If you don't find your answer here, try the Asahi Aurora Classroom. For even more in-depth information, try David Stern's educational files. The COMET program at HAO website with lots of animations is useful (free, but registration is required). Another excellent resource is The Aurora Watcher's Handbook by Neil Davis.
Aurora is a luminous glow of the upper atmosphere caused by energetic particles entering the atmosphere from above.
This definition differentiates aurora from other forms of airglow, and from sky brightness that is due to reflected or scattered sunlight. Airglow features that have "internal" energy sources are
more common than aurora. For example, lightening and all associated optical emissions like sprites should not be considered aurora.
On Earth, the energetic particles that make aurora come from the geospace environment, the magnetosphere. These energetic particles are mostly electrons, but protons also make aurora. The electrons travel along magnetic field lines. The Earth's magnetic field looks like that of a dipole magnet where the field lines are coming out and going into the Earth near the poles. The auroral electrons are thus guided to the high latitude atmosphere near each pole. As the electrons penetrate into the upper atmosphere, the chance of colliding with an atom or molecule increases the deeper they go. Once a collision takes place, the atom or molecule takes some of the energy of the energetic particle and stores it as internal energy while the electron goes on at a reduced speed. The process of storing energy in a molecule or atom is called "exciting" the atom. An excited atom or molecule can return to the non-excited state (ground state) by sending off or emitting a photon, i.e. by making light.
For more information:
David Stern's educational files: The Polar Aurora
The composition and density of the atmosphere and the altitude of the aurora determine the possible light emissions.
When an excited atom or molecule returns to the ground state, it sends out a photon with a specific energy. This energy depends on the type of atom which has been excited, and on the level of excitement. We perceive the energy of an emitted photon as color. The upper atmosphere consists of air just like the air we breathe. At very high altitudes there is atomic oxygen in addition to normal air, which is made up of molecular nitrogen and molecular oxygen. The energetic electrons in aurora are strong enough to occasionally split the molecules of the air into nitrogen and oxygen atoms. The photons that come out of aurora have therefore the signature colors of nitrogen and oxygen molecules and atoms. Oxygen atoms, for example, strongly emit photons in two typical colors: green and red. The red is a brownish red that is at the limit of what the human eye can see, and although the red auroral emission is often very bright, we can barely see it.
Photographic film and digital cameras have different sensitivities to colors than the eye, therefore you often see more red aurora in photos than with the unaided eye. Since there is more atomic oxygen at high altitudes, the red aurora tends to be on top of the regular green aurora. The colors that we see are a mixture of all the auroral emissions. Just like the white sunlight is a mixture of the colors of the rainbow, the aurora is a mixture of colors. The overall impression is a greenish-whitish glow. Very intense aurora gets a purple edge at the bottom. The purple is a mixture of blue and red emissions from nitrogen molecules.
Photo by Jan Curtis
The bottom edge of the aurora is typically at around 100km (60 miles) altitude.
The aurora extends over a very large altitude range. The altitude where the emission comes from depends on the energy of the energetic electrons that make the aurora. The more energy, the bigger the punch, and the deeper the electron gets into the atmosphere. Very intense aurora from high energy electrons can be as low as 80 km (50 miles). The top of the visible aurora peters out at about 200-300 km (120-200 miles), but sometimes high altitude aurora can be seen as high as 600 km (350 miles). This is about the altitude at which the space station flies.
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The aurora is caused by energetic charged particles from the magnetosphere.
The immediate cause of aurora is precipitating energetic particles. These particles are electrons and protons that are energized in the near geospace environment. This energization process draws its energy from the interaction of the Earth's magnetosphere with the solar wind.
The magnetosphere is a volume of space that surrounds the Earth. We have this magnetosphere because of Earth's internal magnetic field. This field extends to space until it is balanced by the solar wind.
The solar wind is the outermost atmosphere of our sun. The sun is so hot that it boils off its outer layers, and the result is a constantly outward expanding, very thin gas. This solar wind consists not of atoms and molecules but of protons and electrons (this is called a plasma). Embedded in this solar wind is the magnetic field of the sun. The density is so low that we can almost call it a vacuum. However tenuous it is, when this solar wind encounters a planet, it has to flow around it. When this planet has a magnetic field, the solar wind sees this magnetic field as an obstacle, as protons and electrons cannot move freely across a magnetic field. These charged particles are constrained to move almost always only along the magnetic field. Likewise, when they are forced to move in a specific direction, a magnetic field will move with them or will be bent into the direction of the flow. Whether the magnetic field forces the plasma motion or whether the plasma motion bends the magnetic field depends on the strength of the field and the force of the motion. When the solar wind encounters Earth's magnetic field, it will thus bend the field unless the field gets too strong. The strength of the magnetic field falls off with distance from Earth. The distance at which the solar wind and the magnetic field of the Earth balance each other is about 10-12 Earth Radii (1 RE is 6371 km). For comparison, the moon is at about 60 RE, geostationary satellites are at about 6 RE. A plot that shows the actual distance in real-time can be found at this website. The inside of this volume that is bounded by the solar wind is called the magnetosphere.
