For an extensive collection of auroral pictures, visit Spaceweather.com's Aurora Galleries|
Above, a diagram of the structure of the magnetosphere. Aurorae occur near the poles, where solar wind particles and ionized atoms trapped in the van Allen radiation belts pour into the upper atmosphere, particularly near the "cusps" in the magnetic field. The high-energy particles bombard atoms and molecules in the upper atmosphere, causing them to fluoresce at wavelengths corresponding to the energy of the collisions. Most aurorae occur within 50 to 100 miles of the surface of the Earth in the lower upper atmosphere, and involve green radiation by oxygen atoms. But red and other colors can be produced, typically at greater heights ranging up to 300 miles above the surface. From the ground it can be hard to tell whether a given color lies "above" another because of its actual position, or the perspective produced by the observer's viewing position. In this respect images taken from space (as shown about halfway down on this page) are particularly useful, as they clearly show that the green auroral emissions originate closer to the surface of the Earth. (Another important characteristic of such images, as far as this site is concerned, is that most truly spectacular auroral images are copyrighted by the "aurora chasers" who took them, and cannot be used on any site without permission of the original authors. For that reason, many of the images shown below are presented only as large thumbnails, such as would be found on search engine listings. To view the original images, click through to the site where their Credits and Copyrights are shown.)
The cause of the solar wind is gases escaping from the solar corona, primarily at coronal holes
. The Solar Dynamics Observatory image of the solar corona, above, shows a large coronal hole (the dark region at upper left), and magnetic field lines calculated from SDO measurements. The closed loops (in white) represent regions where the solar atmosphere is held in by its magnetic field. The open lines (in pale yellow) represent regions where the magnetic field lines extend into space, allowing portions of the atmosphere (that is, the solar wind) to escape into space. Coronal holes represent regions where field lines have become detached from the Sun, and when they are numerous the solar wind is denser and more energetic. As holes rotate onto our side of the Sun the resulting increase in the solar wind's strength can cause more intense auroral displays (although the most intense storms are caused by Coronal Mass Ejections
). (Image courtesy of SDO (NASA) and the [AIA, EVE, and/or HMI] consortium (field line imaging by Karel Schrijver, Lockheed Martin SAL); original shown here)
The aurora borealis, as seen near Bear Lake, Alaska. This image is posted on Wikimedia Commons with a statement that it was taken by a member of the U.S. military as part of his official duties. If that is correct it is in the public domain, and can be shown on non-commercial sites such as this one as long as credit is given where credit is due, namely to Senior Airman Joshua Strang, USAF.
A red and green aurora borealis, as seen near Fairbanks, Alaska. This image was posted on Wikimedia Commons by the photographer, Mila Zinkova.
Above, the Aurora Australis as photographed in Antarctica (the contrast has been slightly increased relative to the original). This image is copyrighted, but was posted on Wikimedia Commons by the photographer (Samuel Blanc) with the provision that it can be used elsewhere as long as his copyright is noted.
Above, a frame from a time-lapse video of the Aurora Australis taken from the International Space Station while passing over the Indian Ocean on March 4, 2012. In the video stars can be seen "rising" through and above the atmosphere as the ISS moves around the Earth, while the circumpolar aurora flickers and glows beneath the spacecraft. Click on the image to play the video (if you have a slow connection click here
for a shorter version).
The images in this box are copyrighted by their creators and cannot be shown at normal size without their permission; so I have created thumbnails to illustrate what their captions explain, and provided links to pages where larger versions can be viewed.
This fisheye image shows a particularly bright and colorful aurora observed on November 7, 2004 near Quebec, Canada. The colors are caused by atoms high in the atmosphere (primarily oxygen and hydrogen, in this case, although nitrogen often contributes to auroral displays as well) energized by collisions with high-energy electrons emitted from the Sun. (Credit & © Philippe Mousette (Obs. Mont Cosmos), apod041109)
This fisheye image shows a particularly bright and colorful aurora observed in July, 2004 near Quebec, Canada. The colors are caused by atoms high in the atmosphere (probably primarily oxygen and hydrogen in this case) energized by collisions with high-energy electrons emitted from the Sun. (Credit & © Philippe Mousette (Obs. Mont Cosmos), apod040730)
This March 31, 2001 aurora, imaged in Iceland (one of whose many lava fields is shown beneath the celestial sights), is partially obscured by clouds. Although both clouds and aurorae occur in our atmosphere, they are at very different altitudes. Clouds are almost always in the troposphere, which extends only 6 or so miles above the Earth's surface, while auroral displays begin at 60 miles and may occur at heights up to a few hundred miles above the surface. It should be noted that although aurorae are most easily observed at high latitudes, they can be seen elsewhere; and in fact the same auroral display was photographed at Kitt Peak National Observatory in Arizona. (Credit & © Sigurdur H. Stefnisson, apod010402)
Aurorae are usually observed at high latitudes, near the magnetic poles; but when the Sun is particularly active they may be seen at mid-latitudes as well. This image, from October 29, 2003 shows an auroral display in Okarche, Oklahoma. (Credit & © Dave Ewoldt, apod031113)
As previously noted, auroral displays are caused by streams of energetic particles from the Sun. One common cause of aurorae which are observed over wide areas, such as this display observed near Des Moines, Iowa, on May 15, 2005 is coronal mass ejections, in which hundreds of millions of tons of solar material are ejected into space by a rapidly expanding bubble of magnetic field energy. Such ejections tear holes in the solar magnetic field and corona (coronal holes), allowing high-energy particles to stream away from the Sun even after the mass ejection is over. (Credit & © Stan Richard, apod050525)
