Magnetic field strength in an intermediate-velocity ionized filament in the First Galactic Quadrant

TL;DR

This study measures the magnetic field in an intermediate-velocity ionized filament in the Galaxy, finding it dynamically significant and proposing a new method to estimate the Alfven speed from observational data.

Contribution

It introduces a novel approach to estimate a lower limit of the Alfven speed using rotation and emission measures, highlighting the magnetic field's importance in the interstellar medium.

Findings

Magnetic field strength is approximately 2.8 microgauss.

Lower limits to Alfven speed are comparable to or exceed the sound speed.

Magnetic fields are dynamically important in the studied filament.

Abstract

We investigate the magnetic field in an intermediate-velocity filament for which the H$\alpha$ intensity in the WHAM survey correlates with excess Faraday rotation of extragalactic radio sources over the length of the filament from b ~ 20 degr. to b ~ 55 degr. The density-weighted mean magnetic field is 2.8 +/- 0.8 microgauss, derived from rotation measures and an empirical relation between H-alpha emission measure and dispersion measure from Berkhuijsen et al. (2006). In view of the uncertainties in the derived magnetic field strength, we propose an alternative use of the available data, rotation measure and emission measure, to derive a lower limit to the Alfven speed, weighted by electron density n_e^1.5. We find lower limits to the Alfven speed that are comparable to, or larger than the sound speed in a 10^4 K plasma, and conclude that the magnetic field is dynamically important. We discuss the role of intermediate-velocity gas as a locus of Faraday rotation in the interstellar medium, and propose this lower limit to the Alfven speed may also be applicable to Faraday rotation by galaxy clusters.