IRIS NUV diagnostics for Ellerman bombs: spectral properties, thermodynamics, and formation height

TL;DR

This study identifies spectral signatures in IRIS NUV spectra, especially Mg II triplet enhancements, as reliable indicators of Ellerman bombs, enabling large-scale detection without Hα observations.

Contribution

The paper develops a new detection criterion based on IRIS NUV spectra, particularly Mg II triplet features, for identifying Ellerman bombs in the solar atmosphere.

Findings

Mg II triplet wing enhancement is a key EB signature.

A detection criterion based on Mg II triplet successfully identifies 14 of 18 EBs.

Mg II line shapes can indicate EB formation height.

Abstract

Context. Ellerman bombs (EBs) are observational signatures of small-scale magnetic reconnection, key to understanding the lower solar atmosphere. While their role in active regions has been widely studied using the H$\alpha$ line, near-ultraviolet (NUV) spectra routinely observed by the Interface Region Imaging Spectrograph (IRIS) offer a promising alternative for EB identification, enabling large-scale studies. Aims. We aim to identify the most important spectral signatures of EBs in the IRIS NUV spectra. With this, we seek to develop a robust criterion for their detection solely using the IRIS NUV spectra. In parallel, we determine the typical atmospheric stratification associated with EBs. Methods. We used four coordinated observations between the Swedish 1-m Solar Telescope (SST) and IRIS. Using the H$\alpha$ line as a reference, we detected 18 different EBs and studied their associated IRIS NUV spectra. In addition, we used the IRIS$^{2+}$ inversion tool to infer the temperature, line-of-sight velocity, and non-thermal broadening from the EB spectra. Results. The defining feature of EBs in the IRIS NUV is the enhancement of the wings of the subordinated Mg II triplet in between the Mg II h&k lines. Inversions reveal that these signatures are produced by localized temperature increase of $\Delta$ T~1650 K around log$\tau$=-3.8. Using only the Mg II triplet signatures, we found a detection criterion that successfully recovered 14 of 18 H$\alpha$-detected EBs. In addition, the shape of the Mg II h&k lines in relation to the Mg II triplet can serve as a proxy for the EB formation height. Conclusions. The NUV spectrum observed by IRIS is a good candidate for detecting EBs, opening the doors to large-scale studies across the extensive IRIS database, removing the dependence on H$\alpha$ observations.