1) What is aurora?
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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. |
 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.Links for further and more detailed information:
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The composition and density of the atmosphere
and the altitude of the aurora determine the possible
light emissions.
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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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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. | ||
| Links for further and more detailed
information: |
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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 shuttle usually flies. Links for further and more detailed information: |
| 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.  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 (1.6Mb) that illustrates this process. Links for further and more detailed information: |
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 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.Links for further and more detailed information: |
| There is always some aurora at some place on Earth. | ||
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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 4 substorms following each other (3.8
Mb) following each other, observed from the IMAGE
satellite. 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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The best places are high northern latitudes during
the winter, Alaska, Canada, and Skandinavia. 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. Links for further and more detailed information: |
| Almost all planets in the solar system have aurora of some sort. | ||
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| 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,
or 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 get a very distorted auroral oval which might be
near the equator, like 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. 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: |
Yes, there are, and they are just like the northern
aurora. 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.Links for further and more detailed information: |
| 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: |
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. 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. 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: |
| Yes, but limited to the high altitude atmosphere. |
| 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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