystals may then reflect ground lights in columns not unlike a Sun-pillar."[7]

These "light pillars [are] extending up from bright parking lot lights in Oulu, Finland."[7]

Def. a "visual phenomenon created by the reflection of light from ice crystals with near-horizontal parallel planar surfaces"[8] is called a light pillar.

Theoretical auroras[edit]

The Aurora Borealis, or Northern Lights, shines above Bear Lake. Credit: United States Air Force photo by Senior Airman Joshua Strang.
This is an aurora forecast based on the OVATION model for 13 May 2015. Credit: NOAA Space Weather.
This is NOAA's WSA-Enlil Solar Wind Prediction graphic, from 21:00 UTC (5 p.m. ET) on May 12, 2015. Credit: NOAA, with notations by Scott Sutherland.

Def. an "atmospheric phenomenon created by charged particles from the sun striking the upper atmosphere, creating coloured lights in the sky"[9] is called an aurora.

On the lower right is an aurora forecast based on the OVATION model for 13 May 2015.

"If you're living in northern regions of Atlantic Canada, Quebec or Ontario, and anywhere across the Prairies, northeastern British Columbia and northward from there, there was a very good chance you saw something last night, as shown [on the lower right]."[10]

"The Sun has been fairly quiet lately, with a few spits and spots worth mentioning, but amid that relative quiet, a strong wind has been blowing. A few days ago, a large coronal hole rotated into view on the face of the Sun. This region - the dark "open eye" of the winky face the Sun is making in the image above - is where the Sun's magnetic field lines have opened up, allowing charged solar particles to stream away from the surface at very high speed."[10]

The image on the left is NOAA's WSA-Enlil Solar Wind Prediction graphic for the effects of the solar wind stream from the coronal hole.

"The wide band of yellow-orange-red, with Earth's dot is right smack dab in the middle of it, is that fast stream of solar particles - a region of the solar wind called a "coronal hole high speed stream" (CH HSS). The reason why the particles of the solar wind are moving so quickly in this region is because they're traveling through space that has been 'swept clear' of other particles by a band of denser solar plasma that swept past us earlier in the week."[10]

"When those fast-moving solar particles interact with Earth's magnetic field, it can have the same effect as when a dense cloud of plasma (a coronal mass ejection, or CME) washes over us - it can cause a geomagnetic storm, resulting in a heightened auroral activity."[10]

"The CME shown in the image resulted from a large dark filament of solar material blasting away from the Sun's western limb. Since this solar plasma was launched out into space well ahead of Earth, it is not expected to affect us."[10]

Electromagnetic interactions[edit]

"[P]ulsating auroras [are] a certain type of aurora that appears as patches of brightness regularly flickering on and off".[11]

"A drop in the number of low-energy electrons, long thought to have little or no effect, corresponds with especially fast changes in the shape and structure of pulsating auroras."[11]

"Without the combination of ground and satellite measurements, we would not have been able to confirm that these events are connected."[11]

"Pulsating auroras are so-called because their features shift and brighten in distinct patches, rather than elongated arcs across the sky like active auroras. However, their appearance isn't the only difference. Though all auroras are caused by energetic particles--typically electrons--speeding down into Earth's atmosphere and colliding brilliantly with the atoms and molecules in the air, the source of these electrons is different for pulsating auroras and active auroras."[11]

"Active auroras happen when a dense wave of solar material--such as a high-speed stream of solar wind or a large cloud that exploded off the sun called a coronal mass ejection--hits Earth's magnetic field, causing it to rattle. This rattling releases electrons that have been trapped in the tail of that magnetic field, which stretches out away from the sun. Once released, these electrons go racing down towards the poles, then they interact with particles in Earth's upper atmosphere to create glowing lights that stretch across the sky in long ropes."[11]

"On the other hand, the electrons that set off pulsating auroras are sent spinning to the surface by complicated wave motions in the magnetosphere. These wave motions can happen at any time, not just when a wave of solar material rattles the magnetic field."[11]

"The hemispheres are magnetically connected, meaning that any time there is pulsating aurora near the north pole, there is also pulsating aurora near the south pole. Electrons are constantly pinging back and forth along this magnetic field line during an aurora event."[12]

"The electrons that travel between the hemispheres are not the original higher-energy electrons rocketing in from the magnetosphere. Instead, these are what's called low-energy secondary electrons, meaning that they are slower particles that have been kicked up out in all directions only after a collision from the first set of higher-energy electrons. When this happens, some of the secondary electrons shoot back upwards along the magnetic field line, zipping towards the opposite hemisphere."[11]

The "most distinct change in the structure and shape of the aurora happened during times when far fewer of these secondary electrons were shooting in along hemispheric magnetic field lines."[11]

"It turns out that secondary electrons could very well be a big piece of the puzzle to how, why, and when the energy that creates auroras is transferred to the upper atmosphere."[11]

"We need targeted observations to figure out exactly how to incorporate these low-energy secondary electrons into our models. But it seems clear that they may very well end up playing a more important role than previously thought."[11]

"Measurements of the number and energies of electrons were made by two satellites that happened to be passing overhead during these pulsating aurora events: Reimei, a JAXA satellite tasked with studying auroras, and a satellite from the U.S. Department of Defense's Defense Meteorological Satellite Program. The ground-based all-sky cameras--used to study both auroras and meteors--are operated at Poker Flat Research Range in Fairbanks, Alaska and the European Incoherent Scatter Scientific Association Radar Facility in Tromsø, Norway."[11]

Plasma meteors[edit]

These three frames from an animation of THEMIS/ASI images shows auroras colliding. Credit: Toshi Nishimura/UCLA.
This animation shows the collision of auroras. Credit: Toshi Nishimura/UCLA.

