Narrow-line-width UV bursts in the transition region above Sunspots observed by IRIS

TL;DR

This paper identifies and characterizes a new class of small-scale, narrow-line-width UV bursts above sunspots in the solar transition region, observed by IRIS, revealing their dynamics, durations, and potential role in mass transport.

Contribution

It reports the discovery and detailed analysis of Narrow-line-width UV bursts (NUBs), a novel small-scale structure in the solar transition region above sunspots.

Findings

Six NUBs identified in two datasets.

NUBs have durations from 10 to over 36 minutes.

All NUBs exhibit Doppler shifts of 15-18 km/s.

Abstract

Various small-scale structures abound in the solar atmosphere above active regions, playing an important role in the dynamics and evolution therein. We report on a new class of small-scale transition region structures in active regions, characterized by strong emissions but extremely narrow Si IV line profiles as found in observations taken with the Interface Region Imaging Spectrograph (IRIS). Tentatively named as Narrow-line-width UV bursts (NUBs), these structures are located above sunspots and comprise of one or multiple compact bright cores at sub-arcsecond scales. We found six NUBs in two datasets (a raster and a sit-and-stare dataset). Among these, four events are short-living with a duration of $\sim$10 mins while two last for more than 36 mins. All NUBs have Doppler shifts of 15--18 km/s, while the NUB found in sit-and-stare data possesses an additional component at $\sim$50 km/s found only in the C II and Mg II lines. Given that these events are found to play a role in the local dynamics, it is important to further investigate the physical mechanisms that generate these phenomena and their role in the mass transport in sunspots.