Page 108 - Intro to Space Sciences Spacecraft Applications

Spacecraft Environment
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about 10 Re, but the tail formed “behind” the earth extends far beyond the
orbital distance of the moon (rmmn = 60 Re) Within the magnetopause rests
the magnetosphere which defines the cavity containing the (deformed)
geomagnetic field.

Geomagnetic Storms. The overall kinetic energy of the solar wind is
increased significantly during times of increased solar activity (increases
in sunspots and solar flares). When this additional ejected plasma (at
speeds up to lo6 km/sec) encounters the earth, it causes a fluctuation of
the geomagnetic field as illustrated in Figure 4-10.
From several minutes to several hours after a solar disturbance, the addi-
tional high-energy particles arrive, causing a compression of the earth’s mag-
netic field measured as a sharp increase in the field strength at the earth’s sur-
face. This sudden commencement phase of the geomagnetic storm is
followed by a two- to eight-hour period during which the magnitude of the
field strength remains high. After this initial stage, when the solar wind
returns to its prestorm intensity, the geomagnetic field drops to an intensity
level significantly lower than normal due to currents set up by increased
motions of charged particles trapped within the field, as described in the next
section. This main phase lasts about 12 to 24 hours after which a recovery
phase begins, lasting typically a few days, during which the field gradually
dissipates the excess charges and returns to its normal level of intensity.
The drastic alteration of the earth’s magnetic field associated with geo-
magnetic storms can affect the operation of many space and terrestrial

INITIAL
PHASE MAIN RECOVERY
+50 y I PHASE
-- u- TIME

SUDDEN
COMMENCEMENT
-
-200 y
Figure 4-10. Geomagnetic “storm.” The intensity of the earth’s magnetic
field may be greatly affected by major solar disturbances.