# Nonlinear galactic dynamos and the magnetic pitch angle

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## Date

2015

## Authors

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## Publisher

American Astronomical Society

## Abstract

Pitch angles p of the large-scale magnetic fields B of spiral galaxies have previously been inferred from
observations to be systematically larger in magnitude than predicted by standard mean-field dynamo theory. This
discrepancy is more pronounced if dynamo growth has saturated, which is reasonable to assume given that such
fields are generally inferred to be close to energy equipartition with the interstellar turbulence. This “pitch angle
problem” is explored using local numerical mean-field dynamo solutions as well as asymptotic analytical solutions.
It is first shown that solutions in the saturated or kinematic regimes depend on only five dynamo parameters, two of
which are tightly constrained by observations of galaxy rotation curves. The remaining three-dimensional
(dimensionless) parameter space can be constrained to some extent using theoretical arguments. Predicted values
of∣p∣ can be as large as ~40, which is similar to the largest values inferred from observations, but only for a small
and non-standard region of parameter space. We argue, based on independent evidence, that such non-standard
parameter values are plausible. However, these values are located toward the boundary of the allowed parameter
space, suggesting that additional physical effects may need to be incorporated. We therefore suggest possible
directions for extending the basic model considered.

## Description

## Keywords

Astronomy, Physics, Cosmology, Galaxies

## Citation

Chamandy, L. et al. (2015). Nonlinear galactic dynamos and the magnetic pitch angle. Astrophysical Journal, 808 (1). 10.1088/0004-637X/808/1/28