Abstract
Collisionless 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 shocks.
However, there have only been a limited number of observations of these
shocks using in situ measurements at Venus, Earth and in inter-planetary
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
pickup O.