# The Galactic Magneto-Ionic Medium Survey: Moments of the Faraday Spectra

## TL;DR

This study analyzes the Galactic magnetic field using Faraday rotation data from the GMIMS survey, revealing different magnetic structures and distances in the northern and southern skies through moment analysis of Faraday spectra.

## Contribution

It introduces a comprehensive moment analysis of Faraday spectra from GMIMS data, revealing distinct magnetic field structures and distances in different Galactic regions.

## Key findings

- Weak vertical magnetic field from Galactic North to South
- Different volume sampling between northern and southern surveys
- Faraday spectral widths vary with latitude, indicating different line-of-sight properties

## Abstract

Faraday rotation occurs along every line of sight in the Galaxy; Rotation Measure (RM) synthesis allows a three-dimensional representation of the interstellar magnetic field. This study uses data from the Global Magneto-Ionic Medium Survey, a combination of single-antenna spectro-polarimetric studies, including northern sky data from the DRAO 26-m Telescope (1270-1750 MHz) and southern sky data from the Parkes 64-m Telescope (300-480 MHz). From the synthesized Faraday spectral cubes we compute the zeroth, first, and second moments to find the total polarized emission, mean and RM-width of the polarized emission. From DRAO first moments we find a weak vertical field directed from Galactic North to South, but Parkes data reveal fields directed towards the Sun at high latitudes in both hemispheres: the two surveys clearly sample different volumes. DRAO second moments show feature widths in Faraday spectra increasing with decreasing positive latitudes, implying that longer lines of sight encounter more Faraday rotating medium, but this is not seen at negative latitudes. Parkes data show the opposite: at positive latitudes the second moment decreases with decreasing latitude, but not at negative latitudes. Comparing first moments with RMs of pulsars and extragalactic sources and a study of depolarization together confirm that the DRAO survey samples to larger distances than the Parkes data. Emission regions in the DRAO survey are typically 700 to 1000 pc away, slightly beyond the scale-height of the magneto-ionic medium; emission detected in the Parkes survey is entirely within the magneto-ionic disk, less than 500 pc away.