Magnetic Helicity and Large Scale Magnetic Fields: A Primer
  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.
