GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 13, PAGES 1827-1830,
JULY 1, 2000
The magnetosphere on May 11, 1999, the day the solar wind almost
disappeared: I. Current systems
G. Le and C. T. Russell
Institute of Geophysics and Planetary
Physics, University of California, Los Angeles, California
S. M. Petrinec
Lockheed Martin Advanced Technology Center, Space
Physics Laboratory, Palo Alto, California
Received November 30, 1999, revised , accepted March 13,
We have examined magnetic field data from Polar MFE to study the
magnetospheric current systems on May 11, 1999, when the solar wind density was
well below 1 cm-3 and the IMF was generally weakly northward.
We find that the magnetosphere was much more dipolar than usual but the ring
current did not disappear. In the inner magnetosphere, the residual field, the
observed magnetic field at Polar with the internal field removed, was dominated
by the ring current contribution. The magnetic field data agreed well with the
Tsyganenko 1996 model with a weak ring current but little magnetopause current.
Weak field-aligned currents were observed both in the northern and southern
hemisphere on May 11, as expected for northward IMF. The cusp signature of a
depressed magnetic field, normally seen by Polar when passing through the high
altitude polar cusp, was not seen on May 11, indicating that the energy density
of the plasma in the cusp was much smaller than usual, as would be expected if
the solar wind source were of lower density.
© 2000 by the American Geophysical Union.
Paper number 1999GL010774.
© 2000 by American Geophysical Union
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