Ionospheres of Venus and Mars: a comparative study

Abstract

Magnetometer measurements on the Pioneer Venus Orbiter (PVO) have shown that Venus is a non-magnetic planet and similar measurements on the Mars Global Surveyor (MGS) have demonstrated that, except for some very localized magnetic fields of crustal origin, Mars intrinsic magnetic field is rather weak. As a consequence, solar wind interacts directly with the atmospheres and ionospheres of these planets. Much has been learnt about the ionosphere of Venus from the large database generated by the various aeronomy experiments on the PVO, especially during the solar maximum. For example, the major ion in the upper ionosphere (topside) of Venus is O⁺, and is controlled by diffusion, while O₂⁺ dominates in the bottomside (below 200 km) and is in photochemical equilibrium. Mars ionosphere is relatively less explored and most of the information has come from radio occultation experiments, which have provided a large data base on the electron density profiles for various solar activity conditions. Only two direct ion measurements exist due to Viking Landers, which correspond to solar minimum conditions and have shown that O₂⁺ dominates in most part of the Mars ionosphere. However, its height distribution is in variance with that expected theoretically from the diffusive equilibrium state. During high solar activity when Venus ionosphere is robust, solar wind interaction results in a sharp boundary of the ionosphere, called the ionopause. However, during solar minimum, when the ionosphere is weak, the solar wind compresses the Venus ionosphere to its limiting altitude in the photodynamical regime forming a thick ionopause. The diffusion region (or the topside ionosphere) is then nearly extinct. This thick ionopause is also seen at times of high solar wind dynamic pressure (Psw) during solar maximum with O⁺ still as the dominant ion there. The diffusion region is nearly extinct again. Since Mars ionosphere is weak, particularly during low and moderate solar activity conditions, one expects the solar wind to compress its ionosphere in the photodynamical regime forming a thick ionopause. Consequently, the diffusion region (or the topside ionosphere) would be nearly extinct most of these times. The observed Viking O₂⁺ profiles and the radio occultation Ne profiles, which are indeed greatly compressed provide evidence in this regard. Thus the observed topside ionospheric profiles of Venus and Mars basically represent the ionopause regions, only exception being the solar maximum when an extended topside ionosphere exists at Venus.