At the interface of the solar wind and the magnetosphere, energy can be transfered into the magnetosphere by a number of processes. Most effective is a process called reconnection. When the magnetic field in the solar wind and the magnetic field of the magnetosphere are anti-parallel, the fields can melt together, and the solar wind can drag the magnetospheric field and plasma along. This is very efficient in energizing magnetospheric plasma. Eventually, the magnetosphere responds by dumping electrons and protons into the high latitude upper atmosphere where the energy of the plasma can be dissipated. This then results in aurora.
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The magnetic field confines the motion of auroral electrons. Think of it as painted magnetic field lines.
The electrons that make the aurora are charged particles, and they are not free to move in just any direction. Magnetic fields impede motion of charged particles when they try to cross the magnetic field. Charged particles can move freely only parallel to the magnetic field (either in the direction of the field or against it). When the solar wind encounters the outer reaches of Earth's magnetic field, the field is distorted by the motion of the plasma (see the previous question). Near the Earth the magnetic field is too strong and the motion of the electrons is guided by the field. When an electron spirals along the magnetic field into the atmosphere, it stays on or near this field line even when it makes a collision. Therefore the aurora looks like rays or curtains
There is always some aurora at some place on Earth.
Above: Weak aurora, with a small, barely visible auroral oval in this image from the POLAR VIS instrument.The bright crescent-shaped light on the left is from the sun illuminating the Earth.
Above: Intense auroral substorm, with aurora over the Great Lakes. Image from the POLAR VIS instrument.
When the solar wind is calm, the aurora might only be occuring at high latitudes and might be faint, but there is still aurora. In order to see the aurora, however, the sky must be dark and clear. Sunlight and clouds are the biggest obstacles to auroral observations. From a camera mounted on a satellite it is possible to look down on the aurora. From that vantage point you'll see an oval-shaped ring of brightness crowning Earth at all times.
When the solar wind is perturbed by a recent flare or other event on the sun, we might get very strong aurora. After the solar wind has transferred a lot of energy into the magnetosphere, a sudden release of this built-up tension can cause an explosive auroral display. These large events are called substorms. A substorm usually starts with a slow expansion of the auroral oval followed by a sudden brightening of a small spot, called the auroral breakup. This spot usually is near that place of the auroral oval that is on the opposite side of the sun, which means near the place where midnight is. This brightening rapidly grows until the entire auroral oval is affected. An observer on the ground where this breakup occurs will see a sudden brightening of the aurora which may fill almost the entire sky within tens of seconds. This aurora will be in the shape of rapidly moving curtains. If you are under the auroral oval west of this breakup, you will see a bright aurora moving toward you from the east that might cover almost the entire sky and move from the eastern to western horizon within minutes. This aurora will often look like a huge spiral of curtains, with many smaller curls within the curtains. After these auroral curtains subside, the sky might be filled with diffuse patches of aurora that turn on and off. The whole substorm typically lasts between 30 and 90 minutes. During periods of high solar activity, we might have several substorms per night. Here is a movie of several substorms following each other, observed from an all-sky camera in Toolik Lake, Alaska. On average, there are about 1,500 substorms per year, but often there can be several days between substorms.
The best places are high northern latitudes during the winter in Alaska, Canada, and Scandinavia.
Auroral zone for a low activity level
Aurora right now
To see aurora you need clear and dark sky. During very large auroral events, the aurora may be seen throughout the US and Europe, but these events are rare. During an extreme event in 1958, aurora was reported to be seen from Mexico City. During average activity levels, auroral displays will be overhead at high northern or southern latitudes. Places like Fairbanks, Alaska; Dawson City, Yukon; Yellowknife, NWT; Gillam, Manitoba; the southern tip of Greenland; Reykjavik, Iceland; Tromsø, Norway; and the northern coast of Siberia all offer a good chance to view the aurora overhead. In North Dakota, Michigan, Quebec, and central Scandinavia, you might be able to see aurora on the northern horizon when activity picks up a little. In the southern hemisphere the aurora has to be fairly active before it can be seen from places other than Antarctica. Hobart, Tasmania, and the southern tip of New Zealand have about the same chance of seeing aurora as Vancouver, BC, South Dakota, Michigan, Scotland, or St. Petersburg. Fairly strong auroral activity is required for aurora viewing in those locations.
