No abstract

A theoretical model has been constructed in which the ion density and the ion and electron temperature distributions are calculated by solving the coupled continuity‐momentum equations and the coupled energy equations. The latest experimental results from the Viking 1 and 2 landers are used to vary some of the parameters in the model in order to obtain agreement between the theoretical and experimental results. It is found that solar EUV radiation alone is not able to maintain the observed high ion temperatures. It was also established that the energy coupling between the electron and ion gas is insufficient to account for the measured ion temperatures even in the presence of very large electron temperatures. Direct heat input to the ion gas, probably due to solar wind‐ionosphere interactions, can result in ion temperature values in reasonable agreement with the observations. The ion densities calculated with the present model agree well with the Viking observations in the chemically controlled region, but at higher altitudes, dynamic transport processes need to be invoked to achieve consistency among the observed and calculated temperature and density values.

Around Venus the planetary ionosphere is directly exposed to the shocked solar wind. The interaction takes place in a broad region surrounding the dayside ionosphere, called the mantle, where the shocked solar wind plasma and the plasma of planetary origin have equally important roles. In this paper both the experimentally determined characteristics and the microphysics of the mantle are discussed in detail. It is shown that as a result of the interaction between the two plasma populations, a modified two-stream instability develops, and waves are excited with a frequency of a few times the lower hybrid frequency. The polarization of the waves is almost perpendicular to the magnetic field. The stabilization of the higher-frequency part of the wave spectrum is the result of transverse wave convection in the particular sheet-like geometry of the mantle. The interaction of these waves with planetary ions and electrons is described within the framework of a nonlinear model in which the saturation of the modified two-stream instability is due to induced scattering of the waves on cold planetary ions. The effective collision frequency between the shocked solar wind protons and planetary ions is also calculated; it is shown how this leads to ion pick up and heating. Other macroscopically observable effects of these processes are electron acceleration along the magnetic field and ionospheric heating. The experimental data collected in the dayside mantle of Venus by the instruments carried onboard the Pioneer Venus Orbiter are compared to our model. It is believed that the observations support the scenario presented. Usw/•ci "' 10 3 km, where Usw "' 100 km S -1 is the solar wind velocity in the mantle and ftc• is the gyrofrequency of the oxygen ions. The typical distance at which E x B pickup takes place is *rUsw/Dc•, and accordingly the pickup ions leave the 21,289 21,290 SHAPIRO ET AL.: SHOCKED SOLAR WIND AND VENUS PLANETARY IONS mantle region. The plasma population of the mantle, which governs its local microphysics, consists of the counterstreaming shocked solar wind and the ionospheric cold plasma. As a result of the interaction between the two plasma populations, a modified two-stream instability (MTSI) develops in the mantle region, and waves are excited with frequencies up to a few times the lower hybrid frequency, fLH = (1/2rr)(12ce12cp) •/2 (where Dee and •cp are the gyrofrequencies of electrons and protons, respectively). For the average mantle magnetic field, f/•H is 30-40 Hz. This physical picture was originally proposed by Sagdeev et al. [1990] to explain the wave activity observed in the Martian magnetosphere during the Phobos 2 mission. This paper showed that the excited waves couple the shocked solar wind plasma to the heavy planetary ions, both in energy and momentum, and they also contribute to the large tailward escape of planetary ions observed by the TAUS instrument carried onboard the Phobos 2 spacecraft [Rosenbauer et al., 1989]. Later, noting the similarity between the sheath-ionosphere boundary l...

The measurements of electron spectra in the Martian magnetosphere by the HARP instrument on board the Phobos 2 orbiter are presented. The energy of the electrons (a few tens of electron volts) is sufficient for the impact ionization of the planetary neutral gas, and the characteristic flux of electrons (∼108 cm−2 s−1) could produce the nightside ionospheric layer with a peak density of a few thousands of electrons per cubic centimeter, which corresponds to densities observed earlier during radio occultations of the Mars 4 and 5 and Viking 1 and 2 spacecraft. The possibility of magnetospheric electron precipitation into the nightside atmosphere of Mars is in agreement with the mainly induced nature of the magnetic field in the planetary magnetotail (as at Venus), while the variability of the Martian nightside ionosphere may be explained by the partial screening of the atmosphere by a weak intrinsic magnetic field of the planet.

Abstract.The coupled, one-dimensional electron and ion energy equations, with a combination of small steady and fluctuating horizontal magnetic fields imposed, are solved for the Mars ionosphere, corresponding to conditions encountered during the Viking mission. A series of calculations with various boundary conditions and heat sources result in a range of electron and ion temperature profiles, which are compared with the results obtained by the RPA's carried aboard the Viking landers. It is shown that solar EUV heating alone does not lead to the observed temperature profiles and that assuming reasonable heat fluxes at the top result in good agreement. It is also found that the introduction of small steady and altitude dependent fluctuating horizontal magnetic fields, which modify the thermal conductivity, leads to electron temperatures in reasonably good agreement with the RPA data, but does not match the observed ion temperatures above about 240 kin. The effects of chemical and Joule heating are also examined and found not to be significant.