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Preliminary results of observations of plasma electrons with energies up to 400 eV, measured on Mars 2 and Mars 3, were previously presented by Gringanz et al. (1973). A summary of the results of further reduction of the data obtained and an attempt at their interpretation is given. (WDM) Two shocklike structures (i.e., abrupt steplike increases in number density, bulk speed, and proton temperature) were found in the data from Martiner 5 at 0.98 AU on June 26 and at 0.85 AU on August 29, 1967. The thickness of these structures' determined from the magnetic field data was more than 1,000 proton Larmor radii. They are also observed by Explorer 33, 34, and 35 at 1 Au, where the thickness were much smaller than in the Mariner 5 data. It is suggested that these two structures were nonlinear magnetoacoustic waves that were in the processes of steepening. It is further suggested that shocks would be formed just beyond 1 AU. These structures were not associated with corotating streams, but they could be associated with impressive solar events in which a flare was followed by type 2 and type 4 radio emission and a rare chromospheric wave. (auth)

The atmosphere of a planet is largely responsible for the degree of influence solar radiation exerts on the planet’s surface. It determines the type of interaction of the interplanetary medium with a planet and it is upon this that the physical characteristics of the circumplanetary space depend. The ionospheres of planets, being the substantial parts of their atmospheres, interact with these by the ionization-recombination cycle as well as by the dynamical processes of heat and mass transfer and by energy exchange between charged and neutral particles. It is very difficult to understand the heat balance in the atmospheres and their dynamics without knowing the structure of the ionospheres and the processes involved in their formation. In addition, this structure of the ionosphere determines the propagation of radio waves near the planet.

The results of Soviet and American spacecraft plasma and magnetic experiments show that a bow shock of Venus forms as a result of the direct interaction of the solar wind with the ionosphere. The shape and the position of the Venus bow shock, in general, correspond to a very weak dissipation of solar wind energy in the ionosphere.

Preliminary results of ion and electron plasma measurements near Venus are presented and discussed. The data were obtained with wide-angle plasma analyzers carried on the Venera 9 and 10 spacecraft. On the basis of 33 bow shock crossings the position of the shock is quite stable and agrees well with theoretical predictions of Spreiter et al. with H/r/sub 0/=0.01 and a stagnation point altitude of approx.500 km. This observation lends strong support to the assumption that the solar wind interacts with the upper ionosphere of Venus and not with a planetary magnetic field. These spacecraft are the first to explore the optical umbra of Venus. Close to the planet a stable population of electrons and an ill-defined population of positive ions were found; this region is called the corpuscular umbra. The corpuscular umbra and the transition region are separated by a zone which contains both positive ions and electrons and is characterized by a flow velocity reduced in comparison with that of the transition region. This zone is called the corpuscular penumbra. The distribution of plasma density behind the bow shock (including the optical umbra of the planet) is given, and the existence of a Venusian plasma magnetic til ismore » revealed.« less

Analysis of the properties of the dayside ionosphere at Venus has shown that two situations are characteristic. One corresponds to the high dynamic pressure of the solar wind, and the other is realized at low dynamic pressure. In both regimes the ionopause manifests itself as a change of the dominant chemical component of plasma rather than the boundary of the magnetized and unmagnetized plasmas (the widely used definition). The nonstationary convection of the magnetized plasma in the Venus dayside ionosphere is analyzed qualitatively. It is argued that the observable large-scale magnetic field in the dayside ionosphere of Venus is the solar wind magnetic field pushed down into the ionosphere during the periods of high solar wind dynamic pressure which evolves under the action of convection and diffusion. The upper boundary of the magnetic belt forms in the vicinity of the upper boundary of the photochemical region. At low dynamic pressures of the solar wind, in a region from approximately 300 km to the ionopause, an upward flux of ionospheric plasma can exist due to motion of plasma to the terminator under the day-to-night pressure gradient. With the plasma convection and the results of the study of the Venus ionopause stability taken into account, the destruction of the large-scale ionospheric field is a probable source of flux ropes in the Venus lower ionosphere rather than small-scale ionopause instabilities.

An analysis of available experimental data and theoretical concepts indicates that the interaction of the solar wind (SW) on the subsolar side with Venus, which has no intrinsic magnetic field, and with Mars, which has a small intrinsic magnetic field, is determined by the solar wind dynamic pressure with a contribution from the neutral planetary atmosphere to this interaction. The pattern of the SW interaction with these planets is different in principle for high and low dynamic pressures of the SW and is related to the varying intensity of ion formation processes (the SW Mass loading effect) in the vicinity of the SW obstacle boundary, which moves for different SW dynamic pressures into regions of different neutral atmosphere density. For moderate or high SW dynamic pressures, the subsolar Martian magnetosphere is also affected by this process. Results of numerical simulations of the SE-Mars interaction for a magnetospheric obstacle boundary at an altitude of 300 km are presented. To estimate the relative role of photoionization and charge exchange processes and their effect on the shock front position, different versions of the mass loading effect were separately calculated. copyright American Geophysical Union 1989

The ionization produced by electrons having energies of several tens of electron volts in the nighttime Venusian atmosphere is investigated on the basis of plasma measurements made on board Venera 9 and 10. We conclude that electron fluxes are sufficient to produce a nighttime ionosphere with a maximal electron density of 10/sup 4/ cm/sup -3/. Calculated density profiles of the Venusian nighttime ionosphere show shapes closely resembling measured ones with closely agreeing values of maximal density and corresponding altitudes as well.