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GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 14, PAGES 2165-2168,
JULY 15, 2000
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
Abstract:
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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