Long‐lasting disturbances in the equatorial ionospheric electric field simulated with a coupled magnetosphere‐ionosphere‐thermosphere model

Abstract

The Magnetosphere‐Thermosphere‐Ionosphere‐Electrodynamics General Circulation model of Peymirat et al. [1998] is used to investigate ionospheric‐wind‐dynamo influences on low‐latitude ionospheric electric fields during and after a magnetic storm. Simulations are performed with time‐varying polar cap electric potentials and an expanding and contracting polar cap boundary. Three influences on equatorial electric fields can be of comparable importance: (1) global winds driven by solar heating; (2) direct penetration of polar cap electric fields to the equator that are partially shielded by the effects of Region‐2 field‐aligned currents; and (3) disturbance winds driven by high‐latitude heating and ion‐drag acceleration. The first two influences tend to have similar magnetic local time (MLT) variations in a steady state, while the disturbance‐wind influence tends to have the opposite MLT variations. The nighttime disturbance winds at upper midlatitudes that affect the global ionospheric wind dynamo are predominantly westward after the simulated magnetic storm. The nighttime winds drive an equatorward dynamo current that tends to charge the low‐latitude ionosphere positively around midnight, which can lead to reductions or reversals of the normal equatorial night‐side east‐west electric fields. The simulations partly support the theories of the so‐called “disturbance dynamo” [Blanc and Richmond, 1980] and “fossil wind” [Spiro et al., 1988], both of which predict long‐lasting disturbances in the equatorial eastward electric field associated with magnetic storms. However, the simulations do not support the element of fossil wind theory that links the disturbance‐wind influence on equatorial electric fields to polar cap contraction following the storm. The simulations show a stronger wind‐produced enhancement of steady state shielding than predicted by the model of Forbes and Harel [1989], due to the fact that the disturbance winds extend well equatorward of the Region‐2 currents.

Observations and modeling of post-midnight uplifts near the magnetic equator

“…Using numerical simulations of the continuity equation, we show how the uplifts can occur despite the fact that the electric field does not change sign. We also focus on an extremely disturbed night during which the uplifts were amplified, and discuss how the uplifts may be enhanced by magnetic activity, which may be related to disturbance dynamo wind systems (e.g., Blanc and Richmond, 1980;Richmond et al, 2003). We also show some ESF statistics from Brazil that seem to support the idea that these uplifts could be the source of a secondary maximum in ESF occurrence rates.…”

“…The ionospheric response to recombination only is wellknown (e.g., Banks and Kockarts, 1973;Rishbeth and Garriott, 1969). However, we present a discussion here in order to illustrate the differences when a finite vertical drift is included.…”

“…A recent study by Richmond et al (2003) has shown that at night the disturbance dynamo tends to drive a current that leads to reductions or reversals in the equatorial zonal electric field. This behavior appears to be consistent with the observations, an uplift and a decrease in the magnitude of the zonal electric field caused by weak auroral electrojet activity.…”

“…Equatorward winds are observed at mid and low latitudes after midnight (e.g., Biondi et al, 1999), although there is an abatement of the wind (e.g., Herrero et al, 1993) associated with the midnight pressure bulge (e.g., Fesen, 1996), which is related to the convergence of winds near the equator (e.g, Faivre et al, 2006). Some analytical solutions to various simplified versions of this continuity equation have been obtained (e.g, Banks and Kockarts, 1973;Duncan, 1956;Dungey, 1956;Martyn, 1956;Rishbeth and Garriott, 1969;Vlasov et al, 2005). However, in general a numerical approach is necessary.…”

Effects of long-lasting interplanetary magnetic field perturbations on the equatorial ionosphere

“…The equatorial ionospheric disturbances during magnetic storms are mainly generated by prompt penetration of the interplanetary electric fields (PPEF) into low-latitude from the auroral region (Kelly et al, 1979;Spiro et al, 1998;Fejer and Scherliess, 1997) and disturbance dynamo electric fields (DDEF) and neutral thermospheric winds (Blanc and Richmond, 1980;Abdu, 1997;Scherliess and Fejer, 1997;Richmond et al, 2003).…”