Planetary ionospheres: the Venus ionopause

Abstract

Pioneer Venus (PV) measurements have confirmed an earlier suggestion that a Venus ionopause is formed by solar wind interactions. Plasma investigators have defined a 'density' ionopause (DI) by the sharp gradient in the plasma density at an altitude where the density falls to 100 ions/cm³. Magnetometer investigators have taken the ionopause altitude at the height where the plasma pressure is balanced by the magnetic-field pressure and have called it the 'pressure' ionopause (PI). The PV data have been examined to test the physical description of the DI and the PI. It is found that the DI is an appropriate definition for the altitude where the ions pause. The PI definition, on the other hand, is more restrictive. The total pressure is higher at the PI than at the DI and from the observations it is inferred that suprathermal ions are present at altitudes above the PI. A horizontal flow velocity of about 27 km s^-1 has been obtained for extremely high PI conditions, producing the DI at its lowest altitude limit of 290 km.