Comparisons of peak ionosphere pressures at Mars and Venus with incident solar wind dynamic Pressure

Abstract

Previous calculations of the potential for pressure balance between the solar wind and the ionospheres of the weakly magnetized planets, Mars and Venus, indicated that the maximum or peak ionospheric thermal pressure is sufficient to stand off the solar wind at Venus but not at Mars. In this study we used radio occultation measurements of electron density profiles from Mariner 6 and 7, the Mariner 9 extended mission, and the U.S. Viking orbiters, together with model ion and electron temperature profiles, to derive thermal pressure profiles in the Mars ionosphere. Similarly, Pioneer Venus Orbiter (PVO) radio occultation data and temperature models were used to obtain ionospheric pressure profiles at Venus. Because the radio occultation data give information for both active and quiet phases of the solar cycle, this method allows one to consider how the balance changes between solar minimum and maximum. The comparison of the Mars peak ionosphere pressures with the incident solar wind dynamic pressure suggests that at solar maximum the Mars ionosphere, like that of Venus, should generally be sufficient to balance the incident solar wind pressure. At solar minimum, when the ionosphere is weakest and the solar wind dynamic pressure is highest, only the peak pressures at high solar zenith angles (SZAs) at Mars appear to be strong enough to balance the incident solar wind pressure. This is similar to the situation at Venus at solar minimum. However, due to the lack of radio occultation data for Mars for SZAs less than 45°, we can only infer what happens near the subsolar point. Nevertheless, our results are somewhat contrary to the frequent assumption that the Mars ionosphere is everywhere and always too weak to withstand the incident solar wind dynamic pressure. When one takes into account the SZA dependence of both the incident solar wind pressure and the peak electron density, and the solar cycle variation of the ionosphere, the situations at Mars and Venus appear to be quite similar. Our previous perceptions for Mars may have been colored by the fact that most of the available Mars ionospheric data are from low or moderate solar activity levels.