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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A9, PAGES 19,023–19,033, 2001

Observations of the seasonal dependence of the thermal plasma density in the Southern Hemisphere auroral zone and polar cap at 1 R E

M. T. Johnson

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota


J. R. Wygant

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota


C. Cattell

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota


F. S. Mozer

Space Science Laboratory, University of California, Berkeley, California


M. Temerin

Space Science Laboratory, University of California, Berkeley, California


J. Scudder

Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa


Abstract

Synoptic maps of the thermal plasma density in the auroral zone and polar cap around 1 R E altitude are produced from measurements of Polar spacecraft floating potential during 1 year of perigee passes. The densities are accurate to around a factor of 2 in this region of space. These measurements, which provide the first comprehensive maps of the thermal plasma density over the polar cap and auroral oval, show clear variations in plasma density due to solar illumination of the ionosphere. Number density increases by a factor of 5 for illuminated verses nonilluminated ionospheric conditions over the entire auroral oval and polar cap. The maps delineate the global extent of the large-scale auroral density cavity near 70° ± 5° invariant latitude. These maps further show that the depth of the density cavity is strongly influenced by solar illumination. The minimum average density (∼0.1 cm−3) occurs in darkness near 1900 magnetic local time.

Received 8 May 2000; accepted 12 October 2000.

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Citation: Johnson, M. T., J. R. Wygant, C. Cattell, F. S. Mozer, M. Temerin, and J. Scudder (2001), Observations of the seasonal dependence of the thermal plasma density in the Southern Hemisphere auroral zone and polar cap at 1 R E , J. Geophys. Res., 106(A9), 19,023–19,033.