THE EFFECTS OF NEUTRAL INERTIA ON IONOSPHERIC CURRENTS IN THE HIGH-LATITUDE THERMOSPHERE FOLLOWING A GEOMAGNETIC STORM

Results of an experimental and theoretical investigation into the effe cts of the time dependent neutral wind flywheel on high-latitude ionos pheric electrodynamics are presented. The results extend our previous work (Deng et al., 19911 which used the National Center for Atmospheri c Research Thermosphere/Ionosphere General Circulation Model (NCAR TIG CM) to theoretically simulate flywheel effects in the aftermath of a g eomagnetic storm. The previous results indicated that the neutral circ ulation, set up by ion-neutral momentum coupling in the main phase of a geomagnetic storm, is maintained for several hours after the main ph ase has ended and may dominate height-integrated Hall currents and fie ld-aligned currents for up to 4-5 hours. We extend the work of Deng et al. to include comparisons between the calculated time-dependent iono spheric Hall current system in the storm-time recovery period and that measured by instruments on board the Dynamics Explorer 2 (DE 2) satel lite. Also, comparisons are made between calculated field-aligned curr ents and those derived from DE 2 magnetometer measurements. These calc ulations also allow us to calculate the power transfer rate (sometimes called the Poynting flux) between the magnetosphere and ionosphere. T he following conclusions have been drawn: (1) Neutral winds can contri bute significantly to the horizontal ionospheric current system in the period immediately following the main phase of a geomagnetic storm, e specially over the magnetic polar cap and in regions of ion drift shea r. (2) Neutral winds drive Hall currents that flow in the opposite dir ection to those driven by ion drifts. (3) The overall morphology of th e calculated field-aligned current system agrees with previously publi shed observations for the interplanetary magnetic field (IMF) B(Z) sou thward conditions, although the region I and region 2 currents are sme ared by the TIGCM model grid resolution. (4) Neutral winds can make si gnificant contributions to the field-aligned current system when B(Z) northward conditions prevail following the main phase of a storm, but can account for only a fraction of the observed currents. (5) DE 2 mea surements provide a demonstration of ''local'' (satellite-altitude) fl ywheel effects. (6) On the assumption that the magnetosphere acts as a n insulator, we calculate neutral-wind-induced polarization electric f ields of approximately 20-30 kV in the period immediately following th e geomagnetic storm.