"A network of cameras deployed around the Arctic in support of NASA’s THEMIS mission has made a startling discovery about the Northern Lights. Sometimes, vast curtains of aurora borealis collide, producing spectacular outbursts of light."[13]

“Our jaws dropped when we saw the movies for the first time. These outbursts are telling us something very fundamental about the nature of auroras.”[14]

"The collisions occur on such a vast scale that isolated observers on Earth — with limited fields of view — had never noticed them before. It took a network of sensitive cameras spread across thousands of miles to get the big picture."[13]

"NASA and the Canadian Space Agency created such a network for THEMIS, short for “Time History of Events and Macroscale Interactions during Substorms.” THEMIS consists of five identical probes launched in 2006 to solve a long-standing mystery: Why do auroras occasionally erupt in an explosion of light called a substorm?"[13]

"Twenty all-sky imagers (ASIs) were deployed across the Alaskan and Canadian Arctic to photograph auroras from below while the spacecraft sampled charged particles and electromagnetic fields from above. Together, the on-ground cameras and spacecraft would see the action from both sides and be able to piece together cause and effect-or so researchers hoped. It seems to have worked."[13]

There is "a common sequence of events. It begins with a broad curtain of slow-moving auroras and a smaller knot of fast-moving auroras, initially far apart. The slow curtain quietly hangs in place, almost immobile, when the speedy knot rushes in from the north. The auroras collide and an eruption of light ensues."[13]

The "fast-moving knot [may be] associated with a stream of relatively lightweight plasma jetting through the [plasma] tail. The stream gets started in the outer regions of the plasma tail and moves rapidly inward toward Earth. The fast knot of auroras moves in synch with this stream. [...] Meanwhile, the broad curtain of auroras is connected to the stationary inner boundary of the plasma tail and fueled by plasma instabilities there. When the lightweight stream reaches the inner boundary of the plasma tail, there is an eruption of plasma waves and instabilities. This collision of plasma is mirrored by a collision of auroras over the poles."[13]

"Millions of kilometers long and pointed away from the sun, the plasma tail is made of charged particles captured mainly from the solar wind. Sometimes called the "plasma sheet," the tail is held together by Earth's magnetic field."[13]

Particles[edit]

This yellow-brown, ground-touching aurora may be more than just electrons. Credit: unknown, or unstated. {{fairuse}}
Typical dayside cusp aurora over Kjell Henriksen Observatory is occurring at 24.11.2014. Credit: F. Sigernes, Kjell Henriksen Observatory, Svalbard. {{fairuse}}

On the right is an image of a yellow-brown, ground-touching aurora that may be more than just due to electrons.

Def. "a particular subset of the [polar auroras] in which energetic particles are accelerated downward into the atmosphere directly from the solar wind"[15] is called a cusp aurora.

"Though cusp auroras are not particularly rare, they are often difficult to spot because they only happen during the day, when sunlight usually drowns out what would otherwise be a spectacular light show. However, because the magnetic North Pole is offset from the geographic North Pole, it’s often possible to see cusp auroras in Northern Europe near the winter solstice."[15]

“The magnetic pole is tilted towards North America, putting this magnetic opening—the cusp—at a higher latitude on the European side. Combine that extra-high latitude with the winter solstice—when nights are longest, especially as you go farther north—and you can sometimes see this daytime aurora with the naked eye.”[16]

Electromagnetic, "or EM, waves [...] accelerate electrons down into Earth’s atmosphere or up out to space. The electrons that are accelerated downward collide with particles in the atmosphere, releasing light and creating the cusp aurora".[15]

"CAPER [Cusp Alfven and Plasma Electrodynamics Rocket], flying on a four-stage Oriole IV sounding rocket, carries three instruments—one to measure low-frequency EM waves, one to measure high-frequency EM waves, and one to measure the number of particles at different energy levels."[15]

The "density of neutral atoms within the atmosphere can change throughout the day because of heating by sunlight, the original understanding was that the heating—and the extra-dense layers of neutral particles—was driven horizontally. However, some satellites have hit speed bumps as they have orbited through Earth’s magnetic cusp—their acceleration briefly slowed, which indicates a small vertical slice of higher-density neutral atoms that are harder to travel through."[15]

“When solar wind electrons collide with atmospheric electrons, they transfer some of their energy, heating the atmospheric electrons. The higher heat means the electron populations expand upward along the magnetic field lines.”[17]

"This upward flow of negatively-charged particles creates a vertical electric field, which in turn pulls up the positively-charged and neutral particles, increasing the atmospheric density in columns rather than horizontal layers."[15]

In the image on the left, dayside cusp auroras are shown as the Moon total eclipses the Sun.

Cosmic rays[edit]

The relatively low energy cosmic rays from the sun (aka solar wind) are also responsible for auroras at Earth's poles. Credit: NASA.

The relatively low energy cosmic rays from the sun (aka solar wind) are also responsible for auroras such as shown in the space image on the right at Earth's poles.

Electrons[edit]

This is an example of Aurora Australis. Credit: Samuel Blanc[1].

Aurorae are mostly caused by energetic electrons precipitating into the atmosphere.[18]

X-rays[edit]

Bright X-ray arcs of low energy (0.1 - 10 keV) are generated during auroral activity. Observation dates: 10 pointings between December 16, 2003 and April 13, 2004. Instrument: HRC. Credit: NASA/MSFC/CXC/A.Bhardwaj & R.Elsner, et al.; Earth model: NASA/GSFC/L.Perkins & G.Shirah.
This image is a composite of the first picture of the Earth in X-rays over a diagram of the Earth below. Credit: NASA, Ruth Netting.

"[L]ow-altitude regions of downward electric current on auroral magnetic field lines are sites of dramatic upward magnetic field-aligned electron acceleration that generates intense magnetic field-aligned electron beams within Earth’s equatorial middle magnetosphere."[19]

"The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km. It owes its existence primarily to ultraviolet radiation from the Sun."[20]

The images [lower right] are superimposed on a simulated image of the Earth. The color code represents brightness, maximum in red. Distance from the North pole to the black circle is 3,340 km (2,080 mi).

"Auroras are produced by solar storms that eject clouds of energetic charged particles. These particles are deflected when they encounter the Earth’s magnetic field, but in the process large electric voltages are created. Electrons trapped in the Earth’s magnetic field are accelerated by these voltages and spiral along the magnetic field into the polar regions. There they collide with atoms high in the atmosphere and emit X-rays".[21]

At right is a composite image which contains the first picture of the Earth in X-rays, taken in March, 1996, with the orbiting Polar satellite. The area of brightest X-ray emission is red.

Energetic charged particles from the Sun energize electrons in the Earth's magnetosphere. These electrons move along the Earth's magnetic field and eventually strike the ionosphere, causing the X-ray emission. Lightning strikes or bolts across the sky also emit X-rays.[22]

Ultraviolets[edit]

A sequence of false-colour images of ultraviolet Aurora shows the development of a magnetospheric substorm. Credit: NASA, Polar, VIS, L. A. Frank, John. B. Sigwarth et al. (University of Iowa).

On the right is a sequence of false-colour images of ultraviolet Aurora showing the development of a magnetospheric substorm, taken about 3 minutes apart with the Earth Camera of the Visible Imaging System (VIS) of the Polar satellite from 3:09 to 3:29 UT.

