Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 472

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Figure 10.2 Distortion of a circular blob of ionization as it convects from the polar cap
through the auroral zone. The first panel shows the initial conditions and the assumed
convection model. [After Robinson et al. (1985). Reproduced with permission of the
American Geophysical Union.]

the ionospheric plasma composition changes from H + at lower latitudes to O +
at higher latitudes. At ionospheric altitudes the width of this high-flow region is
between 150 and 200 km. These meridional scales are in the intermediate range
but, through shear instabilities, can create larger scale longitudinal structures, as
discussed in the next section. Modest small field-aligned currents were detected
during this pass (Providakes et al., 1989).
These large poleward electric field events have come to be called subauroral
ion drifts (SAID) due to their signature in drift meter instruments (Spiro et al.,
1979). There are two ways to think about their origin. The combination of
E B , gradient, and curvature drifts yield eastward zonal pressure gradients

deep in the magnetosphere and thus radially inward currents. These currents
close through poleward electric fields in the low conductivity nighttime subau-
roral ionosphere, creating large poleward electric fields. The second viewpoint
is that these same factors cause ions to precipitate equatorward of electrons

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