The outer scale of turbulence in the magneto-ionized Galactic interstellar medium

TL;DR

This study determines that the outer scale of turbulence in the Galactic interstellar medium is about a parsec, much smaller than previously assumed, with different energy sources dominating in spiral arms and interarm regions.

Contribution

It provides new measurements of the turbulence outer scale in the Galactic interstellar medium using Faraday rotation and depolarization data, revealing smaller scales than prior models.

Findings

Outer scale of turbulence is about a parsec.

Different energy injection sources dominate in spiral arms and interarm regions.

Turbulence spectra vary between spiral arms and interarm regions.

Abstract

We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's spiral arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in spiral arms are present, and show power law behavior with a shallow spectrum. These results suggest that in the spiral arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and super bubble explosions are the main sources of energy on scales on the order of 100 parsecs.