Opticals[edit]

The image shows an aurora australis seen from space. Credit: Author not stated, or wTSmHnV.
NASA's Polar spacecraft took this image of the aurora over Earth's northern hemisphere. Credit: NASA.
When we see an auroral arc – and associated rays – we really seeing a small section of the much larger, permanent aurora called the auroral oval. Credit: NASA.

This image on the right of an aurora australis seen from space was probably captured using an optical device aboard the IMAGE spacecraft.

"NASA's Polar spacecraft took [a] series of images [including the one second down on the right] of the aurora over Earth's northern hemisphere. The images were collected by Polar's Visible Imaging System in February 2000, and they reveal the auroral oval around the polar regions in visible and ultraviolet light. The most intense auroral activity appears in bright red or white."[5]

On the left is an image of the auroral oval. When we see an auroral arc – and associated rays – we are really seeing a small section of the much larger, permanent aurora called the auroral oval.

Visuals[edit]

This yellow aurora near the horizon has many vertical rays, sometimes called "light pillars", though these are probably not from ice crystals. Credit: Unknown, or unstated. {{fairuse}}
These two rays in the foreground are distinctive from the background aurora. Credit: Unknown, or unstated. {{fairuse}}
The image shows rays from an unexpected aurora. Credit: Anderson. {{fairuse}}
This is a white and aqua aurora over Finland. Credit: Unknown, or unstated. {{fairuse}}

The image on the right shows a yellow aurora near the horizon that has many vertical rays, sometimes called "light pillars", though these are probably not from ice crystals.

The second image down on the right shows two distinctive rays in the foreground that terminate in yellow over Queenstown, New Zealand, in July 2012.

"This aurora [on the left] was a bit of a surprise. For starters, on this Friday morning in August 2002, no intense auroral activity was expected at all. Possibly more surprising, however, the aurora appeared to show an usual structure of green rays from some locations. In the [left] image, captured from North Dakota, USA, a picket fence of green rays stretches toward the horizon. Mirroring the green rays is a red band, somewhat rare in its own right. Lights from the cities of Bismarck and Mandan are visible near the horizon. Large sunspot groups indicate that activity from an active Sun is relatively likely, possibly causing other streams of energetic particles to cascade onto the Earth and so causing more auroras."[23]

"The ray structure often seen in arcs and bands marks out the orientation of the magnetic field, nearly vertical at high latitude. The vertical extent of arcs and bands is also along this direction. Though the rays appear to converge upward, they are, in reality, essentially parallel shafts of light."[3]

"If rayed aurora is directly overhead, the point to which the rays appear to converge is the magnetic zenith. A line from that point to the observer marks out the local direction of the earth's magnetic field."[3]

"Standing in the aurora like pickets in a fence, the rays sometimes move sideways across the arcs and bands at high speeds. Sometimes one even sees them appear to move past each other both to the left and the right."[24]

"Rays line up along the direction of the earth's magnetic field, which points nearly vertically and somewhat to the northeast over Alaska and western Canada. To recognize the cross-sectional shapes of the rays, one needs to see them directly overhead in the sky. When they are in that position, they don't look like rays anymore; one reason why it took so long to discover their true shapes."[24]

"Not until very sensitive, high-speed television cameras were aimed at the bottoms of rays overhead was the mystery resolved. [The] rays were tightly wound up spirals only a kilometer or two across. Their form is difficult to recognize with the naked eye because the curled up shapes develop so quickly--sometimes in a second or so--and they often move very rapidly."[24]

"With a television camera capable of taking 30 pictures each second, it was possible to record the development of the spiral-shaped rays and measure their motion. Sometimes they move across the sky at speeds one hundred times that of a jet aircraft. To the observer on the ground, they do not appear to move quite that fast because the rays are so far away."[24]

This is a white aurora at the lower center and an aqua aurora in the upper part of the image on the lowest right.

White auroras[edit]

This is a white aurora. Credit: hensen.
This is a predominantly white aurora over Canada. Credit: Paul Zizka.

The images on the right show white auroras. The image in the section above on the right shows a white aurora combined with an aqua portion.

Multicolored auroras[edit]

This is a multicolor aurora. Credit: tommy-eliassen and leonafaye.
This shows a multicolored aurora over Finland. Credit: S. D. Simonson.

"Auroras are known to be generated by beams of electrons which are accelerated along Earth's magnetic field lines. The fast-moving electrons collide with atoms in the ionosphere at altitudes of between 100 to 600 km. This interaction with oxygen atoms results in a green or, more rarely, red glow in the night sky, while nitrogen atoms yield blue and purple colours."[25]

On the right are two images of multicolored auroras. The second down on the right occurred over Finland in October 2012.

Black auroras[edit]

This image appears to contain a black aurora between the red and green. Credit: Unknown, or not stated.
This aurora over Tromso, Norway, appears to contains a black aurora. Credit: Unknown, or unstated.
This is apparently the first of two images of auroras over Fairbanks, Alaska. Credit: Unknown, or unstated.
This is apparently the second of two images of auroras over Fairbanks, Alaska. Credit: Unknown, or unstated.

"Most people have heard of auroras - more commonly known as the Northern and Southern Lights - but, except on rare occasions, such as the recent widespread apparition on 17 March, they are not usually visible outside the polar regions. Less familiar are phenomena known as black auroras, dark patches which often subdivide the glowing curtains of red and green light."[25]

"Whereas bright auroras are created by electrons plunging downward into the ionosphere, neighbouring black auroras are caused by electrons escaping from the ionosphere - like a kind of anti-aurora. However, until now, scientists have been struggling to explain the relationship between the two auroral types."[25]

"We found strong evidence of a two-way interaction between the ionosphere and the magnetosphere."[26]

"Auroral arcs are created by electric currents. The beam of electrons shooting down towards Earth along magnetic field lines is actually an electric current aligned with Earth's magnetic field. It is called an upward, field-aligned current because the negatively charged electrons are moving downward."[26]

"On the other hand, when a downward magnetospheric current meets the ionosphere, electrons are driven upwards and 'sucked' from the ionosphere, creating a black aurora. However, when the electron density in the ionosphere drops markedly the black aurora becomes less intense."[26]

"This evacuation of the ionosphere is essential in shaping the black auroras. The process is much more important on Earth's nightside than on the dayside because sunlight creates new electrons which fill the 'hole'."[26]

The "two-way electrodynamic coupling between the magnetosphere and ionosphere [...] is made possible by a horizontal drift of ions in the ionosphere, known as the Pedersen current, which closes the current system."[26]

"According to convention, negatively charged electrons flow downward, from the magnetosphere to the ionosphere, in an upward field-aligned current. Electrons flow upward, from the ionosphere to the magnetosphere, in a downward field-aligned current."[25]

The third and fourth images down on the right are apparently two successive images of the same aurora showing changes with time and black auroras.

