A model of the ionospheric tail rays of Venus

Abstract

Distinctive structural patterns in the "tail rays" that form the outermost reaches of the Venus nightside ionosphere were observed on the Pioneer Venus Orbiter. The measurements further suggested that the tail rays are the vehicle for significant escape of atmospheric oxygen, but the manner in which they fit into the scenario of solar wind scavenging and the physics behind their formation remain unexplained. In this paper a model of tail ray generation and morphology is proposed which is based on knowledge of the magnetic field structure in the low‐altitude magnetosheath and near‐wake, where the tail rays are observed. The model applies specifically to those regions where the plasma pressure in the tail rays is less than the field pressure. It is shown that many of the observed structural patterns can be explained if it is assumed that initially low energy ionospheric O+ ions are picked up by an externally imposed convection electric field from a thin source region around the terminator. Gravity is found to play a significant role in determining the ion trajectories and hence the modeled tail ray structures. The model can produce single, double, triple, or quadruple thin tail rays, all of which are observed. The energies of the particles are also consistent with the available data. The reported tail ray dependence on solar EUV flux and solar wind dynamic pressure naturally fits into the proposed concept. The implication for ion escape is that no special mechanisms are required to explain Venus tail rays. They may be simply interpreted as the low‐altitude, low‐energy manifestation of the standard ion pickup process at a weakly magnetized planet.