OPPOSITE ION DISPERSIONS OBSERVED QUASI-SIMULTANEOUSLY IN THE POLAR CUSP BY THE DE-2 AND AUREOL-3 SATELLITES

On 26 August 1982 the satellites Dynamics Explorer 2 (DE-2) and Aureol -3 (A-3) crossed around the same time the southern polar cusp. DE-2 de tected the cleft in the prenoon sector (11.5 H MLT) constituted by a p oleward ion dispersion with two steps at around 50 and 1.5 keV, respec tively. Around 1.5 H MLT eastward, A-3 detected a diffuse ion precipit ation, immediately followed by an equatorward dispersion (energy Incre ases when the latitude increases). The convection pattern detected by DE-2 was directed westward in the cleft with a maximum, around 1600 m/ s, centred on the low energy step. The convection velocity measured by A-3 had a component directed poleward where the diffuse ion precipita tion was observed and a convection with an equatorward component where the ion dispersion is observed. The IMP-8 IMF data showed a gap at th e time of the cusp crossings, however using the IMF-Bz component aroun d that time and the AE index we infer that the IMF-Bz would have been positive during the cusp crossings. We are explaining the observations by the merging of the interplanetary magnetic field with the geomagne tic lobe field lines which produces the dispersion observed at high la titude on A-3 and the low energy step observed by DE-2. At the same ti me a viscous diffusion on the frontside of the magnetosphere produces the diffuse ion precipitation observed by A-3 and the high energy step observed by DE-2.