Violets[edit]

The aurora borealis imaged shows blue, violet, and purple colors. Credit: Ragnar Sigurdsson.
This multicolored aurora has a strong violet band above the pink band. Credit: Black Swamp Storm Intercept Team.

The aurora borealis imaged on the right shows blue, violet, and purple colors with the Milky Way in the background.

The second aurora on the right contains an intense violet band above the pink band.

Blues[edit]

This is a blue aurora borealis that occurred over Iceland. Credit: Daniel Nelson.
The image contains an extensive blue aurora over Canada. Credit: Unknown, or unstated.
This is a blue aurora with some purple at the lower left. Credit: Micha.

The image on the right shows blue aurora borealis that occurred over Iceland.

The second image down on the right shows an extensive blue aurora above the green over Canada.

The image on the left shows an extensive blue aurora.

Cyans[edit]

This aurora borealis is a greenish-blue or cyan. Credit: beautiful-portals.tumblr.com.
This aurora contains a band of aqua-blue. Credit: Unknown, or unstated.

The aurora borealis on the right is probably the usual green aurora but appears greenish-blue or cyan.

Greens[edit]

The image shows Aurora Borealis at its finest in Alaska. Credit: jewishbrick.
This green aurora appears to have a black aurora at its center. Credit: Sebastian Saarloos.
This green aurora has the appearance of coming at the viewer. Credit: Unknown, or unstated.
This is a loop aurora over Summit, Alaska. Credit: Jason Ahrns.

This aurora borealis on the right that occurred over Alaska is almost all green.

The second green aurora down on the right is over Urenroe, Russia. It shows the radiation pattern of being directly overhead.

"Last night Earth experienced a geomagnetic storm and aurora were visible in the Northern U.S. states. [This image on the left] of [an] aurora [was] captured on March 17, 2015, around 5:30 a.m. EDT in Donnelly Creek, Alaska by Sebastian Saarloos. These aurora might have been caused by the fast solar wind streaming from two solar coronal holes."[27]

An earlier green aurora is shown second down on the left from apparently January 2015.

Yellows[edit]

This photo of yellow and green auroras shows convincingly that yellow is a distinctive result of the auroral process. Credit: Belinda Witzenhausen.
This image captures an unusual aurora from Urengoi, Russia. Credit: Unknown.
This aurora image from Alaska shows distinctive yellow associated with the horizon. Credit: Unknown.
This is another aurora from Alaska containing yellow. Credit: Unknown.
This orange and yellow aurora occurred above Beaghmore Stone Circles. Credit: Martin McKenna.

Any doubt that a yellow aurora can occur should be put to rest with the image on the right.

The image on the left shows individual rays of radiation apparently impacting an upper atmospheric layer to produce a bead-like pattern.

The second image down on the left shows yellow of an aurora near the horizon with apparently the midnight Sun off to the left.

The third image on the left contains yellow aurora that is closer to true yellow.

The second image down on the right shows a yellow aurora following the skyline with an orange aurora above.

"On February 25th 2014 a violent X4.9-class solar flare erupted from a large sunspot group which had just rotated into view around the SE limb of the solar disk. The CME it unfurled was a massive full halo feature in the form of an expanding cloud of highly charged particles and plasma en route to the inner planets at a staggering velocity of over 2000km/sec. At this speed the CME would sweep across 93 million miles of space and impact planet Earth in only two days. However there was bad news as the source of this flare - and subsequent CME event - was located so close to the limb of the sun that the CME was very unlikely to impact Earth because it was located too far from the meridian and hence was not termed geoeffective which meant there was no chance of any Earth directed component at all. A few hours later a more detailed look by spaceweather scientists followed which offered some cautious optimism for in some of their forecasting models there was a slight chance that the CME could hit Earth a glancing blow with a possibility of minor geomagnetic storms on Feb 27th however the consensus was that the CME would probably miss entirely or if there was a hit then it wasn't expected to be significant."[28]

"The Bz is the secret to a good aurora show, this is [where] its at, the Bz (pronounced Bee Sub Zee) is a value indicating the tilt of the Interplanetary Magnetic Field or IMF. If the Bz is N then you can forget about a good show, even if the KP is good it won't make a difference, however if the Bz tilts S then the Earth and Sun's magnetic fields become aligned and in effect what you are doing is opening a gate way [...] allowing the highly charged solar particles to interact with the Earth's magnetosphere undisturbed - this open channel will manifest as a strong geomagnetic storm. The fact that it was - 20 got me extremely excited, this value meant the aurora was going to be strong and would be seen from far more southern latitudes than usual."[28]

Oranges[edit]

This is an aurora borealis photographed as occurring above Finland. Credit: Pekka Parviainen.
This is an orange aurora over New York. Credit: Unknown, or unstated.
This is an extensively orange aurora that occurred over Maine. Credit: Unknown, or unstated.

The aurora imaged on the right occurred over Finland in early October 2002. Note the pastel orange colors.

The second image down on the right shows a reddish-orange aurora observed over New York in October 2011.

To compare and contrast with the orange-containing aurora on the right is the extensively orange aurora on the left which also occurred over Finland.

Reds[edit]

This is a red aurora borealis. Credit: Isarl.
A view of an all-red aurora is captured in Independence, Mo., on October 24, 2011. Credit: Tobias Billings.

On the right is an example of a red aurora borealis.

"A coronal mass ejection (CME) shot off the sun late in the evening of October 21 [2011] and hit Earth on October 24 at about 2 PM ET. The CME caused strong magnetic field fluctuations near Earth's surface – technically, this level of magnetic fluctuation rated a 7 out of 9 on what is called the "KP index" – that resulted in aurora that could be seen in the US as far south as Alabama. This image [on the left] was captured in Independence, Mo. Such completely red aurora are not as common as green aurora, however they can happen during strong solar activity and they occur a little more often at low latitudes such as where this was taken."[29]

"The strength, speed, and mass of this CME also pushed the boundary of Earth's magnetic fields – a boundary known as the magnetopause – from its normal position at about 40,000 miles away from Earth in to about 26,000 miles. This is the area where spacecraft in geosynchronous orbit reside, so these spacecraft were briefly orbiting outside of Earth's normal environment, traveling through material and magnetic fields far different from usual."[29]

Hydrogens[edit]

Main sources: Chemicals/Hydrogens and Hydrogens
The spectrum shows the lines in the visible due to emission from elemental hydrogen. Credit:Teravolt.

