This web site is a compendium of information from several web sites listed at the end of this article.
Particles from the Sun
The sun is a sphere of hydrogen and helium over a million kilometers in diameter. The gravitational pressure at the centre of the sun is so great that nuclear fusion occurs there. In a process just like that of a hydrogen bomb, hydrogen atoms are forced together by extreme gravity to create helium atoms. This process creates energy that makes it's way to the surface of the sun and is released out into space as light, infrared radiation, X-rays, and other radiation.
The
force of this energy release separates many of the sun's
hydrogen atoms into separate electrons and protons that are free to move about
independently of one another. A mass of such separated particles is called
a
plasma. The picture at left shows some of the plasma flowing
along magnetic lines near the surface of the sun. (That loop is bigger than
the Earth!)
The light and other radiation leaving the sun pushes some of the plasma away from the sun and out into space. This flow of particles is called the solar wind. It moves quickly, over a million kilometers an hour, but it is very thin, only a few particles per cubic centimeter.
Conditions on the surface of the sun vary greatly so sometimes the solar
wind is relatively calm and other times it is very strong.
Click on the blue image below to see a time-lapse movie of the solar
wind taken over two weeks. (It's a 1 Mb download.) The sun is at the centre
but blocked out so you can see 
the
much fainter solar wind. Notice the comet that passes by the sun near the end
of the movie ... but that's another topic.
As you can see in the movie, the strength of the solar wind varies considerably. If the earth happens to lie in the direction of one of those strong bursts, many more particles than usual will reach the earth. If conditions are right, the auroral oval will expand in size and the aurora in intensity.
Click on the image at right for a movie of the sun solar wind (1Mb download) ...
Earth's magnetic field
The earth has a magnetic field called the magnetosphere. If there were no solar wind, the magnetic lines around the earth would be symmetrically shaped much like the picture below on the left. However, the solar wind contains electrically charged particles (the electrons and protons) and some of the magnetic field from the sun, called the Interplanetary Magnetic Field or IMF. The solar wind and its IMF apply a pressure on the earth's magnetosphere which distorts its shape, as shown in the picture at right. The magnetosphere is compressed on the sun side and stretched far from earth on the night side.
Below is a more detailed cut-away of the earth's magnetosphere along with the names of some of its parts. The details that are most important to notice for this discussion are the cusps in the magnetosphere and the way in which some of the IMF's magnetic lines can line up with and join the earth's magnetic lines. This is essential to creating aurora.
How solar particles reach the atmosphere
When the charged particles from the sun encounter the earth's magnetosphere most of them are deflected around it in much the same way as water is deflected around an obstacle. For any charged particle, it is always easier to flow along magnetic field lines than to cross them. As the solar wind flows by the earth's magnetosphere, it drags it along, compressing it on the sun side and stretching it out on the night side.
To have aurora the solar wind particles much reach the earth's atmosphere. There are two places in the magnetosphere where this is most likely to happen. The first is straightforward. Notice in the diagram above that there are cusps in the earth's magnetic field above the north and south magnetic poles. The magnetic field lines are nearly perpendicular to the earth's surface there so it is easy for particles to flow down those lines into the atmosphere where they are responsible for some of the aurora we see.
The largest aurora displays come from particles on the night side of the magnetosphere. The process is known as magnetic reconnection. Far out in the tail of the earth's magnetosphere the lines of the earth's and sun's magnetic fields can become parallel to each other. If the polarity of the two fields is opposite to each other, then the lines can join up and allow the particles to flow easily along the joined lines down to the earth's atmosphere. Click on the image below for an animation of this process. (1.7 Mb download)
Click
on this image for an animation of magnetic reconnection (1.7 Mb download)
...
The animation shows the flow along a single magnetic field line for clarity but it actually occurs over a larger region. The process is not always immediate. The charged particles can stay trapped in the tail of the earth's magnetosphere until a disturbance in the magnetic field allows them to flow into the earth's atmosphere.
Polarity of the Sun's Interplanetary Magnetic Field
Magnetic reconnection occurs more easily when the magnetic field lines of the solar wind and earth are parallel to each other and opposite in polarity (known as anti-parallel).
