Convective motions and net circular polarization in sunspot penumbrae

TL;DR

This study models the net circular polarization in sunspot penumbrae considering convective motions and Evershed flow, finding that convection has minimal impact and magnetic field strength near 1000 G is essential for accurate reproduction.

Contribution

It introduces a penumbral model incorporating convection and Evershed flow to explain NCP variations, highlighting the dominant role of the Evershed flow and the necessity of strong magnetic fields.

Findings

Convective motions have a small effect on NCP compared to Evershed flow.

NCP generated by upflows cancels that from downflows.

Magnetic field strength inside filaments must be close to 1000 G.

Abstract

We have employed a penumbral model, that includes the Evershed flow and convective motions inside penumbral filaments, to reproduce the azimuthal variation of the net circular polarization (NCP) in sunspot penumbrae at different heliocentric angles for two different spectral lines. The theoretical net circular polarization fits the observations as satisfactorily as penumbral models based on flux-tubes. The reason for this is that the effect of convective motions on the NCP is very small compared to the effect of the Evershed flow. In addition, the NCP generated by convective upflows cancels out the NCP generated by the downflows. We have also found that, in order to fit the observed NCP, the strength of the magnetic field inside penumbral filaments must be very close to 1000 G. In particular, field-free or weak-field filaments fail to reproduce both the correct sign of the net circular polarization, as well as its dependence on the azimuthal and heliocentric angles.