Earth Field Encounter

As the sunspot number changes through its 11-year cycle, so changes the radial outflow of solar plasma (magnetic fields and ionized particles). The

E. Magnetopause Tail Current

F. Auroral Ionosphere Current

FIGURE 4.5 ► Configuration and table of the principal currents in the magnetosphere during a major geomagnetic storm.

E. Magnetopause Tail Current

F. Auroral Ionosphere Current

FIGURE 4.5 ► Configuration and table of the principal currents in the magnetosphere during a major geomagnetic storm.

Earth responds to this activity, with a similar 11-year cycle of magnetic disturbances. The maximum occurrence of geomagnetic storms is usually delayed by about 1 or 2 years after a sunspot maximum. The gradually changing latitude of the solar active regions during the cycle (Figure 4.1), and the alignment of the Earth's orbit nearer to a plane that includes the solar active regions, seem to be responsible for this delay.

The bursts of solar-eruption particles encounter the magnetic field of the Earth at almost a 45° Archimedes spiral angle. The sunward boundary (standoff position) of the magnetosphere (Figure 4.5 and Plate 3) is forced inward by the bowshock and the solar wind reconfigures the full magnetospheric envelope into the extended teardrop shape. The outer boundary of the magnetosphere is called the magnetopause. The magnetosheath is the region between the bow shock and the magnetopause. Using measurements from special satellites, space scientists establish the wind's Interplanetary Magnetic Field (IMF) direction at the magnetospheric stand-off (encounter) position. When the solar wind burst IMF at the stand-off region turns southward with respect to the Earth's northward main field, the oppositely directed field lines interconnect, releasing energy, and solar particles then enter the magnetosphere to start a magnetic storm. The interaction causes a considerable quantity of energy to be stored in the magnetospheric tail region. When the loading of this tail energy becomes a sufficient size, particles are dumped into the high-latitude regions of the Earth as field-aligned currents to cause more magnetic storms.

During the magnetic storm, charged particles (primarily solar electrons, protons, and a little helium) originating in the solar wind can follow complicated paths within the magnetosphere. These paths are defined by the distribution of particle types, their energy, and the shape of the Earth's field. Special currents (called partial ring currents) form gathering locations near 3 to 7 Re, where field-aligned currents of charged particles flow to (and from) the high-latitude ionosphere. Figure 4.5 illustrates the six principal currents around the Earth during magnetic storms. Solar particles gradually accumulate in the Van Allen belts (see Section 3.3, p. 88).

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