Direct Imaging of Fine Structure in the Chromosphere of a Sunspot Umbra

TL;DR

This study uses high-resolution Hinode CaII H line images to reveal the complex, dynamic fine structures and oscillation patterns in the chromosphere of a sunspot umbra, including newly observed filamentary features.

Contribution

It reports the first detection of filamentary structures within a sunspot umbra and analyzes their dynamic behavior and oscillation characteristics.

Findings

Umbral flashes are highly structured and dynamic.

Filamentary structures within the umbra are observed for the first time.

Different umbral cores exhibit distinct oscillation frequencies.

Abstract

High-resolution imaging observations from the Hinode spacecraft in the CaII H line are employed to study the dynamics of the chromosphere above a sunspot. We find that umbral flashes and other brightenings produced by the oscillation are extremely rich in fine structure, even beyond the resolving limit of our observations (0.22"). The umbra is tremendously dynamic, to the point that our time cadence of 20 s does not suffice to resolve the fast lateral (probably apparent) motion of the emission source. Some bright elements in our dataset move with horizontal propagation speeds of 30 km/s. We have detected filamentary structures inside the umbra (some of which have a horizontal extension of ~1500 km) which, to our best knowledge, had not been reported before. The power spectra of the intensity fluctuations reveals a few distinct areas with different properties within the umbra that seem to correspond with the umbral cores that form it. Inside each one of these areas the dominant frequencies of the oscillation are coherent, but they vary considerably from one core to another.