But at the same time, a gust in the solar wind can squeeze the magnetosphere, forcing some of the magnetosphere's particles earthward along the magnetic field lines. Particles energized enough to burrow as deep as the upper atmosphere produces the dazzling aurora borealis and magnetic storms. Imagers aboard Polar, orbiting around the Earth's north and south poles, provide TV pictures of the aurora borealis. In effect, the aurora acts as a mirror that reflects activities in the tail. Spann and his colleagues believe that by looking in this mirror, they can get a better understanding of what is happening in the tail, and get a direct measure of how much energy is pumped into the ionosphere by events such as CME and the solar wind.
Earth has two auroral ovals, each more of less centered on the magnetic poles.
The size of the oval varies with the energy pumped into it by the magnetotail. It's somewhat like extending your hands in a rubber band and pulling it open. Its takes more strength to enlarge the opening. We are now looking at how much of that energy is deposited on the the dayside relative to the rest of the oval. Spann is studying three different periods, a quiet magnetospheric period, a series of three pressure pulses sweeping past the Earth, and a relatively active event in September 1998.