Fine structures in the atmosphere above a sunspot umbra

TL;DR

This paper reports the discovery of transient jet-like structures called umbral microjets in sunspot umbrae, observed with high-resolution imaging, revealing their properties, alignment with magnetic fields, and relation to umbral dots.

Contribution

It provides the first detailed observations and characterization of umbral microjets, highlighting their properties and possible magnetic field alignment.

Findings

Umbral microjets are elongated, jet-like features in sunspot umbrae.

They are typically less than 1 arcsecond long and last about one minute.

Microjets are aligned with magnetic field lines and associated with umbral dots.

Abstract

We present simultaneous photospheric and chromospheric observations of the trailing sunspot in NOAA 10904, obtained with the Swedish Solar Telescope (SST) La Palma, Canary Islands. Time series of high resolution \ion{Ca}{ii}\,$H$ images show transient jet-like structures in sunspot umbrae are elongated, which we call umbral microjets. These jets are directed roughly parallel to nearby penumbral microjets, suggesting that they are aligned with the background magnetic field. In general, first a bright dot-like structure appears, from which a jet later emerges, although some jets appear without an associated chromospheric dot. Bright photospheric umbral dots are associated with umbral microjets arising in the outer umbra. Nevertheless, a one-to-one correspondence between jet-like events and underlying umbral dots is not seen. They are typically less than 1\arcsec ~long and less than 0\farcs3 wide. The typical lifetime of umbral microjets is around one minute. The brightness of these structures increases from the center of the umbra towards the umbra-penumbra boundary along with the brightness of the local background.