Supersonic Evershed flow outside Sunspots

TL;DR

This paper reports the discovery of supersonic, horizontally directed magnetic flows outside sunspots, extending the known Evershed flow into the moat region with distinctive spectral signatures.

Contribution

It provides the first clear spectropolarimetric evidence of supersonic Evershed flows outside sunspots, identifying specific spectral signatures for these horizontal field channels.

Findings

Discovery of supersonic flows outside sunspots

Identification of spectral signatures with satellite lobes in Stokes V

Detection of five horizontal field channels in the studied sunspot

Abstract

We report on the discovery of mostly horizontal field channels just outside sunspot penumbrae (in the so-called `moat' region) that are seen to sustain supersonic flows (line-of-sight component of 6 km s{-1}). The spectral signature of these supersonic flows corresponds to circular polarization profiles with an additional, satellite, third lobe of the same sign as the parent sunspot' Stokes V blue lobe, for both downflows and upflows. This is consistent with an outward directed flow that we interpret as the continuation of the magnetized Evershed flow outside sunspots at supersonic speeds. In Stokes Q and U, a clear signature of a transverse field connecting the two flow streams is observed. Such an easily detectable spectral signature should allow for a clear identification of these horizontal field channels in other spectropolarimetric sunspot data. For the spot analyzed in this paper, a total of 5 channels with this spectral signature have been unambiguously found.