Physical Properties of a Sunspot Chromosphere with Umbral Flashes

TL;DR

This study provides the most detailed spectro-polarimetric analysis of sunspot umbral flashes, revealing fine structures, temperature variations, and magnetic field stability, with new insights into wave power distribution across sunspot regions.

Contribution

It offers the first high-resolution, multi-inversion analysis of umbral flashes, detailing temperature, magnetic field, and wave power variations at sub-arcsecond scales.

Findings

Umbral flashes exhibit fine structures with hot and cool material intermixed.

Shock fronts are approximately 1000 K hotter than surrounding material.

Magnetic field strength in the umbra remains stable during flashes, while penumbral waves cause some fluctuations.

Abstract

We present new high-resolution spectro-polarimetric Ca II 8542 observations of umbral flashes in sunspots. At nearly 0.18", and spanning about one hour of continuous observation, this is the most detailed dataset published thus far. Our study involves both LTE and non-LTE inversions (but includes also a weak field analysis as a sanity check) to quantify temperatures, mass flows and the full magnetic field vector geometry. We confirm earlier reports that UFs have very fine structure with hot and cool material intermixed at sub-arcsecond scales. The shock front is roughly 1000 K hotter than the surrounding material. We do not observe significant fluctuations of the field in the umbra. In the penumbra, however, the passage of the running penumbral waves alter the magnetic field strength by some 200 G (peak-to-peak amplitude) but it does not change the field orientation (at least not significantly within our sensitivity of a few degrees). We find a trend of decreasing high-frequency modulation power for more inclined magnetic fields for the line-of-sight velocity and magnetic field strength. In the outer penumbra we find an absence of high frequency power while there is increasingly more power at high frequencies towards the umbra.