On the injection scale of the turbulence in the partially ionized very local interstellar medium

TL;DR

This paper investigates the injection scale of turbulence in the very local interstellar medium, finding it to be around 194 au, and discusses how ion-neutral damping affects the turbulence cascade.

Contribution

It introduces a model for the turbulence injection scale in the partially ionized VLISM considering ion-neutral decoupling and damping effects.

Findings

Turbulence in VLISM is sub-Alfvénic.

Maximum injection scale is approximately 194 au.

Neutral viscosity damps turbulence at around 261 au.

Abstract

The cascade of magnetohydrodynamic (MHD) turbulence is subject to ion-neutral collisional damping and neutral viscous damping in the partially ionized interstellar medium. By examining the damping effects in the warm and partially ionized local interstellar medium, we find that the interstellar turbulence is damped by neutral viscosity at $\sim 261$ au and cannot account for the turbulent magnetic fluctuations detected by Voyager 1 and 2. The MHD turbulence measured by Voyager in the very local interstellar medium (VLISM) should be locally injected in the regime where ions are decoupled from neutrals for its cascade to survive the damping effects. With the imposed ion-neutral decoupling condition, and the strong turbulence condition for the observed Kolmogorov magnetic energy spectrum, we find that the turbulence in the VLISM is sub-Alfv\'{e}nic, and its largest possible injection scale is $\sim 194$ au.