High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 397

which is shown in Fig. 8.7b. By joining points of equal potential deduced from
the lower plot with a line that is locally parallel to the observed flow, it is pos-
sible to deduce the flow pattern, as shown by the dashed lines in the upper plot.
Typical electric field values of 10 to 50mV

/ m (corresponding to flow velocities

of 200 to 1000m s) in the sunward flow regions lead to maxima in the potential

of the order of plus and minus 30 kV and a total potential drop across the open
field line region of the polar cap of about 60 kV. This observed convection pat-
tern is in qualitative agreement with our expectations from earlier consideration
of the solar wind-magnetosphere interaction. However, only a fraction of the
magnetospheric potential drop across a distance equal to the dimension of the
magnetosphere appears across the ionospheric polar cap. For example, using our
previous estimate for the solar wind electric field of 2mV

/

m, we obtain a total
potential of about 500 kV across a magnetosphere of typical width 40 R e . This
indicates that the region of direct connection of the earth's magnetic field to the
IMF is much smaller than the width of the magnetosphere.
The next series of figures shows more examples of the measurements used
in the derivation of high-latitude convection patterns. They illustrate the most
important variations in the plasma flow pattern when the IMF has a southward
component

/

( B z < 0 ) . The following points should be noted.

1. Average auroral zone pattern. The average electric field pattern in the auroral oval
determined from 500 h of balloon electric field measurements is presented in Fig. 8.8a.
The dominant variation is diurnal with the meridional component considerably larger
than the zonal component. When collated with respect to high and low K p values,
the pattern does not change much in shape but the amplitude of the dominant feature
decreases by about a factor of 2 as K p varies from 6 to 0.
2. Seasonal dependence. In Fig. 8.8b the plasma motion in a direction almost parallel to
the earth-sun line is shown for two dawn-dusk satellite passes. The figure illustrates
that the plasma velocity is considerably more structured in the winter (Southern)
hemisphere (top panel) than in the summer hemisphere. In both cases, however, the
data are consistent with a two-cell convection pattern. This is characterized by two
large-scale reversals (shown by the heavy arrows) that separate regions of antisun-
ward flow (dawn-to-dusk electric field) from regions of sunward flow (dusk-to-dawn
electric field). As shown in Fig. 8.8b, passes only 45 minutes apart can show average
fields twice as large in the winter hemisphere as in summer.
3. Variability. Figure 8.8c shows the plasma drift velocity perpendicular to the magnetic
field measured along a satellite track during two passes through the high-latitude
ionosphere using a vector representation. Notice that for these quite similar IMF
conditions, the latitudinal extent of the sunward flow in the auroral zones is quite
different. Also the magnitude of the antisunward flow in the polar cap is quite different
in these two cases.
4. Response to B y . Figure 8.8d again shows the ion drift velocity vector perpendicular
to the magnetic field for two passes through the high-latitude ionosphere. In the first
pass (top panel) B y is negative, and in the bottom panel B y is positive. Notice that
when B y is positive, the sunward flow from the dusk side passes across local noon in
the auroral zone before flowing antisunward. When B y is negative, the sunward flow
from the dawn side passes across local noon before flowing antisunward.