MAGNETIC-FIELD AND PLASMA-FLOW STRUCTURE NEAR THE MAGNETOPAUSE

It has been experimentaly established that there exists a normal compo nent of magnetic field on the magnetopause and that open field lines c onnect polar cap with interplanetary medium. To explain this informati on, it is necessary to develop a quantitative theory of an open magnet osphere and to study the structure of the magnetopause. The magnetoshe ath and the magnetosphere are considered as a unitary system. The diss ipative MHD approach is used outside the magnetopause, and the magneto static approach is used inside. The frozen-in condition is valid anywh ere except at the magnetopause which is considered to be a thin dissip ative boundary layer separating the magnetosheath and the magnetospher e. The magnetopause thickness is proportional to R(m)(-1/2) (where R(m ) is the magnetic Reynolds number) and is about 100 times less than st and-off distance. The magnetic field near the magnetopause is formed d ue to mutual diffusion of the magnetospheric magnetic field in the mag netosheath and IMF into the magnetosphere. The magnetospheric magnetic field component normal to. the magnetopause is proportional to R(m)(- 1/2) and is about 1 nT at the dayside magnetopause. The related potent ial difference accross the open field lines is about 15 kV, which can be responsible for the background electric field measured in the polar caps. The portion of IMF which penetrates into the magnetosphere is p roportional to R(m)(-1/4) and is about a factor of 10 less than the IM F. The potential difference accross the polar cap is about 100 kV for the southward IMF of -10 nT.