Links between the local and global dynamos

Stenflo Jan, stenflo@astro.phys.ethz.ch, ETH, Zurich, Switzerland

Abstract
Dynamo theories explain how macroscopic cosmic magnetic fields can be built up from an arbitrary seed field, through the interaction between the magnetic field and the turbulent motions in an electrically conducting medium, in combination with a symmetry-breaking mechanism. In a rotating medium the Coriolis force breaks the symmetry between left and right, which makes the turbulence cyclonic and produces helicity. In the Sun these processes, in combination with differential rotation and meridional circulation, create the Sun's 11-yr activity cycle. Recently the term "global dynamo" has been used to refer to these global products of the solar dynamo. Due to the high electrical conductivity of the solar plasma, the magnetic field remains frozen in and can continue to be tangled up by the turbulent motions down to very small scales (below 100 m). It has been known for three decades (since 1982) from Hanle-effect observations that there is indeed a vast amount of "hidden" magnetic flux in the solar atmosphere, which has remained invisible in solar magnetograms because the opposite magnetic polarities cancel their Zeeman-effect contributions on scales much smaller than the telescope resolution. There is now observational evidence that this hidden, tangled magnetic flux is physically associated with the kG flux tubes, which represent the flux that has been produced by the global dynamo but has been subject to the convective collapse instability that makes this flux highly intermittent. Therefore the small-scale, tangled field is continually fed from the flux that is produced on larger scales by the global dynamo. As there is no evidence that the small-scale tangled field can be built up from an arbitrary seed field, but is instead dependent on flux injection at larger scales from the global dynamo, an independent local solar dynamo does not seem to exist. In this sense there is no physical basis for the distinction between a "global" and a "local" dynamo.