Solar wind‐driven plasma fluxes from the Venus ionosphere

Abstract

[1] Measurements conducted with the ASPERA-4 instrument and the magnetometer of the Venus Express spacecraft show that the kinetic pressure of planetary O + ion fluxes measured in the Venus wake can be significantly larger than the local magnetic pressure, and as a result, those ions are not solely being driven by magnetic forces but also by the kinetic energy of the solar wind. Beams of planetary O + ions with those properties have been detected in several orbits of the Venus Express through the Venus wake as the spacecraft traverses by the noon-midnight plane along its near-polar trajectory. The momentum flux of the O + ions leads to superalfvenic flow conditions. It is suggested that such O + ion beams are produced in the vicinity of the magnetic polar regions of the Venus ionosphere where the solar wind erodes the local plasma leading to plasma channels that extend downstream from those regions. The distribution of the number of cases where superalfvenic and subalfvenic conditions are measured along the Venus Express trajectory leads to dominant values when the total kinetic plasma pressure (including that of the solar wind protons) and the magnetic pressure are comparable, thus suggesting a possible equipartition of energy between the plasma and the magnetic field.

Day‐to‐night transport in the Martian ionosphere: Implications from total electron content measurements

“…Specifically, Chaufray et al [] considered an ionospheric plasma flow driven by collisional coupling to the neutral winds and diurnal pressure gradient. Such a scheme is not able to reproduce the observed depletion of nightside TEC on Mars, indicating that an important mechanism is missing in their model, presumably momentum transfer from the SW [e.g., Pérez‐de‐Tejada , ; Pérez‐de‐Tejada et al , ]. However, the MHD calculations of Ma et al [], which did take into account both pressure gradient and momentum transfer via the SW convective electric fields, predicted a day‐to‐night transport rate of only several 10 24 s −1 , about an order of magnitude lower than our result.…”

“…Specifically, Chaufray et al [2014] considered an ionospheric plasma flow driven by collisional coupling to the neutral winds and diurnal pressure gradient. Such a scheme is not able to reproduce the observed depletion of nightside TEC on Mars, indicating that an important mechanism is missing in their model, presumably momentum transfer from the SW [e.g., Pérez-de-Tejada, 1998;Pérez-de-Tejada et al, 2013]. However, the MHD calculations of Ma et al [2004], which did take into CUI ET AL.…”

“…Instead, it is generally thought that momentum transfer from the SW plays an important role in supporting the supersonic ionospheric flow on Venus. [e.g., Pérez‐de‐Tejada , ; Pérez‐de‐Tejada et al , ].…”

Vortex Structure in the Plasma Flow Channels of the Venus Wake

“…It is currently accepted that magnetic forces are sufficient to drive planetary ions away from the Venus wake. From measurements conducted with the Venus Express spacecraft it has been found, however, that conditions in the wake show the opposite since the motion of the plasma particles is super-Alfvenic [6]; namely, their kinetic energy density is larger than the local magnetic energy density. An example of this behavior is shown in Figure 3 where the energy spectra of the solar wind and those of the O+ planetary ions measured in orbit 123 of the Venus Express are shown in the top panels.…”

“…This constraint provides an explanation for the absence Figure 4. (Lower panel) Kinetic pressure of the O+ ions (profiles marked in red) and the H+ ions, together with the profile of the magnetic field pressure measured through the Venus wake during orbit 123 of the Venus Express spacecraft [6]. (Upper panel) Ratio values of the total kinetic pressure of the plasma to the magnetic field pressure derived from the profiles shown in the lower panel.…”