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Fig. 6.11 Measured monthly average 3-h power spectra for magnetic field variations. The data
were gathered at Arrival Heights, Antarctica near McMurdo Station, during June 1986. Taken
from Lanzerotti et al. ( 1990 )
To a certain extent the sources of the natural ULF variations can be divided
into two general classes, depending on whether they are external or internal to the
magnetosphere. It is now generally accepted that the external sources are mainly
due to the interaction of the Earth's magnetosphere with solar wind and with MHD
waves coming from outer space. Under certain orientation of the interplanetary
magnetic field (IMF) and the Earth's magnetic field lines, when partial reconnection
of the field lines occurs, the small quasiperiodic variations of the IMF ( 10 nT)
may result in generation of the ground-based variations with amplitude of about
several hundred Tesla (Pilipenko et al. 2000 ). The energy of turbulent noise
generated in the magnetosheath can penetrate through the magnetopause and thus
can get trapped in the magnetosphere thereby exciting ULF noise and MHD waves.
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