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

Synoptic maps of the thermal plasma density in the auroral zone and polar c ap around 1 R-E altitude are produced from measurements of Polar spacecraft floating potential during 1 year of perigee passes. The densities are accu rate to around a factor of 2 in this region of space. These measurements, w hich provide the first comprehensive maps of the thermal plasma density ove r the polar cap and auroral oval, show clear variations in plasma density d ue to solar illumination of the ionosphere. Number density increases by a f actor of 5 for illuminated verses nonilluminated ionospheric conditions ove r the entire auroral oval and polar cap. The maps delineate the global exte nt of the large-scale auroral density cavity near 70 degrees +/- 5 degrees invariant latitude. These maps further show that the depth of the density c avity is strongly influenced by solar illumination. The minimum average den sity (similar to0.1 cm(-3)) occurs in darkness near 1900 magnetic local tim e.