The best time to watch for aurora is around midnight, but aurora occurs throughout the night. There are very few places on Earth where one can see aurora during the day. Svalbard (Spitzbergen) is ideally located for this. For a 10 week period around winter solstice it is dark enough during the day to see aurora, and the latitude is such that near local noon the auroral oval is usually overhead.
Since clear sky and darkness are essential to see aurora, the best time is dictated by the weather, and by the sunrise and sunset times. The moon is also very bright, and should be taken into account when deciding on a period to travel for the purpose of auroral observation. You might see aurora from dusk to dawn throughout the night. The chances are higher for the three or four hours around midnight.
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Almost all planets in the solar system have aurora of some sort.
If a planet has an atmosphere and is bombarded by energetic particles, it will have an aurora. Since all planets are embedded in the solar wind, all planets are subjected to the energetic particle bombardment, and thus all planets that have a dense enough atmosphere will have some sort of aurora. Planets like Venus, which has no magnetic field, have very irregular aurora; while planets like Earth, Jupiter, and Saturn, which have an intrinsic magnetic dipole field, have aurora in the shape of oval-shaped crowns of light on both hemispheres. When the magnetic field of a planet is not aligned with the rotational axis, we observe a very distorted auroral oval which might be near the equator, such as on Uranus and Neptune. Some of the larger moons of the outer planets are also big enough to have an atmosphere, and some have a magnetic field. They are usually protected from the solar wind by the magnetosphere of the planet that they orbit, but since that magnetosphere also contains energetic particles, some of these moons also have aurorae.
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This is a difficult question to answer. It is easy to say that the aurora makes no audible sound. The upper atmosphere is too thin to carry sound waves, and the aurora is so far away that it would take a sound wave five minutes to travel from an overhead aurora to the ground. But many people claim that they hear something at the same time when there is aurora in the sky. I am aware of only one case where a microphone has been able to detect audible sound associated with aurora (Auroral Acoustics: the web site does not have sound samples, but you'll find a link to an in depth paper there). But one can not dismiss the many claims of people hearing something. The sound is often described as whistling, hissing, bristling, or swooshing. What it is that gives people the sensation of hearing sound during auroral displays is an unanswered question. By searching for an answer to that question, we will probably learn more about the brain and how sensory perception works than about the aurora.
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On Earth, where the magnetic dipole field guides the energetic particles that make the aurora, we get an oval-shaped ring of aurora around both magnetic poles. The particles don't care whether they are going south or north along the magnetic field, so the aurora on the two hemispheres is the same. Of course, when the northern hemisphere has winter and the darkness that's needed to see the aurora, the south pole has bright daylight all day long. So it is only during fall and spring that a person in Antarctica could get on the phone to call someone in Alaska to find out if the aurora looks the same.
If you take photographs of the aurora at these two places at the same time, the large spirals that we sometimes see in the aurora will often look like mirror images of each other.
Proton aurora is a diffuse auroral glow caused by precipitating energetic protons, usually too dark to be visible.
Most visible aurora comes from precipitating electrons. However, the magnetosphere also shoots energetic protons toward the atmosphere. Both electrons and protons are charged particles, and they are not free to move in just any direction (see question 6). The curtain shapes of aurora results from this restriction on the motion of charged particles. When an electron spirals along the magnetic field into the atmosphere, it stays on or near this field line even when it makes a collision. Therefore the aurora looks like rays or curtains. When a proton spirals into the atmosphere along a field line it is just as restricted in its motion. In a collision, however, the proton can catch an electron from the atom or molecule that it collides with, and it is then a neutral hydrogen atom (i.e. a proton and an electron bound together). This hydrogen atom is free to travel in any direction, independent of the magnetic field. It may again turn into a proton in a subsequent collision, and be bound to travel along the direction of the magnetic field. This process can repeat itself several times before all the energy of the initial proton is spent. The effect of this meandering path is that the proton aurora is spread out and gives a very diffuse glow rather than the confined curtains of electron aurora. Because it is so spread out, proton aurora is usually not bright enough to be visible to the human eye. Sensitive instruments and cameras, however, can see this aurora.
Black aurora is defined as gaps between diffuse aurora.
