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.

Measurements of electron density and temperature by the Pioneer Venus orbiter electron temperature probe have been employed to examine the characteristics and morphology of ionospheric holes in the antisolar ionosphere of Venus. The holes apparently exist as north‐south pairs which penetrate the ionosphere vertically down to altitudes as low as 160 km. Magnetic field measurements show that the holes are permeated by strong radial fields whose pressure is sufficient to balance the plasma pressure of the surrounding ionosphere. The electron temperature in the holes is substantially cooler than the surrounding ionosphere, except in the lowest density regions of the holes where the temperatures greatly exceed the ionosphere temperature. The low temperatures and the low densities of the holes are consistent with the strong radial magnetic fields which inhibit horizontal transport of plasma and thermal energy from the surrounding ionosphere. Plasma depletion processes associated with magnetotail electric fields may be important in the formation of the holes.

No abstract

Measurements of electron density and temperature by the Pioneer Venus orbiter electron temperature probe (OETP) are used to describe the dynamic behavior of the Venus ionosphere and to begin to relate this complex behavior to variations in the solar wind and the ionosheath magnetic field, parameters that are also measured by orbiter instruments. The average ionopause height rises from about 330 km at the subsolar point to 700 km at the dusk terminator and 1000 km at the dawn terminator, in both cases exhibiting a stronger dependence upon solar zenith angle than that reported from Venera 9 and 10 occultation data. The ionopause on the dayside tends to expand and contract with changes in solar wind pressure, becoming asymptotic to about 290 km at pressures above 4 × 10−8 dyn/cm² and rising to over 1000 km for pressures below 5 × 10−9 dyn/cm². The solar wind pressure, after correction for solar zenith angle, agrees approximately with the magnetic field pressure applied at the ionopause, confirming earlier suggestions that the pressure is conveyed to the ionosphere primarily by the magnetic field rather than by the shocked solar wind plasma. On the nightside the ionopause is much more highly variable, sometimes falling below 200 km or rising above 3500 km. The present Pioneer Venus orbit does not permit the true configuration to be measured. Within the nightside ionosphere itself, we find extreme spatial irregularities in the form of holes, horizontally stratified layers, detached plasma clouds, and dual temperature plasma in regions of low electron density. A scenario is developed to describe the process of ion pickup on the dayside in terms of solar wind pressure discontinuities inducing wavelike structure at the ionopause, which then is penetrated by ionosheath plasma and magnetic fields that remove ionospheric plasma impulsively in the form of detached plasma clouds. The energy released in this process may be responsible for the elevated electron temperatures observed in both the dayside and nightside of the Venus ionosphere.

No abstract