# A Hot Downflowing Model Atmosphere For Umbral Flashes And The Physical Properties Of Their Dark Fibrils

## TL;DR

This study uses NLTE inversions of high-resolution Ca II 8542 A observations to analyze the physical properties of dark fibrils in umbral flashes, revealing distinct atmospheric dynamics and magnetic field variations.

## Contribution

It introduces a detailed modeling of dark fibrils within umbral flashes using NLTE inversions, highlighting their unique velocities, temperature structures, and magnetic field changes.

## Key findings

- Dark fibrils have profiles similar to flashed regions with reduced amplitudes.
- Two solution sets: upflowing atmospheres with deeper transition regions and downflowing, hotter atmospheres.
- Magnetic field reduction in flashed areas suggests shock-driven field line movement.

## Abstract

We perform NLTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca II 8542 A observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier "sunspot plumes" which were invariably found to display strong downflows in sunspots. Finally we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers.