Small-scale energetic phenomena in H<i>ε</i>: Ellerman bombs, UV bursts, and small flares

Aims. We investigated the potential of using H ε 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 H ε 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 H ε , appearing as well-defined structures. UV bursts and small flares are partially optically thin and give rise to diffuse structures. The H ε 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 H α , H ε exhibits greater sensitivity to phenomena such as Ellerman bombs, as its line core experiences higher extinction than the H α wing. Conclusions. H ε 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 H α or H β . Maps of H ε emission show more abundant energetic events than the H α counterpart. Our findings highlight H ε ’s potential to advance our understanding of dynamic processes in the solar atmosphere.