Observations of the chromospheric Evershed flow of sunspot penumbra

TL;DR

This study uses spectral imaging and machine learning to analyze the chromospheric Evershed flow in sunspot penumbra, revealing distinct flow regimes and supporting the uncombed model of sunspot magnetic structure.

Contribution

It introduces a novel application of Self Organizing Map technique to segment and measure plasma velocities in the chromosphere of sunspots, providing new insights into flow dynamics.

Findings

Vertical flows consistent with Evershed flow observed in penumbral filaments.

Inverse Evershed flow detected only in the outer super-penumbra.

Two opposite Evershed regimes coexist without overlap, influencing downflows.

Abstract

The sunspot penumbra is usually observed in the photosphere and it is of particular interest for its magneto-convection which seems to transport the heat from the top of the convection zone into the solar atmosphere. It is well known that the penumbra magnetic field extends in the upper layers of the solar atmosphere forming the so called super-penumbra. Thanks to the application of the Self Organizing Map technique to a spectral dataset containing monochromatic images acquired along the Ca II 854.2 nm and H$\alpha$ 656.28 nm lines, we were able to segment the penumbra and to measure the plasma velocity along the chromospheric portions of penumbral filaments. We found that the head, body and tail of penumbral filaments show vertical flows compatible with the persistence of the Evershed flow. Instead, the inverse Evershed flow has been observed only in the outer portion of the super-penumbra. We found that two opposite Evershed regimes work next to each other, without overlapping and both contribute to the downflow around sunspots. These results confirm the uncombed model of the sunspot penumbra and provide some hints that the downflow around sunspots may be ascribed to the magnetic field dragging the plasma down.