SM32C-02 1345h

Observations of Intense Electric Field Structures at the May 4, 1998 Magnetopause Crossings

J. R. Wygant1(612-626-8921; C A Cattell1; A Keiling1; F. Mozer2; M Temerin2;
W. K. Peterson3; Trattner3; J. Scudder4; C. T. Russell5; N Maynard6

1School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, United States

2Space Sciences Lab, University of California, Berkeley, CA 94720

3Lockheed-Martin Palo Alto Research Lab, Palo Alto, CA 94304

4Department of Physics, University of Iowa, Iowa City, IA 52240

5IGPP-UCLA, Los Angel., CA 90024

6Mission Research Corp., Nashua, NH 03054

Electric fields, magnetic fields, and particle measurements are presented from a sequence of dayside magnetopause crossings observed by the POLAR spacecraft during a strong compression of the earth's magnetopause on May 4, 1998 between 5:41 UT and 8:00 UT at 5.3 to 8.0 Re geocentric distance. The electric fields observed during this period are among the most intense ever observed in the outer magnetosphere and are 1-2 orders of magnitude larger than those typically observed at the magnetopause. They ranged in magnitude up to 250 mV/m and were directed predominantly normal to the direction of the nominal magnetopause surface with both transient and steady state components. The z GSM component of the magnetic field varied from about -300 nT in the magnetosheath to 300 nT in the magnetosphere with a transtion characteristic of a rotational discontinuity. The steady state component of the electric field signature is consistent with a boundary layer ExB flows in the magnetosheath which are tangential to the boundary and directed east ward with a magnitude of 500-1000 km/s. Superimposed on these steady state signatures are intense transient electric and magnetic field pulses lasting periods of about 1 second observed in and near the magnetopause current layer. These pulses have electric field amplitudes >100 mV/m directed normal to the magnetopause surface and magnetic field perturbations of >100 nT directed approximately transverse to the ambient field. Magnetic minimum variance analysis indicates the direction of the k vector is well determined for a signficiant portion number of the pulses and in these cases is directed along the magnetopause normal. This implies the dominate electric field signature is aligned along the k vector and is the electrostatic component of the rotational discontinuity. These discontinuities coincide with 10 % to 100 % density depressions as inferred from high time resolution measurements of the spacecraft potential. The wave Poynting flux associated with the largest Alfvenic discontinuities exceeds 10 ergs/cm**2s and is directed northward along the ambient field. This suggests the possibility that wave Poynting flux may be a signficant portion of the total energy flux radiated way from the reconnection region.