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Magnetosphere on May 11, 1999, the Day the Solar Wind Almost Disappeared: II. Magnetic Pulsations in Space and on the Ground

G. Le,1 P. J. Chi,1 W. Goedecke,1 C. T. Russell,1 A. Szabo,2 S. M. Petrinec,3 V. Angelopoulos,4 G. D. Reeves,5 and F. K. Chun6

1Institute of Geophysics and Planetary Physics, University of California, Los Angeles

2NASA/Goddard Space Flight Center, Greenbelt, Maryland

3Lockheed Martin Advanced Technology Center, Space Physics Laboratory, Palo Alto, California

4Space Sciences Laboratory, University of California, Berkeley, California

5Los Alamos National Laboratory, Los Alamos, New Mexico


We have examined magnetic pulsations in the magnetosphere using magnetic field data from Polar and mid-latitude ground stations on May 11, 1999, when the solar wind density was well below its usual values and the IMF was generally weakly northward. By comparing May 11 with a control day under normal solar wind conditions and with a similar foreshock geometry, we find that the magnetosphere was much quieter than usual. The Pc 3-4 waves were nearly absent in the dayside magnetosphere both at Polar and as seen at mid-latitude ground stations even through the foreshock geometry was favorable for the generation of these waves. Simultaneous observations in the upstream region on May 11 indicate there were upstream waves present in the foreshock, but wave power was an order of magnitude weaker than usual due to an extremely weak bow shock and tenuous solar wind plasma. Since the solar wind speed was not unusual on this day, these observations suggest that it is the Mach number of the solar wind flow relative to the magnetosphere that controls the amplitude of Pc 3-4 waves in the magnetosphere and not the Kelvin-Helmholtz instability associated with the flow of the solar wind plasma past the magnetosphere.

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© 1999 by American Geophysical Union