Evershed flow observed in neutral and singly ionized iron lines

TL;DR

This study compares the Evershed flow velocities derived from neutral and singly ionized iron lines in a sunspot, revealing consistent differences with neutral lines showing slightly larger flow amplitudes across various heights and locations.

Contribution

It provides the first detailed comparison of Evershed flow measurements from FeI and FeII lines, highlighting measurable differences in flow velocities.

Findings

FeI lines show larger flow amplitudes than FeII lines by a few hundred m/s.

The velocity difference persists across different heights and radial distances.

The observed differences may indicate physical effects related to ionization states in the penumbra.

Abstract

The amplitudes of the Evershed flow are measured using pairs of carefully selected FeI and FeII spectral lines located close in wavelength and registered simultaneously. A sunspot belonging to the NOAA 11582 group was scanned using the spectrograph of the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife). Velocities were extracted from intensity profiles using the lambda-meter technique. The formation heights of the observed spectral lines were calculated using semi-empirical models of a bright and dark penumbral filament taking into account the sunspot location at the limb. Our objective is to compare azimuthally averaged amplitudes of the Evershed flow extracted from neutral and ion lines. We find measurable differences in the radial component of the flow. All five pairs of lines show the same tendency, with a few hundred m/s larger amplitude of the flow measured from FeI lines compared to FeII lines. This tendency is preserved at all photospheric heights and radial distances in the penumbra. We discuss the possible origin of this effect.