IONOSPHERIC RESPONSE TO THE SUSTAINED HIGH GEOMAGNETIC-ACTIVITY DURING THE MARCH-89 GREAT STORM

A simulation was conducted to model the high-latitude ionospheric resp onse to the. sustained level of high geomagnetic activity for the grea t magnetic storm period of March 13-14, 1989. The geomagnetic and sola r activity indices and the DMSP F8 and F9 satellite data for particle precipitation and high-latitude convection were used as inputs to a ti me-dependent ionospheric model (TDIM). The results of the TDIM were co mpared to both DMSP plasma density data and ground-based total electro n content (TEC) measurements for the great storm period as well as wit h earlier storm observations. The comparisons showed that the overall structure of the high-latitude ionosphere was dominated by an increase d convection speed within the polar cap that led to increased ion temp eratures. In turn, this enhanced the NO+ density, raised the atomic-to molecular ion transition height to over 300 km, decreased NmF2, incre ased h(m)F(2), and in places either increased n(e) at 800 km or slight ly decreased it. The morphology of the ionosphere under these extreme conditions was considerably different than that modeled for less distu rbed intervals. These differences included the character of the daysid e tongue of ionization that no longer extended deep into the polar cap . Instead, as a result of the ion heating and consequent reduction in NmF2, a large polar hole occupied much of the polar region. This polar hole extended beyond the auroral oval and merged with the night secto r midlatitude trough. The limitations associated with the applicabilit y of the TDIM to the geomagnetic conditions present on March 13 and 14 are discussed. The primary limitations of the TDIM derive from the li mited temporal resolution of the model input parameters and the lack o f a suitably dynamic thermospheric specification for the great storm c onditions. These limitations lead to midlatitude ionospheric storm pha ses that do not follow those observed.