Dependence of the polar cap structure on parameters of the interplanetary medium based on penetration deduced from solar cosmic ray electrons.

The structure of the polar cap boundaries is studied, based on the penetration of solar cosmic ray electrons measured by the low altitude satellite Intercosmos-17. The boundary location is discussed in connection with parameters of the interplanetary medium, such as the solar wind density N, the velocity V, the characteristic solar wind pressure (proportional to NV²) and the interplanetary magnetic field B_z component. From the data obtained during four events, during which solar cosmic ray electrons were present in interplanetary space, one can derive how the boundary of electron penetration in the dayside sector is controlled by the solar wind pressure and by B_z. In the evening-midnight-morning sector electrons are seen in two regions: in the polar cap on field lines belonging to the geomagnetic lobe and on field lines of the plasmasheet. When a geomagnetic substorm is in progress, the boundaries of these two regions are not only crontrolled by parameters of the solar wind and the IMF, but also by the phase of the substorm.