Frequently Asked Questions about Aurora and Answers
Romanian (translation by Web Geek Science),
Ukrainian (translation by Vlad Brown, STD Science),
Russian (translation by Donna Barrier, science.eduboard.com).
This is a compilation of questions from the Aurora Alerts Forum and from various e-mails sent to me. For each question there is a short, one-sentence answer, and a paragraph or two with a bit more explanation. For more detailed and technical explanations there are other web pages, and I tried not to duplicate too much here, but rather included links to these pages. If you have comments, questions, or suggestions, send me a message at lumm@gi.alaska.edu. If you don't find your answer here, you might try the Asahi Aurora Classroom. For even more in depth information, I recommend David Stern's educational files. A very nice and in-depth web site with lots of animations is the COMET program at HAO (free, but registration is required). A good book (it's hard to put web pages on a shelf...) that I recommend is "The Aurora Watcher's Handbook" by Neil Davis.
Links to realtime geophysical data that are related to the aurora and aurora forecast can be found at Real Time Data.
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Aurora is a luminous glow of the upper atmosphere which is caused by energetic particles that enter the atmosphere from above. |
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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. |
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Links for further and more detailed information:
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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. As they 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 with 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 a
photon, i.e. by making light.|
The composition and density of the atmosphere and the altitude of the aurora determine the possible light emissions. |
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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. |
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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. |
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Links for further and more detailed information: |
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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 and on the level of excitement, and we perceive
the energy of a 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.
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The bottom edge is typically at 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 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 station flies. Links for further and more detailed information: |
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Energetic charged particles from the magnetosphere. 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 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. Here is an animation (1.5Mb) that illustrates this process. Links for further and more detailed information: |
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The magnetic field confines the motion of auroral electrons. Think of it as painted magnetic field lines.
Links for further and more detailed information: |
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 gets 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 Earth's magnetic
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. |
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Weak aurora, with a small, barely visible auroral oval in this image from the POLAR VIS instrument. The bright crescant shape light on the left is from the sun illuminating the Earth. |
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Intense auroral substorm, with aurora over the Great Lakes. Image from the POLAR VIS instrument. |
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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, 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 1500 substorms per year, but often there can be several days between substorms. Links for further and more detailed information: |
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Maybe. 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 does not have sound samples, but you'll find a link to a very nice and in depth paper there). But one can not 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 sensory perception works than about the aurora. Links for further and more detailed information: |
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.|
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. Links for further and more detailed information: |
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Gaps between diffuse aurora.
Links for further and more detailed information: |
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.|
Yes, but with less confidence than weather prediction.
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. Using satellite data from the solar wind for a 1-2 hour prediction, we may also see if the conditions for a substorm are right. 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 the intensity and timing of an expected substorm. You can also watch the sky, and if you see typical substorm behavior, for example, a dim and diffuse aurora that slowly moves south, you can predict an auroral breakup a few minutes into the future. Links for further and more detailed information: |
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Yes, but limited to the high altitude atmosphere. |
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Since the aurora takes place at about 90-100 km 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 km. The precipitating particles that cause the light also cause ionization and heating of the ambient atmosphere. The ionization has the consequence that the electric 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 1000 mph), none of these disturbances reach down to where the 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 only happens on a smaller scale limited to the polar regions, thus they are "sub"-storms. Links for further and more detailed information: |
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Dirk LummerzheimLast updated:
Questions? Comments? Please send E-mail to lumm@gi.alaska.edu