Effects of Forcing on Shocks and Energy Dissipation in Interstellar and Intracluster Turbulences

Abstract

Observations indicate that turbulence in the interstellar medium (ISM) is supersonic ($M_{\rm turb}\gg1$) and strongly magnetized ($\beta\sim0.01-1$), while in the intracluster medium (ICM) it is subsonic ($M_{\rm turb}\lesssim1$) and weakly magnetized ($\beta\sim100$). Here, $M_{\rm turb}$ is the turbulent Mach number and $\beta$ is the plasma beta. We study the properties of shocks induced in these disparate environments, including the distribution of the shock Mach number, $M_s$, and the dissipation of the turbulent energy at shocks, through numerical simulations using a high-order accurate code based on the WENO scheme. In particular, we investigate the effects of different modes of the forcing that drives turbulence: solenoidal, compressive, and a mixture of the two. In the ISM turbulence, while the density distribution looks different with different forcings, the velocity power spectrum, $P_v$, on small scales exhibits only weak dependence. Hence, the statistics of shocks depend weakly on forcing either. In the ISM models with $M_{\rm turb}\approx10$ and $\beta\sim0.1$, the fraction of the turbulent energy dissipated at shocks is estimated to be $\sim15~\%$, not sensitive to the forcing mode. In contrast, in the ICM turbulence, $P_v$ as well as the density distribution show strong dependence on forcing. The frequency and average Mach number of shocks are greater for compressive forcing than for solenoidal forcing, so is the energy dissipation. The fraction of ensuing shock dissipation is in the range of $\sim10-35~\%$ in the ICM models with $M_{\rm turb}\approx0.5$ and $\beta\sim10^6$. The rest of the turbulent energy should be dissipated through turbulent cascade.