Some of the sun's magnetic field lines are open, that is, they do not join back together but extend out into space. The surface between these open field lines is known as the interplanetary current sheet and defines the boundary between north and south polarity in the IMF. The sheet is not a flat plane. As the sun rotates the sheet is shaped into swirls (see the purple drawing at right) so that sometimes the earth is above the ICS and sometimes it is below it. Aurora can occur in either orientation but is significantly stronger when the vertical magnetic field component (Bz) of the IMF at the earth has a southward orientation. (Which side of the ICS this is on changes every sun spot cycle when the sun's magnetic polarity reverses.) You can monitor the current orientation of Bz at http://www.sec.noaa.gov/SWN/index.html and http://www.sec.noaa.gov/ace/MAG_SWEPAM_6h.html The more negative (southward) the Bz value, the more likely it is that aurora will occur.
Excited nitrogen and oxygen
Once solar wind particles reach the earth's atmosphere they strike air molecules and give some of their energy to those atoms and molecules. Electrons in the atoms and molecules are temporarily raised to a higher energy level. When the electrons return to their former energy state they release photons of light. It's this combined release of large numbers of photons that creates the glow we call aurora.
The earth's atmosphere is composed mainly of nitrogen and oxygen. Each releases photons at particular wavelengths and altitudes in the atmosphere. Oxygen emits green light and some red, nitrogen red and some blue. The human eye can see the colours only when the aurora is very bright, otherwise it all looks gray. Cameras are better at recording the colour.
Auroral oval
As mentioned in Aurora 101, aurora forms most often in a ring around each of the north and south magnetic poles. The size and shape of the ring is not static. The side opposite the sun tends to be furthest from the magnetic pole, meaning the aurora will be furthest south around midnight for any location. As geomagnetic activity increases, the size of the oval increases as well so that it can be seen from more southerly latitudes. Click on the image below to see a movie of activity in the auroral oval. (3.6 Mb download)
Click
on this image for a movie of the auroral oval (3.6 Mb download) ...
Predicting when aurora will occur
There are a number of instruments that monitor space weather and the factors that affect aurora, but keep in mind that forecasting aurora is about as accurate as forecasting weather. Aurora is more likely when the solar wind is dense, so large ejections from the sun in the direction of the earth will give 2-3 days notice of possible aurora. Closer to earth, the ACE satellite monitors the solar wind about an hour before it reaches earth. Finally, there are monitors on the ground and satellites viewing the earth that give real-time reports of geomagnetic activity and aurora.
A few of the web sites that monitor the factors that affect aurora are listed below.
http://www.spaceweather.com/ - the left hand column provides real time reports near the top and geomagnetic activity forecasts near the bottom. The top of the centre column displays alerts if there are any.
http://www.sec.noaa.gov/SWN/index.html - reports on current geomagnetic conditions, solar wind and an estimate of the current position of the aurora oval.
http://asahi-classroom.gi.alaska.edu/predict.htm - reports on current conditions for the aurora oval, the Kp index (an indication of geomagnetic activity on the ground) and solar activity. Links near the top, such as Science and FAQ, provide general information on aurora.
http://www.sec.noaa.gov/ace/MAG_SWEPAM_6h.html - reports a recent history of ACE satellite observations. Of particular interest is the value of Bz, the red line in the first plot. The more negative the value of Bz (below the dashed white line), the more likely it is to see aurora when the auroral oval is over your location.
http://solar.spacew.com/mailman/listinfo - sign up to receive aurora watches and warnings by email.
https://pss.sec.noaa.gov
Links for some of the material on this page and for further reading:
http://odin.gi.alaska.edu/FAQ/
http://www.gi.alaska.edu/asahi/color.htm
http://www.birchwoodtours.com/aurora/52/default.aspx
http://pluto.space.swri.edu/IMAGE/glossary/magnetosphere2.html
http://sprg.ssl.berkeley.edu/~cyclopi/lesson1.html
http://www.nasa.gov/centers/goddard/news/topstory/2003/1203image_cluster.html
http://pluto.space.swri.edu/IMAGE/glossary/IMF.html
http://ffden-2.phys.uaf.edu/211.fall2000.web.projects/Christina%20Shaw/AuroraColors.html
http://www.gi.alaska.edu/asahi/index.htm
Ready for Aurora 303? Have a look at http://www.meted.ucar.edu/hao/aurora/ The page may take several seconds to load. Use the "begin text version" link on that page if you don't have a high-speed internet connection.