Proton plasma asymmetries between the convective-electric-field hemispheres of Venus' dayside magnetosheath

Abstract

Proton plasma asymmetries with respect to the convective electric field (E) are characterized in Venus' dayside magnetosheath using measurements taken by an ion mass-energy spectrometer and a magnetometer. Investigating the spatial structure of the magnetosheath plasma in this manner provides insight into the coupling between solar-wind protons and planetary ions. A previously developed methodology for statistically quantifying asymmetries is further developed and applied to an existing database of proton bulk-parameter measurements in the dayside magnetosheath. The density and speed exhibit mild asymmetries favoring the hemisphere in which E points towards the planet, while the magnetic-field-strength asymmetry favors the opposite hemisphere. The temperature perpendicular to the background magnetic field has a mild asymmetry favoring the hemisphere in which E points away from the planet; the temperature parallel to the background magnetic field and the temperature anisotropy present no significant asymmetries. Deflection of the solar wind due to momentum exchange with planetary ions is revealed by the O+ Larmor-radius trends of the asymmetries of the bulk-velocity components perpendicular to the upstream solar-wind flow. This interpretation is enabled by comparisons to experimental and numerical studies of solar-wind deflection at Mars, highlighting the benefits of comparative planetology studies.