The dynamic behavior of the Venus ionosphere in response to solar wind interactions

Measurements of electron density and temperature by the Pioneer Venus orbiter electron temperature probe (OETP) are used to describe the dynamic behavior of the Venus ionosphere and to begin to relate this complex behavior to variations in the solar wind and the ionosheath magnetic field, parameters that are also measured by orbiter instruments. The average ionopause height rises from about 330 km at the subsolar point to 700 km at the dusk terminator and 1000 km at the dawn terminator, in both cases exhibiting a stronger dependence upon solar zenith angle than that reported from Venera 9 and 10 occultation data. The ionopause on the dayside tends to expand and contract with changes in solar wind pressure, becoming asymptotic to about 290 km at pressures above 4 x 10(-8) dyn/cm(2) and rising to over 1000 km for pressures below 5 x 10(-9) dyn/cm(2). The solar wind pressure, after correction for solar zenith angle, agrees approximately with the magnetic field pressure applied at the ionopause, confirming earlier suggestions that the pressure is conveyed to the ionosphere primarily by the magnetic field rather than by the shocked solar wind plasma. On the nightside the ionopause is much more highly variable, sometimes falling below 200 km or rising above 3500 km. The present Pioneer Venus orbit does not permit the true configuration to be measured. Within the nightside ionosphere itself, we find extreme spatial irregularities in the form of holes, horizontally stratified layers, detached plasma clouds, and dual temperature plasma in regions of low electron density. A scenario is developed to describe the process of ion pickup on the dayside in terms of solar wind pressure discontinuities inducing wavelike structure at the ionopause, which then is penetrated by ionosheath plasma and magnetic fields that remove ionospheric plasma impulsively in the form of detached plasma clouds. The energy released in this process may be responsible for the elevated electron temperatures observed in both the dayside and nightside of the Venus ionosphere.