THE INTERBALL-TAIL ELECTRON EXPERIMENT - INITIAL RESULTS ON THE LOW-LATITUDE BOUNDARY-LAYER OF THE DRAWN MAGNETOSPHERE

The Toulouse electron spectrometer flown on the Russian project INTERB ALL-Tail performs electron measurements from 10 to 26 000 eV over a 4 pi solid angle in a satellite rotation period. The INTERBALL-Tail prob e was launched on 3 August 1995 together with a subsatellite into a 65 degrees inclination orbit with an apogee of about 30 R-E The INTERBAL L mission also includes a polar spacecraft launched in August 1996 for correlated studies of the outer magnetosphere and of the auroral regi ons. We present new observations concerning the low-latitude boundary layers (LLBL) of the magnetosphere obtained near the dawn magnetic mer idian. LLBL are encountered at the interface between two plasma regime s, the magnetosheath and the dayside extension of the plasma sheet. Un expectedly, the radial extent of the region where LLBL electrons can b e sporadically detected as plasma clouds can reach up to 5 RE inside t he magnetopause. The LLBL core electrons have an average energy of the order of 100 eV and are systematically field-aligned and counterstrea ming. As a trend, the temperature of the LLBL electrons increases with decreasing distance to Earth. Along the satellite orbit, the apparent time of occurrence of LLBL electrons can vary from about 5 to 20 mill from one pass to another. An initial first comparison between electro n and magnetic-field measurements indicates that the LLBL clouds coinc ide with a strong increase in the magnetic field (by up to a factor of 2). The resulting strong magnetic field gradient can explain why the plasma-sheet electron flux in the keV range is strongly depressed in L LBL occurrence regions (up to a factor of similar to 10). We also show that LLBL electron encounters are related to field-aligned current st ructures and that wide LLBL correspond to northward interplanetary mag netic field. Evidence for LLBL/plasma-sheet electron leakage into the magnetosheath during southward IMF is also presented.