# Magnetic Spiral Arms in Galaxy Halos

## TL;DR

This paper investigates the conditions for steady galactic dynamos, analyzing spiral magnetic field modes and predicting observable magnetic sign changes in galaxy halos, supported by recent survey data.

## Contribution

It introduces a scale-invariant analysis of galactic dynamo equations with spiral modes and predicts magnetic sign changes consistent with observations.

## Key findings

- Multiple magnetic sign reversals predicted in galaxy halos.
- Analysis of spiral dynamo modes using scale invariance.
- Predictions align with recent CHANG-ES survey observations.

## Abstract

We seek the conditions for a {\it steady} mean field galactic dynamo. The parameter set is reduced to those appearing in the $\alpha^2$ and $\alpha/\omega$ dynamo, namely velocity amplitudes, and the ratio of sub-scale helicity to diffusivity. The parameters can be allowed to vary on conical spirals. We analyze the mean field dynamo equations in terms of scale invariant logarithmic spiral modes and special exact solutions. Compatible scale invariant gravitational spiral arms are introduced and illustrated in an appendix, but the detailed dynamical interaction with the magnetic field is left for another work. As a result of planar magnetic spirals `lifting' into the halo, multiple sign changes in average rotation measures forming a regular pattern on each side of the galactic minor axis, are predicted. Such changes have recently been detected in the CHANG-ES survey.