Opposite Polarity Magnetic Fields and Convective Downflows in a Simulated Sunspot Penumbra

Tuesday, March 31, 2020

Recent numerical simulations and observations of sunspots show a significant amount of opposite polarity magnetic fields within the sunspot penumbra. Most of the opposite polarity fields are associated with convective downflows.

Graphic image of opposite polarity magnetic fields
Left panels: normalized continuum intensity at 630 nm. Middle panels: Stokes V far wing magnetogram. Right panels: bisector velocity at 80% for Fe i 6301.5 Å. First row: at simulation resolution. Second, third, and fourth rows: at Hinode (0.5 m), 1 m, and 1.5 m resolutions.

We present an analysis of 3D MHD simulations through forward modeling of synthetic Stokes profiles of the Fe I 6301.5 Å and Fe I 6302.5 Å lines. The synthetic Stokes profiles are spatially and spectrally degraded considering typical instrument properties. Line bisector shifts of the Fe I 6301.5 Å line are used to determine line-of-sight velocities. Far wing magnetograms are constructed from the Stokes V profiles of the Fe I 6302.5 Å line. While we find an overall good agreement between observations and simulations, the fraction of opposite polarity magnetic fields, the downflow filling factor, and the opposite polarity-downflow association are strongly affected by spatial smearing and presence of strong gradients in the line-of-sight magnetic fields and velocity. A significant fraction of opposite polarity magnetic fields and downflows is hidden in the observations due to typical instrumental noise. Comparing simulations that differ by more than a factor of two in grid spacing, we find that these quantities are robust within the simulations.

Publication Name: ApJ
First HAO Author's Name: M. Rempel