An empirical model of the Venusian outer environment 1. The shape of the dayside solar wind‐atmosphere interface

Abstract

Solar wind plasma and magnetic field data and ionospheric data obtained from the Pioneer Venus orbiter are considered. It is shown that the variation of the magnetic field pressure within the magnetic barrier is similar to that expected for the solar wind pressure variations along an obstacle's boundary when a more realistic approximation of the shape of the ionosphere is included. Simultaneous solar wind pressure, ionospheric pressure, and magnetic barrier pressure data show that the ionospheric pressure below the ionopause is approximately equal to the solar wind pressure. The magnetic barrier pressure is equal to approximately 2/3 to 3/4 of both the solar wind pressure and the ionopsheric pressure. Estimates of the hot plasma pressure contribution to the total pressure within the magnetic barrier vary from 1/4 to 1/3. Just below the ionopause the ionospheric pressure deviations from the mean ionospheric pressure are significant especially below ∼400 km and appear to be indicative of the adjustment of the ionospheric structure to changing solar wind conditions. A first‐order model of the ionopause pressure variations as a function of height and solar‐zenith angle is suggested. This model provides an estimate of an ‘instant’ ionopause profile for the given solar wind conditions. The mean and ‘instant’ shapes of the ionopause do not appear to correspond to the estimates obtained from ionospheric pressure equilibrium for a given H/r0.