Solar and Galactic Magnetic Halo Structure: Force-Free Dynamos?

Abstract

Magnetic fields may relax dissipatively to the minimum energy force-free condition whenever they are not constantly created or distorted. We review the axially symmetric solutions for force-free magnetic fields, especially for the non-linear field. A new formulation for the scale invariant state is given. Illustrative examples are shown. Applications to both stellar coronas and galactic halos are possible. Subsequently we study whether such force-free fields may be sustained by classical magnetic dynamo action. Although the answer is `not indefinitely’, there may be an evolutionary cycle wherein the magnetic field repeatedly relaxes to the minimum energy condition after a period of substantial growth and distortion. Different force-free dynamos may coexist at different locations. Helicity transfer between scales is studied briefly. A dynamo solution is given for the temporal evolution away from an initial linear force-free magnetic field due to both α 2 and ω terms. This can be used at the sub scale level to create a `delayed’ α effect.

The Filamentary Radio Lobes of the Seyfert–Starburst Composite Galaxy NGC 3079

“…A helical magnetic field morphology is often invoked to explain the RM inversion seen in VLBI scale jets (Kharb et al 2009;Clausen-Brown et al 2011;Pudritz et al 2012). Henriksen (2019) suggested that galaxies possess force-free magnetic fields that are generated by the α 2 dynamo action; cosmic rays are transported in a direction parallel to the magnetic field lines under force-free conditions. Dome-like structures, as seen in NGC 3079, and the active regions in the Sun, can be reproduced by some of the solutions.…”

A Radio Polarimetric Study to Disentangle AGN Activity and Star-Formation in Seyfert Galaxies

“…A simple hydrodynamical model would favor a frustrated jet model, according to which the intrinsically low-power jets gets disrupted by the dense ISM percolating these late-type galaxies, and expands into the ambient medium along the minor axis, which offers the least resistance (Gallimore et al 2006). However, such a model is rendered insufficient to explain the closed dome-like morphology, for which the large-scale magnetic fields that thread the galaxy was invoked (Sebastian et al 2019b;Henriksen 2019), because some of the solutions of a force-free magnetic field of the galaxy resemble the morphology of NGC 3079.…”

Electromagnetic Conversion into Kinetic and Thermal Energies

“…Force-free magnetic fields have and can, therefore, not be sustained against dissipation, but they can be long lived if the current density is small enough; see Ref. [ 21 ] for examples.…”