THE AURORAL OVAL POLAR BOUNDARY POSITION

It is considered that a part of the polar cap, adjacent to the auroral oval, lies on closed magnetic held lines. This has found a reasonable explanation in the framework of a model of the plasma sheet as a coll isionless shock wave (Fridman and Voronov, 1993, J. geophys. Res. A98, 143-151). According to the model, the polar cap can exist even if all of its magnetic field lines intersect the plasma sheet. The global ma gnetospheric electric held controls the dynamics of the auroral oval p olar boundary. An abrupt enhancement of the global magnetospheric conv ection electric field is followed immediately by a thinning of the pla sma sheet and by an equatorward displacement of the boundary. With a d ecrease of the magnetospheric electric field, the boundary moves towar d the cusp. Whether the electric held increases or decreases, the loca l auroral formations inside the auroral oval move equatorward with res pect to the Earth. Copyright (C) 1996 Elsevier Science Ltd.