The ionotail of Venus: Its configuration and evidence for ion escape

Abstract

The Pioneer Venus Orbiter measurements of the plasma and magnetic environment of the near tail of Venus show that the ionosphere becomes increasingly filamentary with increasing altitude, apparently forming cometlike tail rays that extend several thousand kilometers behind the planet. We call this region the ionotail of Venus. The tail rays are envisioned as plumes of high‐beta plasma of ionospheric origin that are surrounded by regions of low‐density, low‐beta plasma. The ionotail appears to be in quasi‐equilibrium, with the plasma pressure in the rays approximately balanced by the magnetic pressure of the region surrounding the rays. The magnetic field in this region is approximately sunaligned as we assume are the tail rays. Magnetic field reversals observed in the tail ray boundaries suggest the presence of strong current sheets there. Unlike the lower ionosphere whose major ion is thermal O+, a detailed study of tail ray plasma between 2000 and 2500 km altitude shows that the major ions are superthermal O+, with energies in the range of 9‐16 eV. The electrons are much cooler, with energies of about 1 eV. A minor, more energetic ion component, having energies exceeding 40 eV is also observed within the tail rays and occasionally between the rays as well. These Pioneer Venus Orbiter measurements reveal an ionotail that is highly dynamic, a region in which solar wind induced magnetic fields configure the ionospheric structures and accelerate the ions beyond the planetary escape velocity. We estimate a total planetary O+ escape rate of 5 × 1025 ions/s, and we infer an H+ escape rate of about half that value, about a factor of 2 below the hydrogen escape rate due to H+ charge exchange with the hydrogen exosphere of Venus.