Decay of a simulated mixed-polarity magnetic field in the solar surface layers

TL;DR

This study uses 3D radiative MHD simulations to analyze how mixed-polarity magnetic fields on the solar surface are removed, revealing an effective turbulent diffusivity and detailed reconnection dynamics.

Contribution

It provides the first detailed simulation-based quantification of magnetic flux removal and reconnection processes in the solar photosphere with implications for magnetic field evolution.

Findings

Flux removal rate corresponds to turbulent diffusivity of 100--340 km^2/s.

Magnetic energy distribution remains invariant above 70 Gauss.

Reconnection and loop escape occur on different timescales depending on loop orientation.

Abstract

Magnetic flux is continuously being removed and replenished on the solar surface. To understand the removal process we carried out 3D radiative MHD simulations of the evolution of patches of photospheric magnetic field with equal amounts of positive and negative flux. We find that the flux is removed at a rate corresponding to an effective turbulent diffusivity, of 100--340 km^2/s, depending on the boundary conditions. For average unsigned flux densities above about 70 Gauss, the percentage of surface magnetic energy coming from different field strengths is almost invariant. The overall process is then one where magnetic elements are advected by the horizontal granular motions and occasionally come into contact with opposite-polarity elements. These reconnect above the photosphere on a comparatively short time scale after which the U loops produced rapidly escape through the upper surface while the downward retraction of inverse-U loops is significantly slower, because of the higher inertia and lower plasma beta in the deeper layers.