The Evershed Flow and the Brightness of the Penumbra

TL;DR

This paper reviews recent high-resolution observations and simulations that suggest the Evershed flow, a magnetized gas motion in sunspot penumbrae, plays a key role in penumbral brightness and filament structure.

Contribution

It synthesizes observational data and heat transfer simulations to propose that hot Evershed flows explain penumbral brightness and filament dark cores, advancing understanding of sunspot dynamics.

Findings

Evershed flow is magnetized and often supersonic.

Simulations show hot Evershed upflows explain penumbral brightness.

Evershed flow is crucial for energy transport in the penumbra.

Abstract

The Evershed flow is a systematic motion of gas that occurs in the penumbra of all sunspots. Discovered in 1909, it still lacks a satisfactory explanation. We know that the flow is magnetized, often supersonic, and that it shows conspicuous fine structure on spatial scales of 0.2"-0.3", but its origin remains unclear. The hope is that a good observational understanding of the relation between the flow and the penumbral magnetic field will help us determine its nature. Here I review advances in the characterization of the Evershed flow and sunspot magnetic fields from high-resolution spectroscopic and spectropolarimetric measurements. Using this information as input for 2D heat transfer simulations, it has been demonstrated that hot Evershed upflows along nearly horizontal field lines are capable of explaining one of the most intriguing aspects of sunspots: the surplus brightness of the penumbra relative to the umbra. They also explain the existence of penumbral filaments with dark cores. These results support the idea that the Evershed flow is largely responsible for the transport of energy in the penumbra.