The Structure and Kinetic Ion Behavior of Low Mach Number Shocks

Low Mach number collisionless shocks are routinely observed in the solar
wind and upstream of planetary bodies. However, most in situ
observations have lacked the necessary temporal resolution to directly
study the kinetic behavior of ions across these shocks. We investigate a
series of five low Mach number bow shock crossings observed by the
Magnetospheric Multiscale (MMS) mission. The five shocks had comparable
Mach numbers, but varying shock-normal angles
($66^{\circ} \lesssim
\theta_{Bn} \lesssim
89^{\circ}$) and ramp widths ($5
\mathrm{km} \lesssim l
\lesssim 100 \mathrm{km}$). The shock
width is shown to be crucial in determining the fraction of protons
reflected and energized by the shock, with proton reflection increasing
with decreasing shock width. As the shock width increases proton
reflection is arrested entirely. For nearly perpendicular shocks,
reflected protons exhibit quasi-periodic structures, which persist far
downstream of the shock. As the shock-normal angle becomes more oblique
these periodic proton structures broaden to form an energetic halo
population. Periodic fluctuations in the magnetic field downstream of
the shocks are generated by fluctuations in dynamic pressure of alpha
particles, which are decelerated by the cross-shock potential and
subsequently undergo gyrophase bunching. These results demonstrate that
complex kinetic-scale ion dynamics occur in low Mach number shocks,
which depend significantly on the shock profile.

11 Sep 2024

17 Sep 2024