Direction Of Magnetic Flux Lines

Fig. 5-6. The Earth's Magnetosphere (From Harris and Lyle [I969D

separates the incoming field lines from the outgoing field lines is called the neutral

Although the solar wind is fairly constant, it is frequently augmented by energetic bursts of plasma emitted by solar flares. When this plasma encounters the geomagnetic field, it compresses the field further giving a rise in field intensity on the surface of the Earth. This rise, called sudden commencement, initiates what is referred to as a magnetic storm [Haymes, 1971]. The initial phase has a typical strength of 50 nT and lasts for about 1 hour. During particularly strong storms, the magnetopause can be compressed to below synchronous altitude. After compressing the field, the plasma burst injects more charged particles into the geomagnetic field. These particles spiral around the field lines in a northsouth direction, reversing direction ("mirroring") at those locations where their velocity is perpendicular to the field line, usually at high latitudes. They will also drift in an eastwest direction, thus developing a ring current at 3 to 5 Earth radii whose magnetic field (up to ~400 nT) opposes the geomagnetic field. This phenomenon causes the main phase of the magnetic storms which lasts for a few hours until the charged particles start to escape from the magnetic entrapment through collisions with the atmosphere. The initial recovery to about 150 nT then takes from 6 hours to 2 days. The field fully recovers in several days.

These phenomena are summarized in Fig. 5-7, which shows the general characteristics of a magnetic storm. The graph, as well as the foregoing discussipn, is an oversimplification. Although a storm is observed simultaneously throughout the world, its characteristics will be different for observers at différent latitudes. The largest storm effects occur in the auroral zones which are 5 deg either side of 67 deg geomagnetic latitude. At that latitude, . ie disturbance can exceed 2000 nT. The amplitude decreases rapidly with latitude to about 250 nT at 30 deg latitude and increases to several hundred nT at the equator. At the higher latitudes, the storm is characteristically much more irregular than that shown in Fig. 5-7 [Chernosky, Fougere, and Hutchinson, 1965].

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