Geoscience Reference
In-Depth Information
9
Ionospheric Response to Electric
Fields
In this chapter we describe some additional features of the high-latitude ionosphere
that distinguish it from the lower-latitude regions. We have defined the high-latitude
ionosphere as that region of latitudes in which (at least some of the time) the plasma
flows in magnetic flux tubes that either have only one foot on the ground or are
so extended that the effect of connecting to the conjugate hemisphere is negligible
on the local plasma. From our knowledge of the high-latitude convection pattern,
we know that such a definition can include the ionosphere at invariant latitudes
above about 50
◦
. In this region the ionospheric plasma may be subject to “bound-
ary conditions” in the outer magnetosphere that allow rapid expansion along the
magnetic field lines. We also know that within this region the plasma velocities per-
pendicular to the magnetic field can be of the order of 1 km/s. This gives the bulk
thermal plasma some very different properties from those found at lower altitudes,
and it is necessary to consider the details of ionospheric plasma motion, both par-
allel and perpendicular to the magnetic field, in order to understand them. The high
plasma velocities that occur in this region also greatly affect neutral atmospheric
dynamics.
9.1 Ionospheric Effects of Parallel Plasma Dynamics
9.1.1 Ionospheric Composition at High Latitudes
The effects of plasma motion along the magnetic field can be seen most dramat-
ically in the ionospheric composition near an altitude of 1000 km as a function
of latitude. At this altitude the ionosphere has essentially three ion species: O
+
,
H
+
, and He
+
. Figure 9.1 shows the quiet-time average distribution of these
ion species for satellite passes across the polar region from the dayside (
∼
1635
magnetic local time) to the nightside (
0446 magnetic local time). The field-
aligned flow velocities and number flux of the light ions are also given. We
obtained the data near equinox with the ISIS II satellite, which was located at
1400 km altitude. These data show quite abrupt decreases in the H
+
and He
+
∼
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