F-region storms and thermospheric circulation

Abstract

This paper puts together various current ideas on the effects of magnetic storms on the ionospheric F-layer, principally at midlatitudes. A major factor is a large-scale circulation in the thermosphere, with equatorward winds driven by the heating resulting from joule dissipation and particle precipitation, different from the quiet-day pattern associated with the diurnal bulge. At midlatitudes this circulation produces upward drift in the F-region which tends to increase NmF2; but it also transports, from lower heights and higher latitudes, air with an enhanced molecular gas concentration which tends to decrease NmF2. Recovery from the midlatitude storm proceeds as molecular diffusion restores the gas composition to normal. Though in principle the circulation can cause both ‘positive’ and ‘negative’ F2-layer effects at midlatitudes, in practice other factors such as electromagnetic drift and ionosphere-magnetosphere plasma flux have important roles, and special considerations apply at low latitudes. Recent satellite experiments provide an excellent opportunity for investigating the storm circulation theory.