Convection and the Origin of Evershed Flows

TL;DR

Numerical simulations suggest that the Evershed flow and penumbral fine structures are caused by overturning convection confined to magnetic field gaps, with flow direction influenced by magnetic field inclination.

Contribution

This paper reviews how overturning convection within magnetic gaps explains the Evershed flow and penumbral filament structures, highlighting unresolved physical parameters affecting sunspot brightness and filament properties.

Findings

Evershed flow is the outward radial component of overturning convection.

Dark penumbral filaments are caused by magnetic field cusps above gaps.

Flow and filament properties depend on physical parameters like plasma beta and entropy.

Abstract

Numerical simulations have by now revealed that the fine scale structure of the penumbra in general and the Evershed effect in particular is due to overturning convection, mainly confined to gaps with strongly reduced magnetic field strength. The Evershed flow is the radial component of the overturning convective flow visible at the surface. It is directed outwards -- away from the umbra -- because of the broken symmetry due to the inclined magnetic field. The dark penumbral filament cores visible at high resolution are caused by the 'cusps' in the magnetic field that form above the gaps. Still remaining to be established are the details of what determines the average luminosity of penumbrae, the widths, lengths, and filling factors of penumbral filaments, and the amplitudes and filling factors of the Evershed flow. These are likely to depend at least partially also on numerical aspects such as limited resolution and model size, but mainly on physical properties that have not yet been adequately determined or calibrated, such as the plasma beta profile inside sunspots at depth and its horizontal profile, the entropy of ascending flows in the penumbra, etc.