Magnetic Helicity and Large Scale Magnetic Fields: A Primer
arXiv:1402.0933 · doi:10.1007/s11214-014-0038-6
Abstract
Magnetic fields of laboratory, planetary, stellar, and galactic plasmas commonly exhibit significant order on large temporal or spatial scales compared to the otherwise random motions within the hosting system. Such ordered fields can be measured in the case of planets, stars, and galaxies, or inferred indirectly by the action of their dynamical influence, such as jets. Whether large scale fields are amplified in situ or a remnant from previous stages of an object's history is often debated for objects without a definitive magnetic activity cycle. Magnetic helicity, a measure of twist and linkage of magnetic field lines, is a unifying tool for understanding large scale field evolution for both mechanisms of origin. Its importance stems from its two basic properties: (1) magnetic helicity is typically better conserved than magnetic energy; and (2) the magnetic energy associated with a fixed amount of magnetic helicity is minimized when the system relaxes this helical structure to the largest scale available. Here I discuss how magnetic helicity has come to help us understand the saturation of and sustenance of large scale dynamos, the need for either local or global helicity fluxes to avoid dynamo quenching, and the associated observational consequences. I also discuss how magnetic helicity acts as a hindrance to turbulent diffusion of large scale fields, and thus a helper for fossil remnant large scale field origin models in some contexts. I briefly discuss the connection between large scale fields and accretion disk theory as well. The goal here is to provide a conceptual primer to help the reader efficiently penetrate the literature.
32 pages, 18 figures (many with multiple panels); this version updated with a number of small corrections made in the proofs preparation stage; in press by Space Science Reviews and to eventually appear in associated ISSI book volume on "Multiscale Structure Formation and Dynamics in Cosmic Plasmas"; A. Balogh, Editor
References in corpus (21)
- The case for a distributed solar dynamo shaped by near-surface shear
- On the Origin of Cosmic Magnetic Fields
- Current status of turbulent dynamo theory: From large-scale to small-scale dynamos
- Galactic dynamo and helicity losses through fountain flow
- Evolution of Primordial Magnetic Fields from Phase Transitions
- Large-scale dynamos in turbulent convection with shear
- Magnetic field production during preheating at the electroweak scale
- Turbulent magnetic Prandtl number and magnetic diffusivity quenching from simulations
- Galactic dynamos supported by magnetic helicity fluxes
- Advection/Diffusion of Large-Scale B-Field in Accretion Disks
- On Self-Sustained Dynamo Cycles in Accretion Discs
- Magnetic Fields from QCD Phase Transitions
- Fluctuation dynamos and their Faraday rotation signatures
- Radio Circular Polarization Produced in Helical Magnetic Fields in Eight Active Galactic Nuclei
- A Local Model for Angular Momentum Transport in Accretion Disks Driven by the Magnetorotational Instability
- Magnetic Jets from Swirling Disks
- Magnetorotational instability driven dynamos at low magnetic Prandtl numbers
- Turbulent dynamo with advective magnetic helicity flux
- Multifrequency Polarimetry of the Nrao 140 Jet: Possible Detection of a Helical Magnetic Field and Constraints on its Pitch Angle
- Magnetic helicity fluxes in an alpha-squared dynamo embedded in a halo
- Magnetic helicity flux in the presence of shear