Hydrogen has two emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas at 397.007 nm of the Balmer series (Hε) and 434.05 nm Hγ.[30]

Nitrogens[edit]

Main sources: Chemicals/Nitrogens and Nitrogens
The spectrum shows the lines in the visible due to emission from elemental nitrogen. Credit:Kurgus.

Nitrogen has two emission lines that occur in plasmas at 455.368 and 455.545 nm from N VII.[30]

There is an "(0,2) vibrational component of the B-x electronic transition of N2(+) at 470.9 nm."[31]

Oxygens[edit]

Main sources: Chemicals/Oxygens and Oxygens
The spectrum shows the lines in the visible due to emission from elemental oxygen. Credit:Teravolt.

Oxygen has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 406.963, 406.99, 407.22, 407.59, 407.89, 408.51, 435.12, 441.489, and 441.697 nm from O II, and 434.045 nm from O VIII.[30]

"Electron temperatures are generally derived from the ratio of auroral to nebular lines in [O III] or [N II]."[32] "[B]ecause of the proximity of strong night-sky lines at λ4358 and λλ5770, 5791, the auroral lines of [O III] λ4363 and [N II] λ5755 are often contaminated."[32]

Argons[edit]

Main sources: Chemicals/Argons and Argons
This shows an Argon spectra using a 600lpm diffraction grating. Credit: teravolt.

Argon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 426.653, 428.29, 433.12, 434.8064, 437.075, 437.967, 442.60, and 443.019 nm from Ar II.[30]

Atmospheres[edit]

Joule heating "is simply the process by which an electrical current flowing through a resistive media increases the temperature or heats the media. Examples of this are an electric toaster coil or the heating element in an electric stove, oven or space heater.”[33]

“Electric currents, driven by the solar wind when encountering Earth’s magnetic field, exist in and around the region where aurora occur. These invisible currents heat the thin air of the upper atmosphere of Earth through the Joule heating process.”[33]

“This process is different than the energetic radiation which cause the spectacular visible glow of the dancing northern lights and the scientific community is trying to determine the relative importance of each.”[33]

“Satellite drag is difficult to predict without a precise understanding of the state of the thermosphere which limits the ability to forecast satellite trajectories. This is especially true when large amounts of electromagnetic energy are dumped into the thermosphere and dissipated through the Joule heating process.”[33]

“One of the results of heating in and around the aurora is an expanded thermosphere. This expanded gas can increase the drag on satellites (those under or about 620 miles altitude) by 1,000% or more for a few days which shifts their orbits significantly.”[33]

Sun[edit]

Main sources: Stars/Sun and Sun (star)
Four images of a filament on the sun from August 31, 2012 are shown here in various wavelengths of light as captured by NASA’s Solar Dynamics Observatory (SDO). Credit: NASA/SDO/AIA/GSFC.

"On August 31, 2012 a long filament of solar material [in the four panels on the lower right] that had been hovering in the sun's atmosphere, the corona, erupted out into space at 4:36 p.m. EDT. The coronal mass ejection, or CME, traveled at over 900 miles per second. The CME did not travel directly toward Earth, but did connect with Earth's magnetic environment, or magnetosphere, with a glancing blow, causing aurora to appear on the night of Monday, September 3 [shown in the first image on the right]."[34]

"Four images of a filament on the sun from August 31, 2012 are shown here [at the lower right] in various wavelengths of light as captured by NASA’s Solar Dynamics Observatory (SDO). Starting from the upper left and going clockwise they represent light in the: 335, 171, 131 and 304 Angstrom wavelengths [ultraviolets]."[34]

Earth auroras[edit]

Main sources: Earth/Auroras and Auroras
This image shows an aurora australis viewed from the International Space Station. Credit: International Space Station.
An auroral curtain unfolds at dawn over Lake Superior. Credit: Bob King.
The Auroral Oval is an instantaneous position of luminosity below which the Earth rotates. Credit: rgk.
The Auroral Zone is the position of maximum probability of observing auroral luminosity. Credit: rgk.

"This striking aurora image [on the right] was taken during a geomagnetic storm that was most likely caused by a coronal mass ejection from the Sun on May 24, 2010. The ISS was located over the Southern Indian Ocean at an altitude of 350 kilometers (220 miles), with the astronaut observer most likely looking towards Antarctica (not visible) and the South Pole."[35]

"The aurora has a sinuous ribbon shape that separates into discrete spots near the lower right corner of the image. While the dominant coloration of the aurora is green, there are faint suggestions of red left of image center. Dense cloud cover is dimly visible below the aurora. The curvature of the Earth’s horizon (the limb) is clearly visible, as is the faint blue line of the upper atmosphere directly above it (at image top center). Several stars appear as bright pinpoints against the blackness of space at image top right."[35]

The second image down on the right shows an "auroral curtain unfolds at dawn over Lake Superior in Duluth, Minnesota, on July 15, 2012. At right, Venus and Jupiter shine near the Hyades in Taurus."[36]

"The Auroral Oval [in the third image down on the right is] the instantaneous position of luminosity below which the Earth rotates. Typically around 67 degrees geomagnetic latitude around midnight and moving to 78 degrees geomagnetic latitude at midday. The picture below shows the auroral oval in winter approaching midnight in Albany, NY."[37]

"The Auroral Zone: the position of maximum probability of observing auroral luminosity. Viewing conditions are best around midnight and the aurora also tends to be brighter in this section of the oval. Consequently the highest probability of viewing aurora occurs around the geomagnetic latitude of 67 degrees."[37]

"Note that the Auroral Zone passes through central Alaska, but in the picture [fourth down on the right] taken just after sunset in Alaska, the Auroral Oval lies to the north. The aurora may be visible at this latitude only during winter; at other times of the year the aurora is not usually visible here because of daylight. The Auroral Oval will pass over central Alaska later in the evening."[37]

Moon[edit]

Main sources: Wanderers/Moon and Moon
This aurora image from Alaska shows the Moon off to the left. Credit: Unknown or unstated.
This image shows a massive aurora after a CME with the Moon on the right. Credit: David Cartier, Sr.

The aurora image on the right shows the Moon off to the left, a yellow to green aurora ribbon on the right and apparent black auroras as gaps between light pillars.

"Swirls of green and red appear in an aurora over Whitehorse, Yukon on the night of September 3, 2012. The aurora was due to the interaction of a coronal mass ejection (CME) from the sun with Earth's magnetosphere. The CME left the sun on August 31 and arrived on September 3."[34]

Mars[edit]

Main sources: Wanderers/Mars and Mars
For five days in December, the Mars Atmosphere and Volatile Evolution (MAVEN) detected an ultraviolet glow blanketing the northern half of the Red Planet. Credit: Bruce Jakosky, NASA.

