Observations of Ellerman bomb emission features in He i D₃ and He i 10 830 Å^⋆

¹ Institute for Solar Physics, Dept. of Astronomy, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
e-mail: tine.libbrecht@astro.su.se
² Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain

Received: 7 July 2016
Accepted: 4 October 2016

Abstract

Context. Ellerman bombs (EBs) are short-lived emission features, characterised by extended wing emission in hydrogen Balmer lines. Until now, no distinct signature of EBs has been found in the He i 10 830 Å line, and conclusive observations of EBs in He i D₃ have never been reported.

Aims. We aim to study the signature of EBs in neutral helium triplet lines.

Methods. The observations consisted of ten consecutive SST/TRIPPEL raster scans close to the limb, featuring the Hβ, He i D₃ and He i 10 830 Å spectral regions. We also obtained raster scans with IRIS and made use of the SDO/AIA 1700 Å channel. We used Hazel to invert the neutral helium triplet lines.

Results. Three EBs in our data show distinct emission signatures in neutral helium triplet lines, most prominently visible in the He i D₃ line. The helium lines have two components: a broad and blueshifted emission component associated with the EB, and a narrower absorption component formed in the overlying chromosphere. One of the EBs in our data shows evidence of strong velocity gradients in its emission component. The emission component of the other two EBs could be fitted using a constant slab. Our analysis hints towards thermal Doppler motions having a large contribution to the broadening for helium and IRIS lines. We conclude that the EBs must have high temperatures to exhibit emission signals in neutral helium triplet lines. An order of magnitude estimate places our observed EBs in the range of T ~ 2 × 10⁴−10⁵ K.