High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 422

by magnetic activity. The maximum Region 1 current intensities occur between
about 0800 and 1000 h on the morning side and between about 1400 and 1600 h
on the afternoon side. At these times the Region 1 currents are about a factor
of 2 to 3 greater than the corresponding Region 2 currents. During equinox and
winter, the Region 1 and Region 2 currents show relatively little dependence
on local time except for the two Region 1 current maxima. In the summer,
however, it is found that the Region 1 currents can exceed the Region 2 currents
at all local times, although the region of maximum difference remains on the
dayside. The Region 2 currents show a much higher degree of variability with
magnetic activity than do the Region 1 currents. This can lead to situations on
the nightside in which the Region 2 currents exceed the Region 1 currents during
disturbed periods.
The magnitude of the Region 1 currents shows a strong dependence on the
interplanetary magnetic field, as might be expected from arguments similar to
those applied for the polar cap potential difference. Figure 8.27 shows the depen-
dence of the morningside and afternoonside Region 1 currents on the magnitude
of the solar wind electric field. These data show that the field-aligned currents
increase as the electric field associated with the IMF increases from zero. Simi-
lar conclusions concerning the polar cap potential difference were drawn from
Fig. 8.20, so it is not surprising that the driving currents should show the same
behavior. As in the case of the polar cap potential difference, the Region 1 current
density does not reduce to zero when the solar wind electric field reduces to zero.
This observation supports the existence of a mechanism in addition to merging
(perhaps viscous interaction) that also provides driving currents for ionospheric
convection.
8.6 Horizontal Currents at High Latitudes
Equations (8.7) and (8.12) represent the fundamental relationships between hor-
izontal and field-aligned currents and the electric field and conductivity in the
ionosphere. The discussion of the dependence of the ionospheric conductivity
on altitude given in Chapter 2 shows that the region over which substantial
currents flow perpendicular to the magnetic field lines is restricted to the range
from about 90 to about 130 km in the sunlit ionosphere and may extend up
to 300 km when no appreciable local ionization source is present. This current
flow is almost horizontal at high latitudes and produces a magnetic signature
that can be observed on the ground. It has been shown, however, that ground
magnetometers respond primarily to Hall currents and thus cannot be directly
converted to electric fields (Piddington, 1962).
Ground-based magnetometers function somewhat differently from those on
satellites, but their output, three mutually perpendicular components of the mag-
netic perturbation from a normal steady baseline, is the same. These magnetic
field perturbations are usually resolved along the geographic north, east, and