Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D₃ spectro-polarimetry

Institute for Solar Physics, Dept. of Astronomy, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
e-mail: tine.libbrecht@astro.su.se

Received: 11 June 2018
Accepted: 30 October 2018

Abstract

Context. Magnetic reconnection during flares takes place in the corona, but a substantial part of flare energy is deposited in the chromosphere. However, high-resolution spectro-polarimetric chromospheric observations of flares are very rare. The most used observables are Ca II 8542 Å and He I 10830 Å.

Aims. We aim to study the chromosphere during a C3.6 class flare via spectro-polarimetric observations of the He I D₃ line.

Methods. We present the first SST/CRISP spectro-polarimetric observations of He I D₃. We analyzed the data using the inversion code HAZEL, and estimate the line-of-sight velocity and the magnetic field vector.

Results. Strong He I D₃ emission at the flare footpoints, as well as strong He I D₃ absorption profiles tracing the flaring loops are observed during the flare. The He I D₃ traveling emission kernels at the flare footpoints exhibit strong chromospheric condensations of up to ∼60 km s⁻¹ at their leading edge. Our observations suggest that such condensations result in shocking the deep chromosphere, causing broad and modestly blueshifted He I D₃ profiles indicating subsequent upflows. A strong and rather vertical magnetic field of up to ∼2500 G is measured in the flare footpoints, confirming that the He I D₃ line is likely formed in the deep chromosphere at those locations. We provide chromospheric line-of-sight velocity and magnetic field maps obtained via He I D₃ inversions. We propose a fan-spine configuration as the flare magnetic field topology.

Conclusions. The He I D₃ line is an excellent diagnostic to study the chromosphere during flares. The impact of strong condensations on the deep chromosphere has been observed. Detailed maps of the flare dynamics and the magnetic field are obtained.