Observations of Energetic Ions in the Magnetosheath on May 4, 1998

S.-W. Chang 1, J. D. Scudder 1, J. F. Fennell 2, R. Friedel 3, C. T. Russell 4, R. P. Lepping 5, and H. E. Spence 6


1 The University of Iowa, Department of Physics and Astronomy, 203 Van Allen Hall, Iowa City, IA 52242

2 The Aerospace Corporation, Mail Stop M2-259, Los Angeles, CA 90009

3 Los Alamos National Laboratory, NIS-2, MS 436, Los Alamos, NM 87544

4 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095

5 NASA Goddard Space Flight Center, Code 696.0, Greenbelt, MD 20771

6 Boston University, Department of Astronomy and Space Physics, 725 Commonwealth Ave., Boston, MA 02215

The NASA GGS/POLAR spacecraft crossed the Earth's magnetopause and went into the magnetosheath for a long period of time on May 4, 1998, as indicated by the plasma and field measurements on POLAR. Very intense energetic ion fluxes were detected by the spacecraft. The ion spectrum is continuous from 20 eV to 200 keV with a break at about 20 keV. The ion total number flux integrated above this energy shows good anticorrelation with the IMF cone angle observed upstream by the NASA GGS/WIND spacecraft. The correlation coefficient for the ion fluxes and IMF measurements shifted by an arbitrary amount of time shows a clear dependence on the time shift. With no correlation at a zero shift and a large shift, the coefficient indicates a minimum (maximum absolute value) at a time lag that is consistent with the estimated solar wind propagation time from the Wind to the Polar spacecraft. These results apply for both H$^+$ and He$^{2+}$ ion species. In contrast, energetic ions observed just prior to this event in the plasma sheet show poor correlation with the IMF cone angle. These magnetosheath observations are consistent with results of the Fermi acceleration at the Earth's bow shock with higher energetic ion fluxes occurring at the quasi-parallel and lower fluxes at the quasi-perpendicular shock geometry. It suggests that the energetic ions observed in the magnetosheath are accelerated at the bow shock which can be a source region of energetic ions in the cusp.