Sometimes we observe diffuse auroral curtains and arcs that have small gaps. These gaps are usually thinner than the arc thickness next to the gap, and they look like a black auroral curtain embedded in the bright auroral glow around them. The black auroras can have curls and other structure. The sense of direction of these curls is opposite to that of regular auroral curtains. Most likely, the electric fields that are present in the upper ionosphere or lower magnetosphere prevent electrons from reaching the atmosphere, or even turn precipitating electrons around.
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Scientists can predict when and where there will be aurora, but with less confidence than weather prediction.
The ultimate energy source for the aurora is the solar wind. When the solar wind is calm, there tends to be minimal aurora; when the solar wind is strong and perturbed, there is a chance of intense aurora. The sun turns on its own axis once every 27 days, so an active region that produced perturbations might again cause aurora 27 days later. The solar wind takes a few (two to three) days to get here on its way from the sun. Observing the sun, and predicting perturbations in the solar wind from events on the sun (such as flares or coronal mass ejections) can thus give you about two to three days' advance prediction. (A movie of the solar wind can be found here.) The accuracy of the prediction depends on how well we understand the solar wind. About an hour before the solar wind reaches us, it passes by a satellite that sends its data back to us. That would give us one to two hours' warning of an upcoming aurora. The accuracy of that prediction depends on how well we understand the interaction of the solar wind with the magnetosphere, and the inner workings of the magnetosphere. There are also satellites inside the magnetosphere which can tell us how the magnetosphere responds to the solar wind. This will only give a prediction a few minutes into the future. All of these predictions are for the global aurora. It is very difficult to predict aurora for a given location.
Looking at the sun, and trying a two- to three-day prediction usually only tells us the probability and the time when an event will occur within a few hours, and we may estimate the size of the auroral oval. That means we may be able to say that the aurora is likely to reach a certain latitude, and that this event will start at a certain time.
Using satellite data from the solar wind for a one- to two-hour prediction, we may also see if the conditions are right for a substorm. In that case we may be able to predict the occurrence of a substorm and predict an estimate of the intensity of an aurora.
Watching the satellite observations from inside the magnetosphere, we can refine prediction of the intensity and timing of an expected substorm. You can also watch the sky, and if there is typical substorm behavior — for example, a dim and diffuse aurora that slowly moves south — we can predict an auroral breakup a few minutes into the future.
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The aurora has an effect on the environment, but it is limited to the high altitude atmosphere.
Since the aurora takes place at about 90 – 100 kilometers (55.2 – 62.3 miles) altitude, only the atmosphere at or above that height is affected by aurora. Some ionization may occur a few tens of kilometers further down, and can have effects on radio wave propagation. Ham radio operators may find that at some frequencies, radio waves will not propagate far. The major effect of the aurora is, however, at the altitude range of 100-200 kilometers (62 – 124 miles). The precipitating particles that cause the light also cause ionization and heating of the ambient atmosphere. A consequence of the ionization is that the electrical properties of the atmosphere change, and currents can flow more easily. Aside from the charged particles that cause the light of the aurora, there are currents flowing between the magnetosphere and the ionosphere inside and in the vicinity of the aurora. These currents also contribute to the heating of the atmospheric gas at auroral altitudes. The heating from these currents is usually much more than by the particle precipitation itself. Once the gas in the aurora is heated, it wants to rise, so that convection can be driven by the aurora.
The currents in aurora not only flow vertically. A current has to be a closed loop, so there are currents flowing to and from the magnetosphere and horizontally in the vicinity of aurora as well. The currents in and around aurora are actually charged particles that move; positive charges in one direction, negative in the other. These moving particles can collide with the neutral gas of the upper atmosphere and drag the gas along. This means that not only vertical convection will be caused by the aurora, but also horizontal winds.
Although the change in temperature and wind inside and near the aurora can be very large, at some altitudes the temperature can rise to its tenfold value, and the wind can blow at several hundred meters per second (more than 1,000 mph), none of these disturbances reach down to where Earth's weather takes place. There is some speculation that long term changes in space weather, i.e. long-term effects of aurora and similar phenomena, may influence the long-term variation of the climate on Earth. This is the subject of ongoing research.
Other phenomena associated with aurora are perturbations in the magnetic field of the Earth. When we have a strong substorm, the magnetic field under the aurora can be decreased by as much as a few percent of its value. That, by the way, is the reason that these strong auroral events are called "substorms." Earth experiences occasional magnetic storms, which are global changes in the magnetic field. The auroral substorm is a similar change in the magnetic field, but it only happens on a smaller scale limited to the polar regions, thus they are "sub"-storms.
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