SM32C-01 1330h

The Extreme Compression of the Magnetosphere of May 4, 1998

X. W. Zhou

Christopher T. Russell1 (310-825-3198;; J. Newbury1; J. H. Shue2; P. Song3; S. M. Petrinec4; J. Wygant5; J. D. Scudder6; F. S. Mozer7; J. G. Luhmann 7

1Earth and Space Sciences University of California Los Angeles 405 Hilgard Avenue, Los Angeles, CA 90095-1567, United States

2Stelab University of Nagoya Japan

3Spare Physics Research Laboratory University of Michigan, Ann Arbor, MI 48109, United States 4Lockheed Martin, Palo Alto, CA 94304, United States

5Dept. of Physics Astronomy University of Minnesota, Minneapolis, MN 55455, United States

6Dept. Of Physics Astronomy University of Iowa, Iowa City, 1A 52242, United States

7Space Sciences Laboratory University of California Berkeley, Berkeley, CA 94720, United States

On May 4, 1998 the velocity and density of the solar wind were high and the interplanetary magnetic field at times strong and southward. The POLAR spacecraft crossed the dayside magnetopause well inside geosynchronous orbit, at 5.3 RE and a solar zenith angle of 19 degrees. After this crossing, POLAR spent most of the rest of its outbound orbit in the magnetosheath and for brief periods crossed into the solar wind at distances from 7.3 RE and a solar zenith angle of 32 degrees to a distance of 8.5 RE and a solar zenith angle of 45 degrees. This corresponds to subsolar distances of only 6.8 to 7.5 RE for the shock. These observations are important not solely because they are POLAR's only observations to date of the subsolar magnetopause and the bow shock. The first magnetopause crossing is a spectacular example of a rotational discontinuity with a field change of 500 nT in the north-south direction with no change in magnitude on either side of the current layer but a depressed field strength within it. The normal component across the current sheet is about 50 nT pointing inward as expected for POLAR's northern hemisphere location. The six bow shock crossings are all examples of supercritical shocks. The 3-axis electric field experiment on board provides the opportunity to test assumptions about the electric field across the bow shock for the first time. This paper introduces the POLAR observations on this day, and the other papers of this collaboration, and then examines the magnetopause crossing in some detail.