For "five days in December, the spacecraft detected an ultraviolet glow blanketing the northern half of the Red Planet. The light show, similar to the northern lights on Earth, coincided with a fierce solar storm, when the sun flooded the solar system with charged particles."[38]

“Nobody expected to see auroras in the northern hemisphere. This changes our view of how the sun interacts with Mars.”[39]

"The European Space Agency’s Mars Express orbiter detected auroras in the Martian southern hemisphere in 2005, but they were concentrated over isolated magnetic spots on the surface."[38]

Jupiter[edit]

Main sources: Wanderers/Jupiter and Jupiter
This is a schematic of Jupiter's magnetosphere and the components influenced by Io (near the center of the image). Credit: John Spencer.
Image of Jupiter aurora in UV was taken with the Space Telescope Imaging Spectrograph (STIS) of NASA's Hubble Space Telescope. Credit: John T. Clarke (University of Michigan), ESA, NASA.

The image at right represents "[t]he Jovian magnetosphere [magnetic field lines in blue], including the Io flux tube [in green], Jovian aurorae, the sodium cloud [in yellow], and sulfur torus [in red]."[40]

"Field-aligned equatorial electron beams [have been] observed within Jupiter’s middle magnetosphere. ... the Jupiter equatorial electron beams are spatially and/or temporally structured (down to <20 km at auroral altitudes, or less than several minutes), with regions of intense beams intermixed with regions absent of such beams."[19]

"This is a spectacular NASA Hubble Space Telescope close-up view [on the left] of an electric-blue aurora that is eerily glowing one half billion miles away on the giant planet Jupiter. Auroras are curtains of light resulting from high-energy electrons racing along the planet's magnetic field into the upper atmosphere. The electrons excite atmospheric gases, causing them to glow. The image shows the main oval of the aurora, which is centered on the magnetic north pole, plus more diffuse emissions inside the polar cap."[41]

"Though the aurora resembles the same phenomenon that crowns Earth's polar regions, the Hubble image shows unique emissions from the magnetic "footprints" of three of Jupiter's largest moons. (These points are reached by following Jupiter's magnetic field from each satellite down to the planet)."[41]

"Auroral footprints can be seen in this image from Io (along the left hand limb), Ganymede (near the center), and Europa (just below and to the right of Ganymede's auroral footprint). These emissions, produced by electric currents generated by the satellites, flow along Jupiter's magnetic field, bouncing in and out of the upper atmosphere. They are unlike anything seen on Earth."[41]

"This ultraviolet image of Jupiter was taken with the Hubble Space Telescope Imaging Spectrograph (STIS) on November 26, 1998. In this ultraviolet view, the aurora stands out clearly, but Jupiter's cloud structure is masked by haze."[41]

Io[edit]

Main source: Io
Gases above Io's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). Credit: NASA/JPL/University of Arizona.

"This eerie view of Jupiter's moon Io in eclipse [on the right] was acquired by NASA's Galileo spacecraft while the moon was in Jupiter's shadow. Gases above the satellite's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). The vivid colors, caused by collisions between Io's atmospheric gases and energetic charged particles trapped in Jupiter's magnetic field, had not previously been observed. The green and red emissions are probably produced by mechanisms similar to those in Earth's polar regions that produce the aurora, or northern and southern lights. Bright blue glows mark the sites of dense plumes of volcanic vapor, and may be places where Io is electrically connected to Jupiter."[42]

Saturn[edit]

Main sources: Wanderers/Saturn and Saturn
Composite infrared image of Saturn at 4 µ (H3+, taken on November 10, 2006) shown in blue and 5 µ shown in red colour (taken on June 15, 2008). Credit: NASA/JPL/University of Arizona.

"[M]agnetospheric electron (bi-directional) beams connect to the expected locations of Saturn’s aurora"[43].

"Powered by the Saturnian equivalent of (filamentary) Birkeland currents, streams of charged particles from the interplanetary medium interact with the planet's magnetic field and funnel down to the poles.[44] Double layers are associated with (filamentary) currents,[45][46] and their electric fields accelerate ions and electrons.[47]"[48]

"Energetic particles, crashing into the upper atmosphere cause the aurora, shown in blue [in the image on the right], to glow brightly at 4 microns (six times the wavelength visible to the human eye). The image shows both a bright ring, as seen from Earth, as well as an example of bright auroral emission within the polar cap that had been undetected until the advent of Cassini. This aurora, which defies past predictions of what was expected, has been observed to grow even brighter than is shown here. Silhouetted by the glow (cast here to the color red) of the hot interior of Saturn (clearly seen at a wavelength of 5 microns, or seven times the wavelength visible to the human eye) are the clouds and haze that underlie this auroral region."[49]

Uranus[edit]

Main sources: Wanderers/Uranus and Uranus
This composite image combines 2011 Hubble observations of the aurorae in visible and ultraviolet light. Credit: NASA, ESA, and L. Lamy (Observatory of Paris, CNRS, CNES).

"These are among the first clear images, taken from the distance of Earth, to show aurorae on the planet Uranus. Aurorae are produced when high-energy particles from the Sun cascade along magnetic field lines into a planet's upper atmosphere. This causes the planet's atmospheric gasses to fluoresce. The ultraviolet images were taken at the time of heightened solar activity in November 2011 that successively buffeted the Earth, Jupiter, and Uranus with a gusher of charged particles from the Sun. Because Uranus' magnetic field is inclined 59 degrees to its spin axis, the auroral spots appear far from the planet's north and south poles. This composite image combines 2011 Hubble observations of the aurorae in visible and ultraviolet light, 1986 Voyager 2 photos of the cyan disk of Uranus as seen in visible light, and 2011 Gemini Observatory observations of the faint ring system as seen in infrared light."[50]

Brown dwarfs[edit]

"Astronomers have discovered the first aurora ever seen in an object beyond our Solar System. The aurora -- similar to the famous "Northern Lights" on Earth -- is 10,000 times more powerful than any previously seen. They found the aurora not from a planet, but from a low-mass star at the boundary between stars and brown dwarfs."[51]

"All the magnetic activity we see on this object can be explained by powerful auroras. This indicates that auroral activity replaces solar-like coronal activity on brown dwarfs and smaller objects."[52]

