Selected upper atmospheric storm effects

Abstract

In the polar ionosphere, a prominent increase in the electron temperature is observed beneath the magnetospheric cleft. During geomagnetic storms this temperature peak is displaced towards lower latitudes, sometimes by as much as 18° (and possibly more). This displacement is attributed to the reconfiguration of the magnetosphere during magnetic substorms. The magnitude of this temperature enhancement is only weakly dependent on the level of geomagnetic activity. At middle latitudes, a significant increase in the electron temperature is observed during geomagnetic storms. This is attributed to a general decrease in the electron density. Thus a close linear correlation exists between both quantities, with the rate of temperature increase possibly depending on height and/or level of solar activity. The decrease in the electron density, in turn, is caused by changes in the neutral gas composition and, specifically, by a decrease in the atomic oxygen to molecular nitrogen density ratio. A simple estimate indicates that the relative changes in both parameters should be of the same magnitude, and this is indeed observed. The same density decreases encountered by satellites in the topside ionosphere are also observed by ground-based ionosondes. Their measurements indicate that larger negative ionospheric storms are initiated in the night sector of the Earth. Such ground-based measurements also allow us to search for differences in the ionospheric response to an isolated short-duration geomagnetic storm and recurrent high-intensity long-duration continuous auroral activity (HILDCAA). As it turns out, the same kind of ionospheric disturbance effects are observed during both events, with the only difference being that during HILDCAA these perturbations last much longer. Ionospheric holes are one of the most spectacular disturbance effects observed at equatorial latitudes. These holes are marked by a steep drop in the electron density to very low values. Also their bottom is rather flat and almost without any structure. Different explanations of this phenomenon have been offered, none of which is generally accepted. Evidently, more comprehensive measurements are needed. This also applies to many other poorly understood aspects of upper atmospheric storms.