Transport of ionospheric O⁺ ions across the Venus terminator and implications

Abstract

Vector velocities of O+ ions in the Venus ionosphere are reported for the dawn and dusk terminator regions. The velocity vectors are generally directed antisunward and radially inward toward the planet with magnitudes ranging from approximately 1 to 8 km/s. The velocity generally increases with altitude. The velocity may increase to still larger values just below the ionopause, but experimental limitations prevent measuring such an increase. The Alfvén and ion Mach numbers are generally greater than 1. The estimated O+ flux across the terminator is equal within a factor of 2 to the total ion recombination rate on the nightside and is evidently a factor of 10 larger than that which could be exiting down the planet wake. The higher velocity flow near the ionopause is hypothesized to converge on the antisolar axis and, after thermalization, to descend into the ionosphere. The kinetic energy and O+ flux in this high‐velocity stream are sufficient to maintain, respectively, the high ion temperature and peak ion concentration, respectively, measured at solar zenith angles in excess of 150°. The nightside ionospheric variability is hypothesized to result from temporal and spatial variations in the ionospheric terminator and wake flow fields. Ion transport contributes substantially and possibly predominantly to the maintenance of the nightside ionosphere.

Dynamics of pick-up ions in collisionless velocity shears

“…While these variations in the plasma properties of the solar wind together with the transterminator flow seen in the upper ionosphere (Knudsen et al 1980) are consistent with the overall behavior expected from the effects of momentum transport through viscous-like processes (Pérez-de-Tejada 1986), there are unrelated observations that reveal further important conditions within the shear layer. In particular, measurements conducted in the Mars magnetosheath with a particle detector system of the Phobos spacecraft indicate the presence of energetic particle fluxes at the lowest (∼900 km) altitudes from Mars that were approached along trajectories near the flank regions.…”

Ion dynamics in the Venus ionosphere

“…There is some evidence for shock wave formation at an SZA of about 140 ~ ; specifically, the sudden fluctuations observed in the velocity field. Knudsen et al (1980a) suggested that the rapidly increasing ion temperatures occurring in the same region indicates that the cessation of mean nightward flow is due to a recompression shock in the plasma. However, Whitten et al (1986) showed that the same amount of kinetic energy would be converted into heat from an adiabatic compression as from a recompression shock.…”

“…Contours in Figure 17 show a much greater vertical gradient in the velocity on the dawn hemisphere than on the dusk hemisphere. The calculated results for the nightside are probably inaccurate, because of, among other things, a strong downward flow behind the terminator and the measured end of ordered flow at about 140 ~ SZA (Knudsen et al, 1980a).…”

“…Ionospheric changes due to solar wind variations must certainly have a much shorter response time (i.e., Wolff et al, 1982). Statistical studies, however, have shown the median ionosphere to be well-behaved, and to vary smoothly with solar zenith angle (Knudsen et al, 1980a, 1981, 1982a, Miller et al, 1980, 1984Theis et al, 1984). The range of variability in the dayside ionosphere is presented in the statistical treatment of the ionospheric parameters by Miller et aI.…”

“…The change of variability from the dayside to the nightside of Venus is apparently caused by the differing ionization sources on the two hemispheres (Theis et al, 1984). The dayside ionospheric variability is produced primarily by changes in solar EUV radiation (Taylor etal., 1981a, b;Elphic etal., 1984a), while changes in the nightside ionosphere are apparently the result of changes in the ion flux across the terminator (Knudsen et al, 1980a;Cravens et al, 1982).…”