"The astronomers observed the object, called LSR J1835+3259, using the Karl G. Jansky Very Large Array (VLA) at radio wavelengths, along with the 5-meter Hale Telescope on Palomar Mountain and the 10-meter Keck Telescope in Hawaii at optical wavelengths. The combination of radio and optical observations showed that the object, 18 light-years from Earth, has characteristics unlike any seen in more-massive stars."[51]

"Brown dwarfs, sometimes called "failed stars," are objects more massive than planets, yet too small to trigger the thermonuclear reactions at their cores that power stars. The astronomers said their observations of LSR J1835+3259 indicate that the coolest stars and brown dwarfs have outer atmospheres that support auroral activity, rather than the type of magnetic activity seen on more-massive and hotter stars."[51]

"The discovery also has implications for studying extrasolar planets. The aurora the scientists observed from LSR J1835+3259 appears powered by a little-understood dynamo process similar to that seen on larger planets in our Solar System. This process is different from that which causes the Earth's auroral displays -- the planet's magnetic field interacting with the solar wind."[51]

"What we see on this object appears to be the same phenomenon we've seen on Jupiter, for example, but thousands of times more powerful," Hallinan said. "This suggests that it may be possible to detect this type of activity from extrasolar planets, many of which are significantly more massive than Jupiter."[52]

Astrography[edit]

Main source: Astrography
This is a Sun-aligned arc, an auroral feature found within the polar cap. Credit: rssi.
Polar cap (or Theta) aurora has a sun-aligned arc stretching over the pole. Credit: rgk.
A theta aurora — so named because it looks like a Greek letter theta, a circle with a line through the middle — as seen by NASA’s IMAGE satellite. Credit: NASA/R. Fear et al (2014).

"Sun-aligned arcs are auroral features found within the polar cap, as opposed to the auroral oval where the more typical arcs reside. First global image of this phenomenon was reported by Frank et al. (1982); it consists of luminous belt reaching across the polar cap from noon to midnigh (that is why it is also called theta aurora; terms transpolar or polar cap arc are also known). However, first visual observations of such features were by the British Antarctic Expedition already during the austral winter of 1908 (Mawson, 1916). Similarly, first ground-based all-sky-camera observations originate from the International Geophysical Year (IGY) of 1957-1958 (Davis, 1962; Feldstein, 1963)."[53]

"The arc is about a hundred kilometers wide or more, and its luminosity may be comparable to the average emissions within the oval (typically, however, it is less than that). The feature can last for several hours, and it moves slowly across the polar cap in the direction of the IMF By component in the northern hemisphere (Frank et al., 1986). This motion is in the opposite direction in the southern hemisphere for the same sign of By (Craven et al., 1991)."[53]

Hot "plasma funneled into near-Earth space from the sun helps cause these unique aurora."[54]

"Depending on how this interplanetary magnetic field is aligned in relationship to Earth’s magnetic field, there can be various results when the solar wind arrives at near-Earths space. At the point where the two fields meet, Earth’s magnetic field points north. If the interplanetary field points in the opposite direction — south — then something called magnetic reconnection occurs, causing magnetic field lines pointing in opposite directions to suddenly realign into a new configuration."[54]

"But when the interplanetary magnetic field points northward, auroras can occur at even higher latitudes, sometimes resulting in theta aurora."[54]

Data was "collected simultaneously by the Cluster and IMAGE spacecraft on Sept. 15, 2005. While the four Cluster satellites were located in the southern hemisphere magnetic lobe, IMAGE had a wide-field view of the southern hemisphere aurora. As one Cluster satellite observed uncharacteristically energetic plasma in the lobe, IMAGE saw the arc of the theta aurora cross the magnetic footprint of Cluster."[54]

Recent history[edit]

Main sources: History/Recent and Recent history

The recent history period dates from around 1,000 b2k to present.

From 800 b2k to 500 b2k, there were "few recorded auroral sightings due to warfare (e.g. Mongol threat, Crusades), plague (Black Death), religious dogma (generally a bad time had by all)".[37]

From 600 b2k to 500 b2k, there was the "Spörer Minimum (little solar activity)".[37]

From 355 b2k to 285 b2k, there was the "Maunder Minimum (little solar activity)".[37]

Little Ice Age[edit]

Changes in the 14C record, which are primarily (but not exclusively) caused by changes in solar activity, are graphed over time. Credit: Leland McInnes.

The Little Ice Age (LIA) appears to have lasted from about 1218 (782 b2k) to about 1878 (122 b2k).

"In 1716, for virtually the first time, the Royal Society and the Acad6mie des Sciences carried articles on the aurora borealis in their journals. Both societies were then a half century old. However, merely 34 years later the Philosophical Transactions had recorded 200 observations of aurorae, and the Mémoires de l'Académie a similar number. The reason for the late and simultaneous debut was the return of the aurora to the latitudes of London and Paris, in or near which most of the societies' members lived."[55]

"The auroral eventsof the year 1716 most clearly announced that the prolonged solar and auroral calm that we now call the Maunder Minimum [Eddy, 1976a] had ended. But the onset of renewed auroral activity was noted already in the previous solar cycle. In 1707 an aurora was seen in Berlin and recorded in the journal of the Berlin Academy. Curiously, in New England, which is closer to the auroral zone than is London, Paris, or Berlin, the aurora returned suddenly in 1719. Contemporary accounts put the first recorded appearance of an aurora in Italy in the 1720's."[55]

"By the time that Jean Jacques Dorious de Mairan published his landmark treatise on the aurora in 1733, he had accumulated a sufficient record to draw two important conclusions: the auroral occurrence frequency had increased suddenly in 1716 and had remained essentially constant since then, and there were a number of times in the past when auroral occurrences had resumed after long absence [de Mairan, 1733]. He identified 22 such instances in the interval 500 A.D. to 1731 and referred to them as resuptious (reprises)."[55]

The "aurora again went into decline for a period of about 33 years, between 1792 and 1826, at a time when careful, routine observations of it were being made in Europe and America."[55]

"Investigation of secular variations prior to the Maunder Minimum is now possible based on six auroral catalogs that have been published within the last 20 years.The catalogs cover the time period from the fifth century B.C. to the seventeenth century A.D. and combine both oriental and European observations. Features corresponding to the previously recognized Medieval Minimum, Medieval Maximum, and the Spörer Minimum are clearly evident in both oriental and European records. The global synchronicity of anomalies in the auroral occurrence frequency is used to argue that they are caused by changes in the level or state of solar activity.The combined catalogs provide a sufficient number of events in the Middle Ages to resolve aquasi-80-year periodicity in the recorded auroral occurrence frequency. Also in the unusually rich intervals of the Middle Ages, clear quasi-10-year periodicities appear in the recorded occurrence frequency waveform. These are most reasonably interpreted as manifestations of the 11-year solar cycle and indicate that the solar cycle was then operative."[55]

Spatial distributions[edit]

"Descriptors of spatial structure

  1. homogeneous: lacking internal structure
  2. striated: having fine filaments
  3. rayed: having rays within a basic form".[37]

Spectral distributions[edit]

Discrete auroras (the bright visible forms} are classified by Color Types. Credit: rgk.

