A Survey of Venus Shock Crossings Dominated by Kinematic Relaxation

S A Pope

doi:10.1029/2020ja028256

Abstract

AbstractCollisionless shocks are one of the most effective particle accelerators in the known universe. Even low Mach number shocks could have a significant role in particle heating and acceleration. Theory suggests that kinematic collisionless relaxation, the process whereby a downstream nongyroptopic ion population becomes thermalized through collisionless gyrophase mixing, is the dominant energy redistribution mechanism in quasi‐perpendicular, low Mach number, and low β shocks. However, there have only been a limited number of observations of these shocks using in situ measurements at Venus, Earth and in interplanetary space. This paper presents the results of the first detailed study using in situ measurements, of the effect of fundamental parameters on the formation of these shocks. All low Mach number shocks occurring during the magnetic cloud phase of an interplanetary coronal mass ejection are identified in Venus Express magnetic field data over the duration of the mission. From the 92 shock crossings identified, 38 show clear evidence of kinematic relaxation. It is shown that kinematic relaxation is dominant at Venus when the angle between the local shock normal and upstream magnetic field is greater 50° and the Alfvén Mach number is less than 1.4. These shocks are also observed across a range of solar‐zenith‐angles indicating that it is likely that any location on the Venus bow shock could form such a structure. Venus Express plasma measurements are used to verify the parameters estimated from the magnetic field and indicate the importance of heavy ions, including potential pickup O+.

Highlights

Understanding collisionless shocks is important for many astrophysical processes
Theory suggests that kinematic collisionless relaxation, the process whereby a downstream nongyroptopic ion population becomes thermalized through collisionless gyrophase mixing, is the dominant energy redistribution mechanism in quasi-perpendicular, low Mach number, and low β shocks
All low Mach number shocks occurring during the magnetic cloud phase of an interplanetary coronal mass ejection are identified in Venus Express magnetic field data over the duration of the mission

Summary

Introduction

Understanding collisionless shocks is important for many astrophysical processes. They are key providers of particle acceleration, both within the heliosphere and further afield. This study confirmed kinematic relaxation as the dominant process for energy redistribution in these quasi-perpendicular low mach number shocks It showed the role of the different ion species (proton and α particles in this case) in forming the downstream distribution. Due to the reliance on single spacecraft measurements with low sample rate and noncontinuous plasma measurements, the shock normal nfor each of the shocks studied is determined from the magnetic field data using both minimum variance analysis (MVA) and the coplanarity theorem (CP) Both of these methods can be subject to errors due to factors such as a small number of data samples across the shock ramp and the presence of non-shock-related structures in upstream and downstream regions. These are compared to model shock altitudes Am at solar minimum for the observed SZA (Zhang et al, 2008), that is, when the bow shock is on average at its most compressed

Observations

19 November 2011

Findings

Conclusion