Evolution of anisotropic distributions of weakly charged heavy ions downstream collisionless quasiperpendicular shocks

Abstract

Hybrid simulations of quasiperpendicular collisionless shocks (QRCS) in multicomponent plasmas have revealed that weakly charged ions can keep anisotropic nonequilibrium distributions at relatively long distances downstream the shock front. However, analytic considerations suggest that such distributions lead to growth of the Alfven ion cyclotron (AIC) instability, which in turn governs isotropisation. Hence, we have conducted targeted hybrid simulations of QRCS with increased accuracy, which appeared to agree with the analytical predictions and suggest that the nonequilibrium distributions eventually relax due to the AIC instability. On another hand, downstream electromagnetic instabilities generated by anisotropic particle distributions may affect the energy balance, therefore influencing macroscopic shock parameters, such as the compression ratio. Simulations of QRCS with parameters close to those of the heliospheric termination shock measured by the Voyager mission, showed that an admixture of just 0.7% (by mass) of oxygen decreases the compression ratio by 3-4%. Thus, one might conclude that along with the effect of the pickup ions, the multi-species composition of the interplanetary plasma can add to the observed decrease of the shock compression.