The Earth’s Magnetic Field - Ultra and Extremely Low Frequency Electromagnetic Fields - page 29

Interaction of the solar wind and frozen-in interplanetary magnetic field with the
Earth's magnetic field in the auroral region can serve as another source mechanism
for the solar-quiet-time magnetic variations. In the daytime the amplitude of
magnetic variations reaches a peak value 1:5 10 2 nT at the magnetic latitude
80 ı in the vicinity of polar electrojet.

1.3.2 Storm Sudden Commencement

When the solar flares give rise to significant perturbations of the solar wind
they produce the magnetic storms and substorms which result in interferences in
communication systems. The solar wind energy flowing to the Earth brings about
the aurora, heats the polar upper atmosphere and the plasma on the Earth's magnetic
field lines thereby energizing a spacious revolving system of magnetospheric
plasma.
During the most active period on the Sun, the solar flares are accompanied by
the energy release as much as 10 25 -10 27 J for the short time interval about 2 10 3 s,
which causes an enhancement of Roentgen and ultraviolet (UV) radiations, genera-
tion of shock waves. The strong solar energy release allows plasma clouds to escape
into the space outside of the Earth's orbit. The sudden increase in Roentgen and
UV radiations produces an excess ionization at the bottom ionosphere followed by
an increase in the ionospheric currents and S q -variations in sunlit hemisphere. The
magnitude and duration of the S q -variations on the ground surface is about 10 nT
and 30 min, correspondingly.
The density and velocity of the solar wind plasma increase significantly behind
the front of interplanetary shock wave so that the shock wave arrival results in
compression of the magnetosphere from the day side and in strengthening the
electric currents at the magnetopause. This has an effect of increasing the Earth's
magnetic field, which has been termed the storm sudden commencement (SSC).
The effect can be observed everywhere reaching a maximum value of several tens
nT at the equator. Sometimes the SSC may initiate a magnetic storm.

1.3.3 Magnetic Storm and Substorms

A magnetic/geomagnetic storm is a temporary perturbation of the Earth's magnetic
field caused by irregular disturbances in the solar wind and on the Sun that may
greatly affect space weather (e.g., see McPherron 1979 ). The magnetic storm
consists of three phases, i.e., an initial/growth phase, a main/active phase, and a
recovery phase. The initial phase is the SSC, which may last from 10 min to 6 h
and more. At this stage the strengthening field is mainly due to the increase in the
electric currents at the magnetopause while the magnetic field is weakly perturbed.