Properties of Turbulence in the Very Local Interstellar Clouds

TL;DR

This study investigates turbulence properties in the Very Local Interstellar Clouds and finds no strong evidence for the anisotropic or ion-dependent temperature characteristics observed in solar corona and wind turbulence.

Contribution

It provides the first detailed analysis of turbulence diagnostics in the Local Clouds, comparing them with well-studied heliospheric turbulence properties and highlighting potential differences.

Findings

No compelling evidence for perpendicular velocity fluctuations.

No clear temperature anisotropy detected.

Ion temperature dependence on ion Larmor radius not observed.

Abstract

We have investigated the degree to which turbulence in the Very Local Interstellar Clouds resembles the highly-studied turbulence in the solar corona and the solar wind. The turbulence diagnostics for the Local Clouds are the absorption line widths measured along 32 lines of sight to nearby stars, yielding measurements for 53 absorption components (Redfield and Linsky 2004). We have tested whether the Local Cloud turbulence has the following properties of turbulence in the solar corona or the solar wind: (a) velocity fluctuations mainly perpendicular to the average magnetic field, (b) a temperature anisotropy in the sense that the perpendicular temperature is larger than the parallel temperature (or at least enhanced relative to expectation), and (c) an ion temperature which is dependent on the ion Larmor radius, in the sense that more massive ions have higher temperatures. Our analysis of the data does not show compelling evidence for any of these properties in Local Cloud turbulence, indicating possible differences with heliospheric plasmas. In the case of anisotropy of velocity fluctuations, although the expected observational signature is not seen, we cannot exclude the possibility of relatively high degrees of anisotropy (anisotropy parameter $\epsilon \sim 0.50 - 0.70$), if some other process in the the Local Clouds is causing variations in the turbulent line width from one line of sight to another. We briefly consider possible reasons for differences between coronal and solar wind turbulence and that in the Local Clouds.