Proton Plasma Asymmetries between Venus' Quasi-Perpendicular and
Quasi-Parallel Magnetosheaths
Abstract
Proton plasma asymmetries between the hemispheres of Venus’ dayside
magnetosheath lying downstream of the quasi-perpendicular
($q_\perp$) and quasi-parallel
($q_\parallel$) 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 $q_{\perp/\parallel}$
asymmetries. The density and bulk-speed exhibit
$q_\parallel$-favored asymmetries, mirroring those
observed at Earth, whereas the magnetic-field strength reveals no
significant asymmetry despite expectations based on simulations. The
temperatures perpendicular ($T_\perp$) and parallel
($T_\parallel$) to the background magnetic field have
$q_\perp$-favored asymmetries while the temperature
anisotropy $T_\perp / T_\parallel$
exhibits a $q_\parallel$-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.