Proton Plasma Asymmetries Between Venus' Quasi-Perpendicular and Quasi-Parallel Magnetosheaths

Proton plasma asymmetries between the hemispheres of Venus' dayside magnetosheath lying downstream of the quasi-perpendicular (q_⊥) and quasi-parallel (q_‖) sides of the bow shock are characterized using measurements taken by a mass-energy spectrometer. This characterization enables comparison to analogous Earth studies, thereby providing insight as to which plasma phenomena, such as turbulent particle heating, contribute in creating the observed plasma asymmetries in planetary magnetosheaths. A database of dayside bow-shock crossings along with magnetosheath proton densities, bulk speeds, temperatures, and magnetic-field strengths is manually constructed by selecting measurements taken during stable solar-wind conditions. Ratios of these magnetosheath proton parameters are calculated as functions of distance from the central meridian and the upstream Alfvén Mach number to quantify the q_⊥/‖ asymmetries. The density and bulk-speed exhibit q_‖-favored asymmetries, mirroring those observed at Earth, whereas the magnetic-field strength reveals no significant asymmetry despite expectations based on simulations. The temperatures perpendicular (T_⊥) and parallel (T_‖) to the background magnetic field have q_⊥-favored asymmetries while the temperature anisotropy T_⊥/T_‖ exhibits a q_‖-favored asymmetry. This trend is opposite to that seen at Earth, suggesting that the different spatial scales of the two planets' magnetosheaths may affect the impact of turbulent processes on global plasma properties.