Survival of 2D Turbulence in the Local Interstellar Medium

Abstract

The evolution of 2D turbulence in the partially ionized local interstellar medium is investigated using 2D MHD simulations coupled with a neutral hydrogen gas via charge exchange. While it has been discussed that Alfvén and compressible waves are damped in a partially ionized plasma due to plasma-neutral drag effects, little is known about how (quasi-)2D turbulence (or magnetic flux ropes) interacts with a neutral gas. We expect that quasi-2D turbulence is unaffected by such an interaction since there is no associated velocity fluctuation, and our nonlinear simulation reveals that magnetic fluctuations in a partially ionized plasma are undamped, whereas the magnetic, kinetic plasma, and neutral spectra are modified to a −7/3 spectrum. The modified spectra can be explained by a Kolmogorov-type scaling law in which the turbulence cascade occurs through magnetic reconnection, as observed in the simulation. Our result suggests that magnetic fluctuations perpendicular to the background magnetic field cascade down to smaller scales without damping, whereas Alfvénic turbulence is damped at the characteristic scale associated with the plasma-neutral drag, and LISM turbulence will be magnetically rather than kinetically dominated. We note that the possible dominance of quasi-2D turbulence in the local interstellar medium can be crucial to the diffusive transport of cosmic rays.