"Discrete aurora (the bright visible form is classified by Color Types). Examples of these color types. Note that the three primary colors (red, blue, and green) can be produced so that the aurora may appear to have a wide range of colors depending on the observer's perspective."[37]

  • "Type A aurora (green with red tops):
  1. energetic electrons flow directly down magnetic field lines
  2. usually red at high altitude (>150 miles)
  3. yellowish green at lower altitude (60-150 miles)
  4. colors due to emission by atomic oxygen
  5. more commonly occurs on day side of geomagnetic pole (visible during polar winter)
  • Type B aurora (green with red lower edges):
  1. energetic electrons from edge of plasma sheet in geotail
  2. crimson and blues at low altitude (<60 miles)
  3. colors due to molecular emissions (nitrogen, oxygen)
  4. yellowish green above 60 miles (vide supra)
  5. more commonly occurs on night side of geomagnetic pole
  • Type C aurora (green):
  1. most commonly observed color Type
  2. essentially Type A or B without visible upper or lower red regions
  • Type D aurora (red):
  1. bombarding particles lack sufficient energy to penetrate below 150 miles altitude
  2. red color due to emission by atomic oxygen (as in Type A)
  • Type E aurora (a rapidly moving Type B aurora)
  • Type F aurora (blue-purple):
  1. auroral altitude is still in sunlight but lower altitudes in darkness
  2. color due to resonance scattering (see airglow) by molecular nitrogen ion N2+
  • Proton aurora:
  1. protons (hydrogen ions) contribute as bombarding particles
  2. additional red and blue from atomic hydrogen emission"[37]
  • "Diffuse aurora (observed by satellite; not typically seen from the ground)
  1. energetic particles steadily "leak" from the plasma sheet
  2. luminosity more spread out and weaker than for discrete aurora "[37]
  • "Polar cap (or Theta) aurora: has a sun-aligned arc stretching over the pole."[37]

Temporal distributions[edit]

"Descriptors of temporal behavior

  1. quiet: steady luminosity
  2. pulsating: rhythmic fading and brightening (every 10 to 100 seconds). Video example
  3. flickering: rapid intensity changes (about 5 to 10 times per second). B&W video shows flickering beginning at about 30 second in.
  4. flaming: luminosity bursts from bottom to top
  5. streaming: varying brightness progressing horizontally".[37]

Sounding rockets[edit]

NASA suborbital sounding rockets carrying university-developed experiments are launched into auroras from the Poker Flat Research Range in Alaska. Credit: NASA/Jamie Adkins.
A NASA Oriole IV sounding rocket with the Aural Spatial Structures Probe leaves the launch pad on Jan. 28, 2015, from the Poker Flat Research Range in Alaska. Credit: NASA/Lee Wingfield.

"The interaction of solar winds and Earth’s atmosphere produces northern lights, or auroras, that dance across the night sky and mesmerize the casual observer. However, to scientists this interaction is more than a light display. It produces many questions about the role it plays in Earth’s meteorological processes and the impact on the planet’s atmosphere."[56]

"To help answer some of these questions, NASA suborbital sounding rockets carrying university-developed experiments -- the Mesosphere-Lower Thermosphere Turbulence Experiment (M-TeX) and Mesospheric Inversion-layer Stratified Turbulence (MIST) -- were launched into auroras from the Poker Flat Research Range in Alaska. The experiments explore the Earth’s atmosphere’s response to auroral, radiation belt and solar energetic particles and associated effects on nitric oxide and ozone."[56]

"This composite shot of all four sounding rockets for the M-TeX and MIST experiments is made up of 30 second exposures. The rocket salvo began at 4:13 a.m. EST, Jan. 26, 2015. A fifth rocket carrying the Auroral Spatial Structures Probe remains ready on the launch pad. The launch window for this experiment runs through Jan. 27."[56]

"On count day number 15, the Aural Spatial Structures Probe, or ASSP [in the second image down on the right], was successfully launched on a NASA Oriole IV sounding rocket at 5:41 a.m. EST on Jan. 28, 2015, from the Poker Flat Research Range in Alaska. Preliminary data show that all aspects of the payload worked as designed and the principal investigator Charles Swenson at Utah State University described the mission as a “raging success.”"[57]

“This is likely the most complicated mission the sounding rocket program has ever undertaken and it was not easy by any stretch. It was technically challenging every step of the way.”[58]

“The payload deployed all six sub-payloads in formation as planned and all appeared to function as planned. Quite an amazing feat to maneuver and align the main payload, maintain the proper attitude while deploying all six 7.3-pound sub payloads at about 40 meters per second."[58]

Satellites[edit]

This auroral image was taken from the International Space Station. Credit: NASA.
The image is taken from the ISS and shows aurora above the ocean off Australia. Credit: Unknown, or unstated.
This view of the Aurora Australis, or Southern Lights, which was photographed by an astronaut aboard Space Shuttle Discovery (STS-39) in 1991, shows a spiked band of red and green aurora above the Earth's Limb. Credit: NASA.
NASA and the CSA created a network of satellites for THEMIS. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab.

Both images on the right was shot from the International Space Station on or about 13 July 2012 and in 2014, respectively going down the page. Note that lights are blurred across the image rather than top to bottom in the first image.

"This view [on the left] of the Aurora Australis, or Southern Lights, which was photographed by an astronaut aboard Space Shuttle Discovery (STS-39) in 1991, shows a spiked band of red and green aurora above the Earth's Limb. Calculated to be at altitudes ranging from 80 - 120 km (approx. 50-80 miles), the auroral light shown is due to the "excitation" of atomic oxygen in the upper atmosphere by charged particles (electrons) streaming down from the magnetosphere above."[5]

To study macroscale interactions during substorms, NASA and the Canadian Space Agency (CSA) created a network of satellites shown in the image on the lower left for “Time History of Events and Macroscale Interactions during Substorms" (THEMIS).

Hypotheses[edit]

Main source: Hypotheses
  1. Auroras are part of an effort to achieve charge balances.
  2. Auroral structures indicate whether they are within the auroral oval, inside it closer to the pole, or outside it.

See also[edit]

References[edit]

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External links[edit]

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