Solar ultraviolet bursts

TL;DR

This paper introduces the concept of ultraviolet (UV) bursts as small, intense, transient brightenings in solar active regions, highlighting their properties, origins, and significance for understanding magnetic reconnection in the low solar atmosphere.

Contribution

It defines UV bursts, compares them with similar phenomena, and discusses their magnetic reconnection origins, providing a comprehensive review of their properties and diagnostic potential.

Findings

UV bursts are associated with small-scale magnetic reconnection events.

They occur in emerging flux regions, sunspot moats, and light bridges.

UV bursts offer unique insights into low-atmosphere magnetic reconnection.

Abstract

The term "ultraviolet (UV) burst" is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016-2017.