Saturation of the magnetosphere during superstorms: new results from the magnetogram inversion technique

Using data on three superstorms, we study new features of the saturation of the polar cap area when the solar wind (SW) increases. The polar cap saturation is shown to occur when the SW dynamic pressure and southward vertical (IMF) component rise. The saturation is realized not only during the passage of interplanetary magnetic clouds, but also at significant enhancement of SW density when the SW thermal pressure is comparable with the pressure of the interplanetary magnetic field. We assume that under such conditions the saturation is caused not only by a decrease in the efficiency of reconnection at the dayside magnetopause, but mainly by finite magnetosphere compressibility — stopping the magnetopause compression due to a rapid earthward growth of the geomagnetic field, i.e. the inner magnetospheric structure of the geomagnetic field. We have found signs of saturation depending on the northward IMF component. We assume that the IMF-dependent saturation exists for both signs of its vertical component due to an increase in the total pressure in the magnetosheath. Moreover, when penetrating into the magnetosphere, the southward IMF component reduces the geomagnetic field and thereby causes additional compression of the magnetopause and, accordingly, an increase in the saturation level of the polar cap area.