Small-scale dynamo in solar surface simulations

Details Small-scale dynamo in solar surface simulations Small-scale dynamo in solar surface simulations

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmng51c..01g&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #NG51C-01

Astronomy and Astrophysics

Astrophysics

[3205] Mathematical Geophysics / Fourier Analysis, [4490] Nonlinear Geophysics / Turbulence, [7524] Solar Physics, Astrophysics, And Astronomy / Magnetic Fields, [7544] Solar Physics, Astrophysics, And Astronomy / Stellar Interiors And Dynamo Theory

Scientific paper

A magneto-convection simulation incorporating essential physical processes governing solar surface convection exhibits turbulent small-scale dynamo action. By presenting a derivation of the energy balance equation and transfer functions for compressible magnetohydrodynamics (MHD), we 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: 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. Comparisons are made between the details of the dynamo mechanism in compressible magneto-convection, Boussinesq convection, and randomly-forced incompressible turbulence. Net energy transfers (kinematic phase): work against magnetic tension (stretching) is 95% of magnetic energy generated; work against magnetic pressure (compression) is 5%. The latter is involved in the breaking down larger-scale field (25%) into smaller-scale field (30%) as part of the cascade. The dominant producer of magnetic energy is the stretching of magnetic field lines against the magnetic tension force (turbulent dynamo action).
Fluid motions at a scale of ~140km create magnetic energy predominately at a scale of ~65km. As the three wave-vectors must form a triad, the scale of the magnetic field being stretched must is 80+/-40km. All 3 scales are in the inertial range: this is turbulent small-scale dynamo.

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