Reversals in Bz component of interplanetary magnetic field and equatorial zonal electric field

Abstract

Sudden northward turnings of the Bz (North-South) component of the interplanetary magnetic field, IMF, are sometimes associated with anomalous reversals in the equatorial East-West electric field. The electric field reversal has been explained earlier as a dusk-to-dawn electric field perturbation, associated with a temporary imbalance between the convection related charge density and the charge on the inner edge of the ring current created by the rapid northward turning of Bz. We suggest that besides the shielding charges on the inner edge of the ring current, the shielding charges in the magnetosphere boundary layer also play a role in the occurrence of Bz related perturbations in the equatorial electric field. Since the time constants for the build up or decay of charges in the boundary layer and on the inner edge of the ring current are about a fraction of an hour, a rapidly varying IMF, if present around the time of the northward swing of Bz, facilitates a prompt and effective decrease of the magnetospheric convection related electric field. As a consequence of this, the shielding charges in the inner magnetosphere produce a dusk-to-dawn electric field perturbation which then penetrates into the low latitude ionosphere. Re-examination of the Bz associated reversals in the equatorial electric field reported in the literature consistently showed the variance of IMF to be quite high just around the times of northward swings of Bz, lending support to the proposed explanation.