Magnetospheric Response Times Following Southward IMF Turnings

Abstract

We analyze the response times of vari ous regions of the magnetosphere ionosphere system to sudden southward turnings of the IMF Our data set consists of events during which the IMF BZ component was observed by WIND to change from a steady northward eld to a southward eld which sub sequently led to a substorm The magnetospheric re sponse to such IMF southward turnings is examined us ing data from the POLAR EFI experiment the GOES magnetometer and ground magnetometers The POLAR EFI was used to investigate changes in the polar cap electric eld which occurred in response to the changing interplanetary electric eld and these re sults are compared with response timings derived from high latitude ground magnetometers POLAR EFI data show responses in the polar cap about min utes after the arrival of the IMF change at the mag netopause Auroral zone magnetograms and geosyn chronous spacecraft measurements are utilized to eval uate the response timing within the closed eld line region In one event examined in detail the start of a substorm growth phase was observed by GOES in the midnight sector of geosynchronous orbit about two minutes before POLAR observed a response in the po lar cap Using superposed epoch analysis we calculate typical response times in the polar cap in the night side plasma sheet and in the ionosphere in order to discuss the various suggested mechanisms for informa tion propagation from the subsolar magnetopause into the magnetosphere We nd that for the set of ten events for which GOES and the CANOPUS array are in the midnight sector the eld at geosynchronous as measured by GOES responds at or before the time of response in the polar cap as measured by POLAR suggesting di erent methods of information propaga tion