The distention of the magnetosphere on May 11, 1999: High latitude Antarctic observations and comparisons with low latitude magnetic and geopotentialdata

We examine the Earth's ionospheric response on May 11, 1999, to an unusuall y tenuous solar wind, focusing on magnetometer, riometer, and optical data from high geomagnetic latitudes in Antarctica. Comparisons are also made wi th POLAR satellite data during a perigee pass over Antarctica, and with geo magnetic data collected at low latitudes. It is shown that the southern hem isphere was geophysically active, even though the K-p index on May 11 range d only from 0 to 0+. Furthermore, despite the fact that the IMF and solar w ind conditions favored northern hemisphere polar rain, low energy electron precipitation did occur in the southern polar cap. Geomagnetic power levels at low, cusp, and polar cap latitudes were also lower on May 11 than on su rrounding days. Although this might be expected, discrete millihertz peaks in ULF power were still evident, especially at cusp latitudes.