Jonathan Pietarila Graham
Max Planck Institute for Solar System Research
February 17, 2010
Foothills Laboratory 2, Room 1022
Turbulent Small-scale Dynamo Action in Solar Surface Simulations
The origin of small-scale quiet-Sun intra-network magnetic field is unclear.
One possible source is a turbulent small-scale dynamo driven by surface convection. We demonstrate that surface convection in realistic solar simulations exhibits such dynamo action. By presenting a derivation of the energy balance equation and transfer functions for compressible magnetohydrodynamics (MHD), we can quantify the source of magnetic energy on a scale-by-scale basis. We rule out the two alternative mechanisms for the generation of small-scale magnetic field in the simulations: the tangling of magnetic field lines associated with the turbulent cascade and Alfvenization of small-scale velocity fluctuations ("turbulent induction''). Instead, we find the dominant source of small-scale magnetic energy is stretching by inertial-range fluid motions of small-scale magnetic field lines against the magnetic tension force to produce (against Ohmic dissipation) more small-scale magnetic field. The scales involved become smaller with increasing Reynolds number, which identifies the dynamo as a small-scale turbulent dynamo.