Nonlinear Galactic Dynamos and the Magnetic Pitch Angle

Abstract

Pitch angles $p$ of the large-scale magnetic fields $\overline{B}$ 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 3-dimensional (dimensionless) parameter space can be constrained to some extent using theoretical arguments. Predicted values of $|p|$ can be as large as $\sim 40^{\circ }$, 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.

11 pages, 5 figures, 1 table, edited to match ApJ version

References in corpus (18)

• The Vertical Structure of Warm Ionised Gas in the Milky Way
• HI holes and high-velocity clouds in the spiral galaxy NGC 6946
• Galactic dynamo and helicity losses through fountain flow
• The Generation and Dissipation of Interstellar Turbulence - Results from Large Scale High Resolution Simulations
• The supernova-regulated ISM. I. The multi-phase structure
• Galactic dynamos supported by magnetic helicity fluxes
• The supernova-regulated ISM. II. The mean magnetic field
• High-resolution radio continuum survey of M33: III. Magnetic fields
• Small-scale magnetic helicity losses from a mean-field dynamo
• Viscous, Resistive Magnetorotational Modes
• Magnetic Fields in a Sample of Nearby Spiral Galaxies
• Non-linear galactic dynamos: A toolbox
• Probing interstellar turbulence in spiral galaxies using HI power spectrum analysis
• Galactic magnetic fields and hierarchical galaxy formation
• New scaling for the alpha effect in slowly rotating turbulence
• Helicity-Flux-Driven Alpha Effect in Laboratory and Astrophysical Plasmas
• The onset of large scale turbulence in the interstellar medium of spiral galaxies
• Power spectra of outflow-driven turbulence

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