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Weak aurora, with a small, barely visible auroral oval in this image from the POLAR VIS instrument. The bright crescent shape light on the left is from the sun illuminating the Earth.

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 at high latitudes and might be faint, but there is still aurora. In order to see aurora, however, the sky must be dark and clear. Sunlight and clouds are the biggest obstacle to auroral observations. If you have a camera on a satellite you can look down on the aurora, and you'll find an oval shaped ring of brightness crowning Earth at all times. When the solar wind is perturbed from 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. On average, there are about 1500 substorms per year, but often there can be several days between substorms.

Photographic film has a different sensitivity to colors than the eye, therefore you often see more red aurora on 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.

The green emission from oxygen atoms has a peculiar thing about it: usually an excited atom or molecule returns to the ground state right away, and the emission of a photon is a matter of microseconds or less. The oxygen atom, however, takes its time. Only after about a 3/4 second does the excited atom return to the ground state to emit the green photon. For the red photon it takes almost 2 minutes! If the atom happens to collide with another air particle during this time, it might just turn its excitation energy over to the collision partner, and thus never radiate the photon. Collisions are more likely when the atmospheric gas is dense, so they happen more often the lower down we go. This is why the red color of oxygen only appears at the very top of an aurora, where collisions between air molecules and atoms are rare. Below about 100 km (60 miles) altitude even the green color doesn't get a chance. This happens when we see a purple lower border: the green emission gets quenched by collisions, and all that is left is the blue/red mixture of the molecular nitrogen emission.

The immediate cause of aurora are 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 constant 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 may well 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 60,000 km away, or 1/10 of the distance to the moon. 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 transferred 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. Here is an animation (1.6Mb) that illustrates this process (Here is that same animation (4.2 Mb) again, coded such that linux users can display it with xanim).

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, Tromso, Norway, and the northern coast of Siberia have a good chance to have 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. On 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 that. 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 sun rise and set 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 3 or 4 hours around midnight.

The ultimate energy source for the aurora is the solar wind. When the solar wind is calm, we tend to have very little aurora, when the solar wind is very strong and perturbed, we have 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 (2-3) 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 a 2-3 days advance prediction. To see a movie of the solar wind click on the image (1.1 Mb mpeg). 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 about 1-2 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 2-3 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.

On Earth, where the magnetic dipole field guides the energetic particles that make the aurora, we get an oval-shaped ring of aurora around the 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. When you do take pictures of the aurora at these two places, the large spirals that we sometimes see in the aurora will often look like mirror images of each other.

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 5 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 is mostly in finnish, and does not have sound samples). But one cannot dismiss the many claims of people hearing something, and this 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 sensual perception works than about the aurora.

Sometimes you can have 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.

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.

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 2-300 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 shuttle usually flies.