Latitude of ionospheric projection of the boundary between open and closed geomagnetic field lines depending on the interplanetary magnetic field Bz value

Within the frames of the solar wind flow about the magnetosphere the latitude of ionospheric projection of the boundary between open and closed geomagnetic field lines is calculated. A case of pure southern direction of the interplanetary magnetic field. The conclusion is made that the equatorial boundary of the observed cusp is not strictly related to the cusp itself. It is shown that the solar wind magnetic field penetrating into the magnetosphere is damping deep into the magnetosphere at distances comparable with the magnetopause slit