RESPONSE OF AURORAL REGIONS TO DIRECTIONAL CHANGES OF THE INTERPLANETARY MAGNETIC-FIELD - CASE-STUDY

On October 6, 1979, the low-altitude polar-orbiting satellites DMSP-F2 and -F4 crossed the auroral electron precipitation region in the oppo site hemispheres at nearly the same universal time (UT) and in the sam e magnetic local-time (MLT) sector near midnight. Three pairs of such nearly simultaneous conjugate crossings took place during a period of enhanced magnetic activity and strongly turning northward or southward of the interplanetary magnetic field (IMF). These conjugate observati ons allowed the study, with time resolution better than six minutes, o f the variation, in response to directional changes of the interplanet ary magnetic field, of the latitudinal position and width of the auror al regions; these are believed to map the central plasma sheet (CPS) a nd boundary plasma sheet (BPS). During the equatorward expansion of th e whole auroral electron precipitation region, its latitudinal width i s observed to decrease markedly when the IMF turns from a northern to a southern direction. In particular, a different response of the equat orward boundary of the auroral oval with respect to the poleward bound ary results from the observations, showing that the speed of the equat orward expansion of the equatorward boundary, measured at a temporal r esolution of less than 6 minutes, is lower than the speed of the polew ard boundary. The BPS/CPS boundary moves coherently with the southward turning of the IMF, with intermediate speed. It follows that the lati tudinal width of the poleward part of the auroral region, assumed to m ap the boundary plasma sheet, decreases more dramatically than the wid th of the equatorward part of the region mapping the central plasma sh eet. These findings could be explained in terms of changes of the tota l open magnetic flux. Actually, the equatorward shift of the poleward boundary of the auroral oval and the subsequent dramatic thinning of t he BPS region seem to be the consequence of a larger number of geomagn etic flux line interconnected with the IMF during a southward IMF cond ition.