MAGNETOSHEATH DYNAMICS DOWNSTREAM OF LOW MACH NUMBER SHOCKS

The dissipation of the how kinetic energy of solar wind ions at the qu asi-perpendicular bow shock results in the formation of ion distributi ons that have large perpendicular temperature anisotropies. These anis otropies provide free energy for the growth of Alfven ion cyclotron (A /IC) and mirror waves. The waves then make the ion distributions gyrot ropic and substantially reduce their anisotropy. Although differences exist, many of the mechanisms governing wave generation and particle i sotropization operate at both high and low Mach number shocks. These m echanisms are easier to study at low Mach number shocks because they p roceed relatively slowly and the turbulence level is lower. Also, in g eneral, the plasma beta is lower at the low Mach number bow shock, all owing for situations in which minority ion species like He++ can suppr ess proton cyclotron waves and thus play a larger role in sheath dynam ics than they do at high Mach number shocks. For these reasons, we use two-dimensional hybrid simulations to model the heating of H+ and He+ ions at the low Mach number bow shock and examine wave excitation an d ion isotropization in the magnetosheath downstream. In agreement wit h observations and theory and in contrast to high Mach numbers, we fin d that the magnetosheath turbulence mostly consists of A/IC waves. Wit hout helium ions, the A/IC wave activity is dominated by parallel prop agating proton cyclotron waves. When helium ions are included at a low density, they tend to absorb these waves, leaving obliquely-propagati ng A/IC waves dominant, though at a lower intensity level. Proton heat ing at the shock is dominated by bulk perpendicular heating of the cor e, while the helium ions are heated only slightly. Instead, initially they gyrate around field lines downstream as a coherent, nongyrotropic bunch. Downstream, the protons are slowly isotropized through pitch-a ngle scattering by A/IC waves. The helium ions undergo perpendicular h eating through absorption of proton waves and become more gyrotropic. The perpendicular heating drives the growth of helium cyclotron waves, which in turn reduce the anisotropy of the helium ions. Far downstrea m of the shock, obliquely-propagating helium cyclotron waves dominate the sheath turbulence.