DIFFUSIVE PENETRATION OF THE IMF B(Y) INTO THE MAGNETOSPHERE

A mechanism of the observed IMF B(y) penetration into the magnetospher e is discussed. It is argued that diffusion of the IMF through the mag netopause prevails when the IMF is sufficiently weak. The diffusive pe netration coefficient alpha defined as the ratio between the internal B(y) penetration and the IMF B(y) is estimated to be of order Rm-1/4, where Rm is the magnetic Reynolds number. A model is developed to desc ribe quantitatively a magnetic field in the outer solar wind flow (wit hin the diffusive magnetopause) and the IMF-related magnetic field in the magnetosphere. The calculated values of the diffusive penetration coefficient are consistent with the observational data. Some differenc e in the results is mainly due to the electric currents inside the mag netosphere which are caused by the penetration of the solar wind elect ric field into the magnetosphere along the open field lines. The furth er improvement of the model implies the incorporation of this current system.