Most aurorae occur at heights of 60 to 100 miles, but occasionally violent solar events such as the X14-class (meaning exceptionally energetic) solar flare of April 15, 2001, send shock waves through the interplanetary magnetic field, and high-speed ions in the solar wind bombard and excite the much thinner gases at great altitudes. This unusual red and pink aurora glows with the light of oxygen ions two hundred miles above the surface of the Earth -- higher than most manned flights venture above the surface. (Credit & © Duane Clausen, apod020115)
An aurora displaying prominent dark gaps, or "black aurorae". Aurorae are caused by negatively charged particles (electrons, or ions with extra electrons) falling into the atmosphere along magnetic field lines which converge near the magnetic poles. The energy of the infalling particles excites the molecules struck by the particles, causing them to fluoresce. Until recently it was thought that gaps in the auroral patterns might be regions where for some reason, no particles were falling inward. However, recent observations suggest that the dark gaps are probably caused by outflows of negatively charged particles along field lines which lie in between the lines where particles are falling inward. (Credit & © Frank Andreassen (nettfoto.no), apod060329)
A time-lapse exposure of an aurora showing the more easily visible brighter regions at the bottom, and fainter portions extending high above the Norwegian landscape. The length of the exposure can be gauged by the streaks caused by artificial satellites (near the top of the image) and the dashed line caused by the blinking lights of a passing aircraft (near the bottom). During the period in mid-September 2010 when this image was taken, many spectacular auroral images were posted on spaceweather.com and (as in this case) APoD, due to a solar eruption which although missing the Earth, substantially increased the number of high-energy particles bombarding the upper atmosphere. (Credit & © Ole Christian Salomonsen, apod100920)
As indicated by the video near the top of this page, aurorae can be observed from space, as well as from the ground. Here is an image of the aurora borealis from the International Space Station, as it flew over Canada. From this vantage point both the horizontal and vertical extent of the display can be easily observed. (The circular feature left of bottom center is the Manicouagan Impact Crater, which is also shown on my page about Meteor Craters
.) (Don Pettit, ISS Expedition 6, NASA, apod030408)
Below, the aurora australis as seen from above by the space shuttle Endeavor during STS47, in 1992 (STS047-20-015, Goddard Space Flight Center)
Below, the aurora australis as observed from the International Space Station on May 29, 2010. (Credit: ISS Expedition 23 Crew, ISAL, NASA, apod100701)
Below, the aurora australis as observed from the International Space Station on April 23, 2003. This image is notable for the extensive red aurora display. Credit ISS Expedition 6 Crew
Below, an even more spectacular auroral display (in this case, the aurora borealis) as observed from the International Space Station on February 2, 2003. This image is notable for the extensive red auroral display. As noted above the green auroral emissions typically occur at altitudes of 60 to 200 miles, while red emissions may extend to heights in excess of 300 miles (well above the height at which the Space Station orbits). The top of this display is certainly near the upper limit for such emissions. Credit ISS Expedition 6 Crew
Another aurora australis, photographed from the space shuttle Discovery during STS39, in April 1991 (the shuttle is visible on the left). The auroral displays shown here were emitted by ionospheric gases at heights of 50 to 80 miles above the surface. (STS-39 Crew, NASA, apod980222)
(Image credit Doug Wheelock, NASA, ISS, 100821spaceweather.com)
Above, an August 13, 2010 auroral display photographed from the International Space Station. Note that most of the display has a bright lower edge where high-energy solar wind particles run into denser gases and are unable to penetrate deeper into the atmosphere, and the fainter upper extension, where there are plenty of solar wind particles but fewer atmospheric atoms to absorb and re-radiate their energy as light. Below, the same image, cropped to show more detail.
Other planets have auroral displays, as well. Above, an ultraviolet image of Jupiter's aurora borealis taken with the Hubble Space Telescope. Unlike the Earth's aurorae, Jupiter's aurorae are strongly affected by the interaction of its magnetic field with its large moons. Flux tubes (regions of greater than normal magnetic field strengths) connect the planet to its moons, intensifying the auroral activity in specific locations. Here, the bright streak on the far left is caused by Io's interaction with Jupiter, and the dots below center and to the right of that are caused by interactions with Ganymede (on the left) and Europa (on the right). (John T. Clarke (U. Michigan), ESA, NASA, apod001219)
Below, an image of Saturn's aurora australis (and at the very top its aurora borealis). (J. Trauger (JPL), NASA, apod990123)
Because Saturn's atmosphere is made of hydrogen, and extends into space further than a denser atmosphere would, the auroral displays extend more than a thousand miles above its cloudy lower atmosphere. The false color image here, taken with ultraviolet radiation, shows radiation by atomic hydrogen in red, since that is the visible light color most associated with hydrogen radiation, while radiation by molecular hydrogen is shown in white.
Aurorae are also visible in the atmosphere of Io, the innermost large moon of Jupiter. Although Io has no atmosphere as such, gases released by its volcanic activity are swept around its orbit by Jupiter's magnetic field, and pile up near the moon as the ionized gases near the moon slow down and concentrate the magnetic field lines. As a result of Jupiter's powerful magnetic field and intense trapped radiation field, the thin gases near the moon produce auroral displays. The display on the left was imaged by the Galileo spacecraft while Io was in Jupiter's shadow, and lit only by its aurorae (in red and green) and the blue glow of volcanic gases electrified by Jupiter's electromagnetic field. (Features on the portion of Io facing the spacecraft at the time are shown on the right.) (Galileo Project, University Of Arizona (PIRL), JPL, NASA, apod981016)