The earth's magnetic field acts as an obstacle to the plasma of solar phenomena, unless the plasma of the solar phenomena or the distorted plasma ahead of the phenomena contains a magnetic field oppositely directed to earth's magnetic field. If that is the case, then energy enters the magnetosphere/ionosphere-system. The motion of the particles is illustrated in the Rice-QuickTime movie.
At present the geomagnetic field strength is 0.31 G ( 31000 nT) at the Earth's surface near the magnetic equator, and twice that at the poles. At times of enhanced energy entry the magnetic field varies by about a percent (Figure 2) of the main field due to currents in the ionosphere and magnetosphere. At high latitudes strong electrojet currents (Figure 3) are flowing in the lower ionosphere, at 90 to 130 km above Earth's surface. In the nearly dipolar field at 3 to 5 RE, particles become trapped on closed field lines and drift around the Earth, thus creating the ring current. The ring current causes a weakening of the field at the Earth's surface (Figure 2). An intense geomagnetic storm is illustrated in Figure 2.
Auroras occur within a band of latitudes known as the auroral oval (Figure 4), the location of which is dependent on geomagnetic activity. Auroras are a result of collisions between atmospheric gases and precipitating charged particles (mostly electrons) guided by the geomagnetic field from the magnetotail. Each gas (oxygen and nitrogen molecules and atoms) gives out its own particular color when bombarded, and atmospheric composition varies with altitude. The auroral altitude range is 80 to 1000 km, but typical auroras are 100 to 250 km above the ground; the color of the typical aurora is yellow-green, from a specific transitions of atomic oxygen. Auroral light from lower levels in the atmosphere is dominated by blue and red bands from spectral line of atomic oxygen. Above 250 km, auroral light (Figure 5 and Figure 6) is characterized by a red spectral line of atomic oxygen. The patterns and forms of the aurora include quiescent arcs, rapidly moving rays and curtains, patches, and veils.
The Magnetosphere by Christopher T. Russell at UCLA.