R. J. Strangeway, C. T. Russell (Both at Institute of Geophysics and Planetary Physics, University of California, Los Angeles. CA 90095-1567, USA. Email:; 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). 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)


During September 24th and 25th 1998 the FAST spacecraft acquired data over the Northern Polar Cap in the noon-midntght local time sector This allowed for detailed observations of the dayside cusp ~ 4000 km altitude. The spacecraft was therefore ideally suited for monitoring the outflow of ionospheric plasma which was also observed at the higher altitude POLAR spacecraft. The interval around the Sudden Impulse (SI) at 23:45 UT on September 24th is characterized by enhanced field-aigned currents in the cusp region. Because the FAST orbital period is ~35 minutes, we can provide snapshots of the cusp region, and FAST passes through the cusp at ~ 00:10 UT on September 25th, some 35 minutes after the S1. The field-aligned currents at this time are signficantly larger than on the previous orbit. Large fluxes of 100 eV oxygen ions are also observed to be flowing out of the ionosphere, with decreasing energy for increasing latitude. These ions form a conic in pitch angle, consistent with the "pressure cooker" model of ion acceleration, where transverse heating overcomes a downward electric field. AS already noted the field-aligned currents are weaker on the previous orbit, and a weaker oxygen conic is also observed. This suggests that the main effect of the increased solar wind dynamic pressure is to enhance already existing ion outflow through an increase in the field-aligned currents and the associated parallel electric fields and ion heating in the cusp