Planer. Spme Sk. Vol. 3X. No. 7. pp 841-850. 1990 Pnnted m Great Bntain. 0032-ofJ33/90 %3.00+0.00 <.” 1990 Pergamon Press plc THE INTERACTION OF IMPULSIVE SOLAR WIND DISCONTINUITIES WITH THE MAGNETOSPHERE : A MULTI-SATELLITE CASE STUDY zyxwvutsrqponmlkjihgfedcbaZ J. LABELLE” Department of Physics and Astronomy, Dartmouth College, Hanover. NH 03755, U.S.A. L. M. KISTLER, R. A. TRE~MANN and W. BA~MJOHANN Max-Planck Institut fur extraterrestrische Physik, D-8046 Garching, F.R.G. D. G. SIBECK Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20707. zyxwvutsrqponmlkjihgfed U.S.A. D. N. BAKER Laboratory for Extraterrestrial Physics, Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. and R. D. BELIAN Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. zyxwvutsrqponmlkjihgfedc jReeeit>ed 12 February 1990) AIrstract-During the magnetic storm of 4-5 September 1984. the outbound ~~PTE~IR~ spacecraft stayed just outside the expanding Earth’s bow shock for a period of 7.5 h. During this interval the solar wind ram pressure and the interplanetary magnetic field remained approximately constant, except for two distinct impulsive pressure pulses lasting a few minutes each. These pulses coincided with discontinuities in which the IMF changed direction by about 90’. and the solar wind kinetic pressure decreased slightly. Accompanying these discontinuities. the following magnetospheric signatures were observed : (1) immedi- ately after each discontinuity. the growth phase of a substorm commenced as evidenced by decreases in the Rux of energetic ions at geosynchronous orbit on both the dayside and nightside, and these were followed by particle injection events on the nightside 20-35 min after the discontinuities; (2) equatorial magnetograms recorded sudden impulses in the magnetic field : and (3) after a delay of 8-12 min. the IRM detected outward motion of the bow shock, and in each case about an hour passed before the outward- moving satellite caught up again with the shock. Overall, the magnetospheric and ground signatures of the first discontinuity were larger than those of the second. It is speculated that the IMF direction and other factors such as the local time and latitude of the around stations may be important factors in determining the magnitude of these signatures. INTRODUCTlON Discontinuities in the solar wind kinetic pressure and magnetic field affect the position of the magnetopause and initiate geomagnetic activity. For example. south- ward turning of the interplanetary magnetic field (IMF) is associated with the onset of reconnection at the dayside, consequent erosion of the dayside mag- netopause. enhanced transfer of magnetic flux to the magnetotail. and release of a portion of this magnetic energy in the form of drifting energetic particles in the magnetosphere, resulting in an associated signature in ground magnetograms. Northward turning of the IMF after prolonged periods of southward IMF also seems to be associated with substorm expansive *Also at: Max-Pianck Institut fur extraterrestrtsche Recently there has been a resurgence of interest Physik. Garching. F.R.G. in the magnetospheric effects of very short-lived phase effects (Rostoker, 1983 ; and references therein). Changes in the solar wind kinetic pressure move the magnetopause inward or outward, thus affecting the tlux of energetic particles observed at any particular location within the magnetosphere and leading to increases or decreases in the H-components of low- latitude ground magnetograms (e.g. Chap. 4 of Akasofu. 1977; Nishida, 1978). The Earth’s bow shock should of course in general follow the mag- netopause in such inward/outward excursions. It has been predicted that the shock might under some con- ditions move relative to the magnetopause in response to interplanetary discontinuities (Auer. 1974; Vijlk and Auer. 1974; Neubauer, 1975), although direct observational evidence for this effect has not been published. 841