Observations of intense ion outflows and cusp-region field-aligned currents during the September 24th and 25th 1998 magnetic storm


R. J. Strangeway, C. T. Russell (both at: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA, Email: strange@igpp.ucla.edu), D. M. Klumpar (Lockheed Martin Advanced Technology Center, 3251 Hanover St., Palo Alto, CA 94304, USA), C. W. Carlson, J. P. McFadden, R. E. Ergun (all at: Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA), R. C. Elphic (Los Alamos National Laboratory, Los Alamos, NM 87545), T. E. Moore (NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA), and W. K. Peterson (Lockheed Martin Advanced Technology Center, 3251 Hanover St., Palo Alto, CA 94304, USA)


Large ionospheric outflows have been observed by the POLAR spacecraft during the magnetic storm on September 24th and 25th 1998. These outflows are also observed at lower altitudes (~4000 km) by the FAST spacecraft. At FAST the outflow region is embedded within a region of strong field-aligned currents. A Sudden Impulse (SI) is observed around 23:45 UT on September 24th, and FAST passes through the near-noon cusp region into the northern polar cap some 20 minutes later. It is on this pass through the cusp that FAST observes the most intense field-aligned currents, with the eastward magnetic field increasing by over 1000 nT on crossing the downward current region. In this region the largest ionospheric outflows are observed, as well as large fluxes of ~ keV ions and electrons, which are presumably magnetosheath particles having entered through the cusp. At higher latitudes, in the polar cap, outflowing ions continue to be observed and the magnetic field perturbation gradually rotates towards the Earth-Sun line. The Interplanetary Magnetic Field (IMF) throughout this pass, and indeed for several hours prior to this, has a strong positive By component. The magnetic field perturbations observed by FAST are as expected for reconnection with such an orientation of the IMF. It should be noted that the field-aligned current signatures are similar on earlier passes through the polar cap, although somewhat weaker. The intensification of the currents after the SI is presumably caused by the shock passage responsible for the SI. The enhanced IMF and dynamic pressure in the post-shock solar wind increases the stresses applied to the polar magnetosphere, and hence intensifies the field-aligned currents. These currents in turn are likely to result in greater heating of ionospheric ions, resulting in the enhanced outflows observed at FAST and POLAR.