HIGH-ALTITUDE POLAR-CAP ELECTRIC-FIELD RESPONSES TO SOUTHWARD TURNINGS OF THE INTERPLANETARY MAGNETIC-FIELD

Interplanetary electric field coupling with the magnetosphere has been analyzed predominantly using data from the Wind magnetometer and the Polar electric field instrument. The coupling was investigated using t he Polar Electric Field Instrument (EFI) to measure the electric field in the northern polar cap immediately following sharp southward turni ngs of the IMF as observed by Wind. Southward turnings were chosen whi ch exhibited a sudden change of the IMF north-south component from B-z > 0 to B-z < 0 (GSM coordinates) after an hour or more of relatively stable conditions, and for which Polar was in the northern polar cap. These Bz changes correspond to B-y changes in the interplanetary elect ric field. For each of the 30 identified events, a time was estimated for the arrival of the IMF change at the magnetopause using the solar wind speed observed by the Wind Solar Wind Experiment (SWE), and Polar electric field data were examined to identify responses. For many of the selected events (about one third), abrupt changes of state in the magnetospheric electric field were evident with timing that matched th e expected solar wind arrival time at Earth. For events for which addi tional data were available, we conducted in-depth examination of the i ndividual events using IMP 8, Geotail, and GOES 9. In one such event, GOES 9 data showed a substorm growth phase and onset which also corres ponded to features in the solar wind observed by Wind, Geotail, and IM P 8. In addition to the individual event studies, a superposed epoch a nalysis of all available events revealed a consistent rise in the mean polar cap electric field about 15 min following sharp IMF southward t urnings.