Stormtime measurements of topside ionospheric upflow from Defense Meteorological Satellite Program

摘要

We have examined characteristics of the vertical ion flux of thermal 0 ± in the topside high-latitude ionosphere before and during a number of geomagnetic storms, including the October and November 2003 events, using measurements of the vertical ion drift and ion number density made by the DMSP F 13 and F 15 spacecraft. Prior to storm onset typical upward fluxes of approximately 10 8-109 cm-2 s-1 are observed in the auroral zones with somewhat smaller downward fluxes in the polar caps. Immediately following storm onset upward fluxes reacanhd d sometimes excee1d0 110 0- c–m2 s– 1and are observed with vertical velocities of 500-1500 m s -/ . At the same time, downward fluxes at the higher latitudes reach unusually high values of 10 9 cm-2 s-1 . Separately integrating the upward and downward fluxes over the high-latitude region (auroral zone and polar cap) for each spacecraft pass allows the observation of variations in the total upflow/downflow during the progression of a geomagnetic storm. A superposed epoch analysis of events from 1995 to 2004 reveals a pattern of sudden onset of upward flux (averaging approximately 14 hours) with a gradually increasing and more extended period of downward flux (lasting about 24 hours). The upward fluxes are stronger when the coincident fluctuations in the z component of the IMF are larger. Strong upward flux events during storm times tend to occur when the solar wind velocity is elevated and the solar wind density is high. Upward flux events can also occur without a coincident geomagnetic storm if the solar wind velocity is above average.