The Earth’s Magnetic Field - Ultra and Extremely Low Frequency Electromagnetic Fields - page 26

electrons and ions of the solar wind plasma directly penetrate into the ionosphere
through the cusp that heat the ionospheric plasma in the cusp and contributes to the
dayside part of the auroral oval. The injection of magnetosheath plasma into the cusp
has been found to trigger the outflow of ionospheric plasma into the magnetosphere.
The longitudinal/field-aligned currents flowing from the magnetosphere along
magnetic field lines are closed through the high-conducting ionosphere. These
persistent currents enter the morning ionosphere and go out from the evening iono-
sphere. The region of the current inflow and issue produces practically continuous
strip along the auroral oval, which results from the plasma sheet and polar cusp
mapping to the high-latitude ionosphere (100-200 km over the Earth surface). In this
region the aurora borealism and magnetic storms are frequently observed due to the
energetic particles precipitating along field lines from the magnetosphere into the
ionosphere.
The plasmasphere is the region, which contains the ionospheric plasma with
enhanced number density n 10 3 cm 3 and thermal energy 1:0 eV. The
plasmasphere is terminated by the so-called plasmapause (Nishida 1978 ). At this
sharp boundary with geocentric distance about 4R e the number density of the
ionospheric plasma decreases abruptly to 0.1-1.0 cm 3 . Inside this region the
ionospheric plasma approximately rotates with the Earth and the Earth's magnetic
field because the magnetic field lines are frozen into the ionospheric plasma. At high
magnetic latitudes the field lines are practically normal to the lower ionosphere.
The terrestrial plasma flows from the ionosphere into the plasmasphere along these
lines forming the so-called polar wind. The sources of the plasmas that occupy
these regions are thus the solar wind and the Earth's ionosphere. The relative
contributions of these two sources to the magnetospheric plasma depend on the
level of geomagnetic activity.
The magnetosphere also contains two Van Allen radiation belts, ring and field-
aligned currents and various large-scale regions depending on plasma parameters,
which vary in time and space. The radiation belts are inner regions of the Earth's
magnetosphere where the Earth's magnetic field maintains the charged particles,
i.e., electrons, protons, ions, and so on (Nishida 1978 ). The kinetic energy of the
trapped species varies from tens keV to hundreds MeV.
Under the influence of Lorentz force the trapped species spiral along the
magnetic field lines from Northern pole towards Southern one and vise versa. The
dynamics of the particle motion in magnetic field is described by

qV ? B D mV 2 =R g ;

?

where q and m are the charge and mass of a particle, correspondingly, and V ? is
the component of particle velocity perpendicular to the magnetic field B. Whence it
follows that the radius R g of the circle or “gyration radius” is

mV ?
qB :

R g D