Modeling seasonal variations of auroral particle precipitation in a global-scale magnetosphere-ionosphere simulation

A variety of observations have shown strong seasonal variations in a vast array of magnetosphere-ionosphere parameters, including field-aligned currents, cross polar cap potential, and precipitating electron energy flux. In this paper we examine how these variations are modeled in the Lyon-Fedder-Mobarry (LFM) global-scale magnetohydrodynamic simulation of the coupled solar wind-magnetosphere-ionosphere system. In order to account for changes in the solar wind conditions caused by the seasonal variation of the Earth's dipole tilt we carefully select the solar wind parameters so that the effective driving conditions are the same across the March, June, and December intervals examined. The seasonal variation of the field-aligned current strengths is in good agreement with observations, with the sunlit hemisphere having more current than the dark hemisphere in the June and December intervals. However, in order to bring the modeled precipitating electron energy flux into better agreement with the observations we need to utilize a modified current-voltage relationship which includes a proxy for illumination effects. We provide a detailed description of the LFM's magnetosphere-ionosphere coupling interface including how illumination effects are incorporated into the model. This methodology for including these effects does not allow for determining if changes in conductance or ionospheric density are responsible for the changes. In addition to improving the agreement with observations the new version of the current-voltage relationship results in enhanced geomagnetic activity in the March interval examined and suppression of activity during the June interval.