Cusp and boundary layer observations by Interball

I. Sandahl, R. Lundin, M. Yamauchi, U. Eklund, J. Safrankova, Z. Nemecek, K. Kudela, R. P. Lepping, R. P. Lin, V. N. Lutsenko and J. A. Sauvaud

Institue of Space and Astronautical Science, Kanagawa, Japan


Interball Tail Probe was launched on August 2, 1995 together with its subsatellite MAGION-4 into a highly elliptical orbit with apogee at 31 RE and inclination 63 degrees. During the course of one year all local times are visited. In this paper we will present initial results obtained from measurements in the cusp, mantle and eveningside plasma sheet in January, 1996. It is found that the cusp was well defined and persistent at altitudes of 4-10 RE. In one case both the main satellite and the subsatellite were in the cusp proper for two uninterrupted hours. We believe this to be the first ever multi-point satellite observation of the high-altitude cusp. The data indicate that the cusp was very stable with a wide entry area and that plasma entry took place at high latitudes rather than at the subsolar point. Pressure pulses, possibly due to Alfven waves were found. Sunward and anti-sunward moving plasma was measured simultaneously during a case of northward IMF but no convection was discovered. Plenty of plasma of cusp/magnetosheath type was also found mixed with plasma sheet plasma both equatorward of the cusp and in the eveningside plasma sheet.

Adv. Space Res., 20, 823-832, 1997