The results of a one-dimensional electromagnetic hybrid simulation of the interaction between the ionospheres of Venus and Mars and the solar wind are presented. Finite electron inertia is retained, allowing for the analysis of the lower hybrid waves propagating along an oblique but fixed angle to the shocked solar-wind magnetic field. The waves are excited by a relative drift between a cold electron beam, created by E 3 B pickup, and the planetary oxygen ions. The free energy source for instability is in the solar-wind proton flow supporting electron drift through the convective electric field. The waves generate a collective friction between the shocked solar-wind flow and planetary ions. PACS numbers: 96.50.Ek, Because Venus and Mars do not possess a significant intrinsic magnetic field, the ionosphere is the main, however weak, obstacle to the solar-wind flow, and the planetary bow shocks are located quite close to the planets. The ionospheres of both planets are directly exposed to the streaming shocked solar wind. Data from the particle and wave instruments onboard the Pioneer Venus Orbiter (PVO) and Phobos-2 spacecraft show the existence of a thin D ϳ 100 km turbulent transition region between the shocked solar wind and the ionospheres of Venus and Mars, referred to as the plasma mantle [1]. In the mantle, plasmas of both solar wind and ionospheric origin are present in comparable densities n 10 2 cm 23 , but with very different temperatures. The solar-wind proton (electron) temperature is approximately 100 (30) eV, and the drift velocity is close to the proton thermal velocity. The planetary oxygen and electron temperature is close to 1 eV. There is also experimental evidence for the presence of superthermal oxygen [2] and electron populations [3,4] at Mars and Venus. Large tailward escape of planetary ions, most probably originating from the dayside mantle, was also observed at Mars [5].Since the mantle thickness is much less than the ion gyroradius, the usual scheme of E 3 B pickup of the planetary ions by the solar wind is not applicable. The scenario that collective friction due to waves must be responsible for the observed tailward ion escape was formulated first for the Mars mantle in [6], and then for the Venus mantle [7]. This point of view is now widely accepted in other papers [8][9][10]. Intense wave activity was observed at the dayside Venusian mantle by the 100 Hz channel of the electric field detector on the PVO, with typical average amplitudes around tens of mV͞m [11]. Even stronger wave activity in the 5 50 Hz frequency range was measured at the boundary of the Mars ionosphere [12].The modified two stream instability (MTSI) [6][7][8] and the ion acoustic current driven instability [13] have both been suggested as explanations of the observed wave activ-ity. The scenarios of wave activity developing from these two instabilities are quite different. The MTSI results in the excitation of sufficiently long wavelength oscillations with the typical wavelength of the order of the solar-wind elect...
A reanalysis of the Pioneer Venus electron temperature data base showed a strong correlation between elevated electron temperatures and induced magnetic fields in the day side ionosphere above about 200 km. These results suggest, although not conclusively, that the elevated temperatures are the result of reduced vertical conductivities caused by the horizontal, induced fields with a possible contribution from energy deposition by magnetosheath electrons moving along the field from the tail region.
Abstract. The planetary ionospheres around the nonmagnetic planets Mars and Venus are directly exposed to the shocked solar wind. An interaction between the solar wind protons and the ionospheric oxygen takes place in a narrow turbulent region referred to as the plasma mantle. In this letter the microphysics of the dayside mantle is investigated numerically using a onedimensional hybrid code that retains the inertia of the electron species. It is shown that lower hybrid waves propagating perpendicular to the magnetic field are destabilized. Wave saturation is caused by electrostatic trapping of the proton species, and the saturated amplitudes are shown to be in reasonable agreement with Pioneer-Venus observations. Oxygen pick-up and acceleration is found to be dominated by wave effects, resulting in significant ion heating, consistent with Phobos observations.
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