Small-scale energetic phenomena in Hε: Ellerman bombs, UV bursts, and small flares

¹ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern 0315 Oslo, Norway
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway

^⋆ Corresponding author; kilian.krikova@astro.uio.no

Received: 21 November 2024
Accepted: 10 February 2025

Abstract

Aims. We investigated the potential of using Hε to diagnose small-scale energetic phenomena such as Ellerman bombs, UV bursts, and small-scale flares. Our focus is to understand the formation of the line and how to use its properties to get insight into the dynamics of small-scale energetic phenomena.

Methods. We carried out a forward modeling study, combining simulations and detailed radiative transfer calculations. The 3D radiative magnetohydrodynamic simulations were run with the Bifrost code and included energetic phenomena. We employed a Markovian framework to study the Hε multilevel source function, used relative contribution functions to identify its formation regions, and correlated the properties of synthetic spectra with atmospheric parameters.

Results. Ellerman bombs are predominantly optically thick in Hε, appearing as well-defined structures. UV bursts and small flares are partially optically thin and give rise to diffuse structures. The Hε line serves as a good velocity diagnostic for small-scale heating events in the lower chromosphere. However, its emission strength is a poor indicator of temperature, and its line width offers limited utility due to the interplay of various broadening mechanisms. Compared to Hα, Hε exhibits greater sensitivity to phenomena such as Ellerman bombs, as its line core experiences higher extinction than the Hα wing.

Conclusions. Hε is a valuable tool for studying small-scale energetic phenomena in the lower chromosphere. It provides more reliable estimates of velocities than those extracted from wing emission in Hα or Hβ. Maps of Hε emission show more abundant energetic events than the Hα counterpart. Our findings highlight Hε’s potential to advance our understanding of dynamic processes in the solar atmosphere.