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Magnetosphere on May 11, 1999, the Day the Solar Wind Almost Disappeared: II. Magnetic Pulsations break 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

Received December 22, 1999, revised April 6, 2000, accepted April 10, 2000


Simultaneous observations by Wind and IMP-8 in the upstream region on May 11, 1999, when the solar wind density was well below its usual values and the IMF was generally weakly northward, 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. Magnetic pulsations in the magnetosphere have been observed in the magnetic field data from Polar and at mid-latitude ground stations. 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. 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.

1 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California
2 NASA/Goddard Space Flight Center, Greenbelt, Maryland
3 Lockheed Martin Advanced Technology Center, Space Physics Laboratory, Palo Alto, California
4 Space Sciences Laboratory, University of California, Berkeley, California
5 Los Alamos National Laboratory, Los Alamos, New Mexico
6 Department of Physics, US Air Force Academy, Colorado

2000 by the American Geophysical Union.
Paper number 2000GL000012.


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2000 by American Geophysical Union

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