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(a)
Figure 8.8a
Hourly averages of auroral zone electric field data from balloon flights. The
error bars are standard deviations of the means, and the solid curves are empirical fits
to the data, satisfying the constraint imposed by
0 that the 24-hour average
westward field be zero. [After Mozer and Lucht (1974). Reproduced with permission of
the American Geophysical Union.]
∇
×
E
=
These observations show that the ionospheric conductivity, the internal state
of the magnetosphere, and the external state of the interplanetary environment
can all affect the high-latitude ionospheric convection pattern. However, when
the IMF has a southward component, a two-cell convection pattern made up
of predominantly antisunward convection at the highest latitudes with sunward
convection occurring at lower latitudes is almost always observed.
In principle, a number of different radar stations and satellites operating at
the same time can provide a simultaneous signature of the convection pattern at
different local times. Alternatively, if some degree of stability in the convection
pattern is assumed, a single radar site can sample all local times over a substantial
latitudinal width of the pattern during a day. These techniques and the previously
mentioned statistical syntheses of data bases are currently being used to elucidate
details of the seasonal dependence in the convection pattern and the factors deter-
mining the convection speed. A climatological model by Weimer (1995, 2001),
driven by solar wind parameters, provides a synthesis of the existing data. The
dependence on
B
y
is well documented for that portion of the convection pattern
on the sunward side of the dawn-dusk meridian and is shown schematically in
Fig. 8.9. The convection pattern is most easily characterized by one small cell
and one large cell. The large cell has an almost circular perimeter and the small
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