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On October 6, 1979, the low-altitude polar-orbiting satellites DMSP-F2 and-F4 crossed the auroral electron precipitation region in the opposite hemispheres at nearly the same universal time (UT) and in the same 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 observations allowed the study, with time resolution better than six minutes, of the variation, in response to directional changes of the interplanetary magnetic field, of the latitudinal position and width of the auroral regions; these are believed to map the central plasma sheet (CPS) and boundary plasma sheet (BPS). During the equatorward expansion of the whole auroral electron precipitation region, its latitudinal width is observed to decrease markedly when the IMF turns from a northern to a southern direction. In particular, a different response of the equatorward boundary of the auroral oval with respect to the poleward boundary results from the observations, showing that the speed of the equatorward expansion of the equatorward boundary, measured at a temporal resolution of less than 6 minutes, is lower than the speed of the poleward boundary. The BPS/CPS boundary moves coherently with the southward turning of the IMF, with intermediate speed. It follows that the latitudinal width of the poleward part of the auroral region, assumed to map the boundary plasma sheet, decreases more dramatically than the width of the equatorward part of the region mapping the central plasma sheet. These findings could be explained in terms of changes of the total open magnetic flux. Actually, the equatorward shift of the poleward boundary of the auroral oval and the subsequent dramatic thining of the BPS region seem to be the consequence of a larger number of geomagnetic flux line interconnected with the IMF during a southward IMF condition.

This paper is an overview of the project TSS-1 and of its basic electrodynamic-science objectives. To this purpose, we first discuss the electrodynamics of conducting tethers in orbit and, secondly, provide a detailed description of the project and its payload. We also add, in the final part of the paper, a short account of the first TSS-1 flight which took place in August 1992.

A brief overview of the magnetospheric research activity carried out recently at IFSI CNR is given in the following.

A comparison between the quiet-time magnetic-field measurements performed on the GEOS 1 spacecraft in the period May 13, 1977–December 13, 1977 and the predictions of four geomagnetic-field models is presented. For the explored magnetospheric region the best approximation to the GEOS 1 measurements is given by the 1976 Olson and Pfitzer model: the difference between its predictions and the experimental data never exceeds 3 per cent in the absolute value of the field and 3 degrees in the field direction.