Ionospheric Response to Electric Fields - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 438

acceleration if the electron population is very cold, as it might be in the winter
polar region. Second, the theoretical descriptions of the polar wind are dependent
on some boundary condition that is usually applied at the top of the modeled
region near 3000 km, where the ionosphere becomes essentially collisionless. The
effects of the acceleration processes and the distribution of the ion species below
this altitude are dependent on their boundary conditions. Ultimately, boundary
conditions must be consistent with conditions in the outer magnetosphere, where
the field tubes are in contact with a much lower-density plasma having a variety
of possible energy distributions that differ significantly from those existing at
lower altitudes.
9.2 Ionospheric Effects of Perpendicular Plasma Dynamics
9.2.1 The Role of Horizontal Transport
In Chapter 8we described in some detail the convective motion of the ionospheric
plasma that results from an electric field of solar wind origin. The large-scale
convection pattern at high latitudes is fixed with respect to the earth-sun line
and the magnetic dipole axis. In this coordinate system the convection pattern
is dependent on latitude and longitude (also referred to as magnetic local time).
The plasma moves both toward and away from the sun and into and out of the
auroral zone. Since the solar radiation and auroral particle precipitation are the
most important ionization sources, the plasma convection velocity is thus one of
the most important factors affecting the plasma distribution and temperature at
all altitudes.
In addition to the ionospheric motion imposed from the magnetosphere, the
earth's atmosphere and portions of its plasma environment corotate about the
geographic axis. We thus have a rather complicated situation in which one source
of motion and ionization is most easily specified in a geographic reference frame
and another is most easily specified in a geomagnetic reference frame. During
the course of a day the geomagnetic pole rotates around the geographic pole
and, in a coordinate system fixed with respect to the sun, there is therefore a
universal time dependence (in addition to latitude and local time dependence)
in both the magnetospheric convection velocity and the auroral zone ionization
rate. At high latitudes the relatively high ion velocities produce frictional heating
due to collisions between the ions and the neutral gas. These collisions also affect
the rate at which ions recombine with the neutral gas. We will deal with these
collisional effects later. We will not deal in detail with the production and loss
processes for plasma in the ionosphere but we will describe the convection effects
in sufficient detail that their effects on the plasma composition, distribution, and
temperature will be recognized.
To understand the net plasma distribution at high latitudes, we will consider
a frame of reference in which the earth-sun line and the geographic pole are
fixed. This frame is essentially fixed in inertial space since its motion, consisting