On the fine structure of the sunspot penumbrae. II. The nature of the Evershed flow

TL;DR

This study models the fine structure of sunspot penumbrae, revealing hot, inclined flux tubes with increasing flow speeds outward, supporting the siphon flow as the driver of the Evershed flow through spectropolarimetric analysis.

Contribution

It introduces a flux tube model in pressure balance to analyze penumbral fine structure and provides observational evidence supporting the siphon flow mechanism for the Evershed flow.

Findings

Flux tubes are hotter and inclined in the inner penumbra.

Flow speed exceeds critical speed at larger radial distances.

Evidence of shock front formation supports siphon flow mechanism.

Abstract

We investigate the fine structure of the sunspot penumbra by means of a model that allows for a flux tube in horizontal pressure balance with the magnetic background atmosphere in which it is embedded. We apply this model to spectropolarimetric observations of two neutral iron lines at 1.56 $\mu$m and invert several radial cuts in the penumbra of the same sunspot at two different heliocentric angles. In the inner part of the penumbra we find hot flux tubes that are somewhat inclined to the horizontal. They become gradually more horizontal and cooler with increasing radial distance. This is accompanied by an increase in the velocity of the plasma and a decrease of the gas pressure difference between flux tube and the background component. At large radial distances the flow speed exceeds the critical speed and evidence is found for the formation of a shock front. These results are in good agreement with simulations of the penumbral fine structure and provide strong support for the siphon flow as the physical mechanism driving the Evershed flow.