Peripheral downflows in sunspot penumbrae

TL;DR

This study uses high-resolution spectropolarimetric data to analyze high-speed, supersonic downflows with strong magnetic fields in sunspot penumbrae, revealing their detailed structure and relation to magnetic field strength.

Contribution

It provides the first high-resolution, height-dependent maps of atmospheric parameters of downflows in sunspot penumbrae, revealing their small-scale structure and magnetic properties.

Findings

High-speed downflows reach over 20 km/s.

Magnetic fields in downflows exceed 7 kG.

Downflow velocity correlates with magnetic field strength.

Abstract

Sunspot penumbrae show high-velocity patches along the periphery. The high-velocity downflow patches are believed to be the return channels of the Evershed flow. We aim to investigate their structure in detail using Hinode SOT/SP observations. We employ Fourier interpolation in combination with spatially coupled height dependent LTE inversions of Stokes profiles to produce high-resolution, height-dependent maps of atmospheric parameters of these downflows and investigate their properties. High-speed downflows are observed over a wide range of viewing angles. They have supersonic line-of-sight velocities, some in excess of 20km/s, and very high magnetic field strengths, reaching values of over 7 kG. A relation between the downflow velocities and the magnetic field strength is found, in good agreement with MHD simulations. The coupled inversion at high resolution allows for the accurate determination of small-scale structures. The recovered atmospheric structure indicates that regions with very high downflow velocities contain some of the strongest magnetic fields that have ever been measured on the Sun.