The ionospheric model of the Venus microwave emission: An obituary

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 ∼1011 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 N2 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 N2 occurs are so much shorter than the time for Herzberg-Herzberg photodissociation on Venus, that N2 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 ne ∼ 106 cm−3 over a region of 50-km thickness. If radiative recombination prevailed, ne ∼ 108 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.