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 weak asymmetries favoring the hemisphere in which E points towards the planet, while the magnetic-field strength has a weak asymmetry favoring the opposite hemisphere. The temperatures perpendicular and parallel to the background magnetic field as well as their ratio present no significant asymmetries. Deflection of the solar wind due to momentum exchange with planetary ions is revealed by (1) the O⁺ Larmor-radius trends of the asymmetries of the bulk-velocity components perpendicular to the upstream solar-wind flow and (2) the E×B_IMF -drift trends of the bulk-velocity component along the cross-flow component of the interplanetary magnetic field (B_IMF). These interpretations are enabled by comparisons to studies of solar-wind deflection at Mars and comet 67P/Churyumov-Gerasimenko, highlighting the benefits of comparative planetology studies.