Geomagnetic Response of the Polar Thermosphere and Ionosphere

Abstract

A self-consistent coupled thermospheric / ionospheric model has been developed by merging the University College London Global Thermospheric Model and the Sheffield University Ionospheric Model. The neutral thermospheric wind velocity, composition, density, and energy budget are computed, including their full interactions with the high-latitude ion drift, precipitation, Joule heating and plasma density. This model has been used to examine thermospheric and ionospheric coupling within the polar cap, polar cusp and auroral oval. Simulations have been performed corresponding to high solar activity, moderate geomagnetic activity (Kp = 3), for the June and December solstices, and for convection electric field patterns corresponding to positive and negative values of the IMF-BY component to examine variations with season, and responses to the interplanetary magnetic field. In the winter polar region, ion transport and the diurnal migration of the polar convection pattern into and out of sunlight, play a major role in the plasma density structure at F-region altitudes. Regions of intense Joule heating, and field-aligned currents and locations of high ion temperatures are very dependent on convection field distributions, so that regions of strong neutral-ionospheric interactions are dependent on the IMF-BY component. In the slimmer polar region, the proportion of molecular to atomic species increases sharply, driven by the summer to winter seasonal thermospheric circulation, augmented by additional geomagnetic forcing. In the winter polar region at 300 km the dominant ion is 0⁺. As a consequence of the seasonal neutral composition change, at levels of moderate geomagnetic activity, molecular (N0⁺ and 0₂ ⁺) and 0⁺ ions are of similar number densities in the summer polar cap. The increased destruction of F-region ions in the summer polar region reduces the mean level of ionization to similar mean winter levels, despite increased solar insolation and ion production. The summer ion temperature at 300 km exceeds the winter values by 500°K, due to the underlying change in neutral temperature. In the lower thermosphere auroral oval the ion density is dominated by auroral precipitation in summer and in winter. Overall, there is a seasonal dependence in the height-integrated Joule heating rate and field-aligned currents (FAC) of about a factor of 2 – 3. Within the polar cusp, extra solar ionization in summer increases the conductivity to produce a threefold increase in peak Joule heating rates. There is a corresponding increase in the summertime cusp ionospheric currents and related FAC distributions. The intense neutral winds significantly modify the distribution of ionospheric currents, Joule heating and FAC, particularly in the dusk sector. Most of the neutral and electrodynamic parameters considered have strong IMF-BY dependence.