The ionospheric model of the Venus microwave emission: An obituary

Abstract

In the ionospheric model of Venus, the observed microwave radiation is attributed to free-free emission of electrons in a dense Cytherean ionosphere. The present paper discusses the mechanisms for formation of such a dense ionosphere, both in the original formulation of Jones and in later formulations which introduce holes in the ionosphere to achieve consistency with the observed radar reflectiveness. Ionization by solar ultra-violet radiation and by the solar proton wind, as measured near Venus by Mariner 2, are considered, assuming that the Cytherean surface magnetic field strength is ≤10 −3 gauss. Both sources of ionization are primarily effective at the level where the neutral particle density is ∼10 11 cm −3, somewhat above the level at which diffusive equilibrium is established. Three-body electron-ion recombination processes are ineffective at these densities. Radiative recombination is plausible only if all the N 2 at this level is converted to N by the Herzberg-Herzberg photodissociation mechanism. But the times for N to diffuse to dense levels where recombination to N 2 occurs are so much shorter than the time for Herzberg-Herzberg photodissociation on Venus, that N 2 must be a predominant atmospheric constituent at the level of the Cytherean ionosphere. If dissociative recombination therefore prevails, characteristics values of the electron density are n e ∼ 10 6 cm −3 over a region of 50-km thickness. If radiative recombination prevailed, n e ∼ 10 8 cm −3 would result, but even this is too small for the ionospheric model to be a valid explanation of the Venus microwave emission. The ionospheric model is accordingly rejected. We conclude that the observed microwave emission arises from the Cytherean surface.