Simulations of the seasonal and universal time variations of the high-latitude thermosphere and ionosphere using a coupled, three-dimensional, model

Abstract

The University College London Global Thermospheric Model and the Sheffield University High-Latitude Ionospheric Convection Model have been integrated and improved to simulate the self-consistent interaction of the thermosphere and ionosphere at high latitudes. For mid- and low-latitudes, equatorward of 65 degrees geomagnetic, the neutral thermospheric code maintains the use of an empirical description of plasma densities. The neutral thermospheric wind velocity, composition, density, and energy budget are computed, including their full interactions with the high-latitude ion drift and the evolution of the plasma densities of O+ , H+ , NO+ , N 2 + , and O 2 + . Two 24 hr Universal Time (UT) simulations have been performed at high solar activity, for a level of moderate geomagnetic activity, at the June and December solstices, to investigate the UT and seasonal response of the coupled system. During winter, the diurnal migration of the polar convection pattern into and out of sunlight, together with ion transport, plays a major role in the plasma density structure at F-region altitudes. Only during those UT periods, when the entire geomagnetic polar region is in total darkness, is the effect of auroral oval precipitation imprinted on the F-region. In summer, the increase in the proportion of molecular to atomic species, created by the global seasonal circulation and augmented by the geomagnetic forcing, is effective in controlling the plasma densities at all Universal Times. The increased destruction of atomic oxygen ions in summer reduces the mean level of F-region ionization to similar mean levels seen in winter, despite the increased level of solar insolation. The UT variation exceeds the seasonal differences, implying a longitudinal dependency in what can be described as a high-latitude winter ionospheric anomaly. Below 200 km summer plasma densities exceed winter values at all times, and are responsible for the larger summer conductivities, Joule heating, and consequently, increased neutral winds and composition disturbance. The summer F-region ion density profile is a broader, flatter feature than in winter, the peak spanning a wider altitude range.