# Tracing interstellar magnetic field using velocity gradient technique: Application to Atomic Hydrogen data

## TL;DR

This paper demonstrates that velocity centroid gradients derived from spectroscopic data can reliably trace magnetic fields in interstellar atomic hydrogen, aligning well with polarization-based measurements, and aiding in separating foreground signals in CMB observations.

## Contribution

The paper introduces and validates a new technique using velocity centroid gradients to trace magnetic fields, showing strong agreement with polarization data in atomic hydrogen.

## Key findings

- Velocity centroid gradients align with polarization-based magnetic field directions.
- The technique effectively traces magnetic field fluctuations in atomic hydrogen.
- Potential to improve foreground separation in CMB polarization studies.

## Abstract

The advancement of our understanding of MHD turbulence opens ways to develop new techniques to probe magnetic fields. In MHD turbulence, the velocity gradients are expected to be perpendicular to magnetic fields and this fact was used by Gonsalvez-Casanova & Lazarian to introduce a new technique to trace magnetic fields using velocity centroid gradients. The latter can be obtained from spectroscopic observations. We apply the technique to GALFA HI survey data and compare the directions of magnetic fields obtained with our technique with the direction of magnetic fields obtained using PLANCK polarization. We find excellent correspondence between the two ways of magnetic field tracing, which is obvious via visual comparison and through measuring of the statistics of magnetic field fluctuations obtained with the polarization data and our technique. This suggests that the velocity centroid gradients can provide a reliable way of measuring of the foreground magnetic field fluctuations and thus provide a new way of separating foreground and CMB polarization signals.