The Earth’s Magnetic Field - Ultra and Extremely Low Frequency Electromagnetic Fields - page 28

The regions of trapped fast particles consist of two radiation belts. The inner
radiation belt discovered by Van Allen in 1958 is a region sandwiched between
magnetic shells at L 1:5 and L 2. This region contains very energetic protons
resulted from collisions between cosmic ray ions and the atmospheric atoms. The
outer radiation belt is bounded by two magnetic shells at L 3 and L 6. The flux
density of the particles moving in this region has a peak value at L 4:5. It should
be noted that the radiation belts have not, in essence, distinct boundaries because
each species has individual “radiation belt” depending on their energy.

1.3 Magnetic Storms

1.3.1 Solar-Quiet-Time Magnetic Variations

Magnetic variations with periods from several seconds to several years are generated
by ionospheric and magnetospheric currents which in turn are influenced by the
solar wind and radiation. The pattern and intensity of these variations depend on
solar wind parameters, latitude, time, and season. The magnetic variations can be
split into three main groups, i.e., a solar-quiet-time magnetic variations, perturbed
variations, and short period oscillations (e.g., Gonzalez et al. 1994 ).
The solar-quiet-time variations are a strictly periodic phenomenon, which
follows the Earth spin, the Earth orbiting around the Sun, and the moon location
with respect to the horizon (lunar tides). These kinds of magnetic variations
result predominantly from the ionospheric neutral winds and from the solar wind
permanently flowing around the magnetosphere. The solar radiation is responsible
for ionization of the upper atmosphere and for heating of the thermosphere, which
in turn results in the diurnal generation of large-scale system of neutral winds at
the ionospheric altitudes. When the neutral winds drag the ions, they produce the
motion of the conducting media through the Earth's magnetic field followed by
the generation of electric currents at altitude range 90-150 km. At middle latitude
these currents give rise to solar-quiet-time magnetic variations (S q -variations)
with amplitude 50 nT. At magnetic equator the amplitude of S q -variations can
enhance up to 2 10 2 nT due to the presence of equatorial electrojet flowing in the
anisotropically conducting ionospheric plasma (Surkov et al. 1997 ; Fedorov et al.
1999 ).
The interaction of solar wind with the magnetosphere produces the eastward
electric current at the magnetopause (Fig. 1.8 ). On the Earth surface in the vicinity
of magnetic equator this current increases the noon magnetic field by the value
25 nT, which can vary with amplitude 4 nT for 24 h. Another result of the
interaction between the solar wind and magnetosphere is the large-scale convection
of plasma inside the magnetosphere that causes the current generation at high-
latitude ionosphere and the magnetic S q -variations with amplitude 10 2 nT for
summer season.