Measurement of Solar Wind Electron Density and Temperature in the Shocked Region of Venus

We present in this paper our efforts to obtain solar wind electron density and temperature values in the shocked solar wind region of Venus. The measurements were made by the retarding potential analyzer (ORPA), one of the instruments mounted on the Pioneer Venus Orbiter space craft (PVO) that orbited Venus during the time interval 1979 to 1992, an interval including solar maximum and solar minimum. It is only recently that we have realized the potential value of these measurements in bettering our understanding of the interaction of the solar wind with the ionospheric shock barrier. Previously there have been no PVO solar plasma measurements in the shocked region. The solar wind instrument included in the PVO package had too long a measurement cycle for it to measure the plasma properties of the shocked region. The magnetic field measurements are the only reported measurements with sufficient spatial resolution to define the magnetic field across and within the shock boundary. We intend to demonstrate that the ORPA operating in its suprathermal electron mode can measure the un-shocked and shocked solar wind electron density and temperature with adequate spatial resolution to reveal much about its behavior at the shock boundary and between the shock boundary and the ionopause which is the effective solar wind barrier. The individual electron density and temperature values are derived from an I-V sweep which is completed within 0.1 seconds and may be considered a point measurement. During one revolution of the spacecraft, which is 12 seconds, the ORPA completes over one hundred of the sweeps, one of which is selected and transmitted to earth. We will present data from several orbits during our presentation. One preliminary conclusion we have come to after examining several orbits with periapsis in the night side ionosphere is that the temperature of the low temperature component of the night side super thermal electron flux, about 14 eV, [Knudsen and Miller,1985] is similar to the temperature of the un-shocked upstream solar wind electrons. This suggests that the un-shocked solar wind electron plasma is supplying the central night side low temperature component of the supra thermal electron flux. Knudsen, W. C., and K. L. Miller, Pioneer Venus suprathermal electron flux measurements in the Venus umbra, J. Geophys. Res.,2695,1985