Ionospheric Response to Electric Fields - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 442

18 6

18 6

II

I

24

24
(a) (b)

Figure 9.7 The model convection shown in Fig. 9.5 can be created by the addition of
a simple two-cell convection pattern shown in (a) with a corotation velocity. The result
shown in (b) produces typical trough flow characteristics along the contour labeled II.
[After Spiro et al. (1978). Reproduced with permission of the American Geophysical
Union.]

the midlatitude trough. Note however that a midlatitude trough is observed in
regions other than the premidnight sector, so it is unlikely that the slow flow
convection trajectories described here are the only source of trough formation.
At other local times the midlatitude trough may be due to low plasma densities
produced elsewhere and transported to the observation region in the complex
flow pattern produced by the offset between the dipole axis and the earth's
rotation axis.
The rather simple concept of opposing flows from corotation and magneto-
spheric sources that can account for the premidnight trough also pertains inside
the polar cap in the postmidnight sector. In the winter season the antisunward
flow in the postmidnight region of the polar cap takes place in darkness and
is directed approximately to the west. This flow is opposed by the eastward
corotation velocity and, therefore, at certain longitudes and universal times can
produce plasma stagnation. In the dark polar cap there are very few sources of
ionization, and, thus, we might expect a plasma depletion to occur in a similar
manner to the midlatitude trough. Figure 9.8 shows the maximum and minimum
O + concentrations in the southern winter hemisphere. These observations, made
on an Atmospheric Explorer satellite near 300 km altitude, clearly show the mid-
latitude trough in the O + concentration and, as discussed above, a large plasma
depletion at latitudes above the auroral zone in the postmidnight polar cap. This
polar cap depletion is called the high-latitude or polar hole and is the site of
the lowest plasma concentration in the winter high-latitude ionosphere. It is a