Relationships between fluid vorticity, kinetic helicity and magnetic field at the small-scale (quiet-network) on the Sun
arXiv:1604.04439 · doi:10.3847/0004-637X/824/2/120
Abstract
We derive horizontal fluid motions on the solar surface over large areas covering the quiet-Sun magnetic network from local correlation tracking of convective granules imaged in continuum intensity and Doppler velocity by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). From these we calculate horizontal divergence, vertical component of vorticity, and kinetic helicity of fluid motions. We study the correlations between fluid divergence and vorticity, and that between vorticity (kinetic helicity) and magnetic field. We find that the vorticity (kinetic helicity) around small-scale fields exhibits a hemispherical pattern (in sign) similar to that followed by the magnetic helicity of large-scale active regions (containing sunspots). We identify this pattern to be a result of the Coriolis force acting on supergranular-scale flows (both the outflows and inflows), and is consistent with earlier studies using local helioseismology. Further, we show that the magnetic fields cause transfer of vorticity from supergranular inflow regions to outflow regions, and that they tend to suppress the vortical motions around them when magnetic flux densities exceed about 300 G (HMI). We also show that such action of magnetic fields leads to marked changes in the correlations between fluid divergence and vorticity. These results are speculated to be of importance to local dynamo action if present, and to the dynamical evolution of magnetic helicity at the small-scale.
arXiv comment12 pages (ApJ style), 9 figures, accepted for publication in The Astrophysical Journal
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