Magnetic field variations associated with umbral flashes and penumbral waves

TL;DR

This study investigates how umbral flashes and penumbral waves affect magnetic field measurements in sunspot chromospheres, revealing that observed magnetic variations are due to height differences rather than actual field changes.

Contribution

The paper demonstrates that apparent magnetic field variations during UF and RPW phases are caused by changes in line formation height, not real magnetic field fluctuations.

Findings

Magnetic field inferred from Ca II line is higher during UFs and RPWs.

Line formation height shifts to higher geometrical levels during UF and RPW phases.

Opacity changes do not account for the observed magnetic field variations.

Abstract

Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Ca II 854.2 nm line. Recent studies have focussed on also explaining the observed polarization profiles, that show even more dramatic variations during the passage of these shock fronts. While most of these variations can be explained with an almost constant magnetic field as a function of time, several studies have reported changes in the inferred magnetic field strength during UF phases. In this study we investigate the origin of these periodic variations of the magnetic field strength by analyzing a time-series of high temporal cadence observations acquired in the Ca II line with the CRISP instrument at the Swedish 1-m Solar Telescope. In particular, we analyze how the inferred geometrical height scale changes between quiescent and UF phases, and whether those changes are enough to explain the observed changes in $B$. We have performed non-LTE data inversions with the NICOLE code of a time-series of very high spatio-temporal resolution observations in the Ca II and Fe I 630.15\630.25 nm lines. Our results indicate that the Ca II line in sunspots is greatly sensitive to magnetic fields at $\log\tau_{500}=-5$ during UFs and quiescence. However, this optical depth value does not correspond to the same geometrical height during the two phases. Our results indicate that during UFs and RPWs the $\log\tau=-5$ is located at a higher geometrical height than during quiescence. Additionally, the inferred magnetic field values are higher in UFs (~270 G) and in RPWs (~100 G). Our results suggest that opacity changes caused by UFs and RPWs cannot explain the observed temporal variations in the magnetic field, as the line seems to form at higher geometrical heights where the field is expected to be lower.