The response of the thermosphere and ionosphere to magnetospheric forcing

Abstract

During the past six years, rapid advances in three observational techniques (groundbased radars, optical interferometers and satellite-borne instruments) have provided a means of observing a wide range of spectacular interactions between the coupled magnetosphere, ionosphere and thermosphere system. Perhaps the most fundamental gain has come from the combined data-sets from the NASA Dynamics Explorer (DE) Satellites. These have unambiguously described the global nature of thermospheric flows, and their response to magnetospheric forcing. TheDEspacecraft have also described, at the same time, the magnetospheric particle precipitation and convective electric fields which force the polar thermosphere and ionosphere. The response of the thermosphere to magnetospheric forcing is far more complex than merely the rare excitation of 1 km s-1wind speeds and strong heating; the heating causes large-scale convection and advection within the thermosphere. These large winds grossly change the compositional structure of the upper thermosphere at high and middle latitudes during major geomagnetic disturbances. Some of the major seasonal and geomagnetic storm-related anomalies of the ionosphere are directly attributable to the gross windinduced changes of thermospheric composition; the mid-latitude ionospheric storm ‘negative phase’, however, is yet to be fully understood. The combination of very strong polar wind velocities and rapid plasma convection forced by magnetospheric electric fields strongly and rapidly modify F-region plasma distributions generated by the combination of local solar and auroral ionization sources. Until recently, however, it has been difficult to interpret the observed complex spatial and timedependent structures and motions of the thermosphere and ionosphere because of their strong and nonlinear coupling. It has recently been possible to complete a numerical and computational merging of the University College London (UCL) global thermospheric model and the Sheffield University ionospheric model. This has produced a self-consistent coupled thermospheric-ionospheric model, which has become a valuable diagnostic tool for examining thermospheric-ionospheric interactions in the polar regions. In particular, it is possible to examine the effects of induced winds, ion transport, and the seasonal and diurnal U.T. variations of solar heating and photoionization within the polar regions. Polar and high-latitude plasma density structure at F-region altitudes can be seen to be strongly controlled by U.T., and by season, even for constant solar and geomagnetic activity. In the winter, the F-region polar plasma density is generally dominated by the effects of transport of plasma from the dayside (sunlit cusp). In the summer polar region, however, an increase in the proportion of molecular to atomic species, created by the global seasonal circulation and augmented by the geomagnetic forcing, controls the plasma composition and generally depresses plasma densities at all U.Ts. A number of these complex effects can be seen in data obtained from ground-based radars, Fabry-Perot interferometers and in the combinedDEdata-sets. Several of these observations will be used, in combination with simulations using the UCL-Sheffield coupled model, to illustrate the major features of large-scale thermosphere-ionosphere interactions in response to geomagnetic forcing.