1 Earth and Space Sciences, University of California Los Angeles, CA 90095-1567
2 Stelab University of Nagoya, Japan
3 Space Physics Research Laboratory, University of Michigan, MI
4 Lockheed Martin, Palo Alto, CA
5 Department of Physics and Astronomy, University of Minnesota, MN
6 Department of Physics and Astronomy, University of Iowa, IA
7 Space Sciences Laboratory, University of California Berkeley, CA
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 19o. 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 32o to a distance of 8.5 RE and a solar zenith angle of 45o. 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.