Anomalous reversals of the zonal equatorial electric field component have sometimes been observed when the interplanetary magnetic field turns northward from a steady southerly direction. We suggest that this reversal is associated with a sudden change in the convection electric field in the magnetosphere and present measurements to support this explanation. Although slower variations in the convection field are shielded from the low latitude ionosphere by polarization charges at the inner edge of the ring current, these charges may require an hour or more to vary. A sudden decrease in the cross-tail electric field will thus be accompanied by a dusk-dawn perturbation electric field across the inner magnetosphere.
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