Storm-time changes in the upper atmosphere at low latitudes

A three-dimensional coupled model of the thermosphere, ionosphere, plasmasphere and electrodynamics has been used to investigate the dynamic and electrodynamic response at low latitudes during a geomagnetic storm. A storm was simulated at equinox and high solar activity, and was characterized by a 12-h enhancement of the high-latitude magnetospheric electric field and auroral precipitation. The deposition of energy at high-latitudes heats the thermosphere and drives equatorward wind surges, and changes the global circulation. The first wave arrives at the equator, 3.5 h after storm onset. The change in the global circulation drives downwelling at low latitudes, which decreases molecular species, and causes a slight positive ionospheric phase. By far the dominant driver of the low latitudes is due to the changes in electrodynamics. The dynamo effect of the altered wind circulation opposes the normal diurnal variation, with downward ion drift during the day and upward drift at night. On the dayside, the equatorial ionization anomaly becomes weaker, the ionospheric F-region peak height is lowered, and the eastward zonal winds are reduced. At night the anomaly is strengthened, the ionosphere is raised, and zonal winds accelerate. The global electrodynamic changes are consistent with earlier results, but the speed of the response was unexpected. The model results showed an equatorial response within 2 h of the storm onset, well before the first gravity waves arrived at the equator. The dynamo action of the mid-latitude wind surges drive an F-region dynamo that can cause charge buildup at the terminators, producing electric fields that immediately leak to the equator. The meridional winds act as the driver of the low-latitude storm response by changing the dynamo action of the winds. In contrast, the zonal winds respond to the redistribution of charge brought about by the electrodynamic changes, rather than acting as a driver of the change. (C) 2002 Elsevier Science Ltd. All rights reserved.