Inferring chromospheric velocities in an M3.2 flare using He I 1083.0 nm and Ca II 854.2 nm

¹ Instituto de Astrofísica de Canarias (IAC), Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
³ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, N-0315 Oslo, Norway
⁴ Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern, N-0315 Oslo, Norway
⁵ Astronomical Institute of the University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁶ School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
⁷ UCL Mullard Space Science Laboratory, Holmbury St Mary, Dorking RH5 6NT, UK

^⋆ Corresponding author: ckuckein@iac.es

Received: 27 March 2025
Accepted: 30 May 2025

Abstract

Aims. Our aim was to study the chromospheric line-of-sight (LOS) velocities during the GOES M3.2 flare (SOL2013-05-17T08:43) using simultaneous high-resolution ground-based spectroscopic data of the He I 10830 Å triplet and Ca II 8542 Å line. A filament was present in the flaring area.

Methods. The observational data were acquired with the Vacuum Tower Telescope (VTT, Tenerife, Spain) and covered the pre-flare, flare, and post-flare phases. Spectroscopic inversion techniques (HAZEL and STiC) were applied individually to He I and Ca II lines to recover the atmospheric parameters of the emitting plasma. Different inversion configurations were tested for Ca II, and two families of solutions were found to explain the red-asymmetry of the profiles: a redshifted emission feature or a blueshifted absorption feature. These solutions could explain two different flare scenarios (condensation vs. evaporation). The ambiguity was solved by comparing these results to the He I inferred velocities.

Results. At the front of the flare ribbon, we observed a thin short-lived blueshifted layer. This is seen in both spectral regions, but is much more pronounced in He I, with velocities of up to −10 km s⁻¹. In addition, at the front we found the coexistence of multiple He I profiles within one pixel. The central part of the ribbon is dominated by He I and Ca II redshifted emission profiles. A flare-loop system, visible only in He I absorption and not in Ca II, becomes visible in the post-flare phase and shows strong downflows at the footpoints of up to 39 km s⁻¹. In the flare the Ca II line represents lower heights compared to the quiet Sun, with peak sensitivity shifting from log τ≃−5.2 to log τ≃−3.5. The loop system's downflows persist for over an hour in the post-flare phase.

Conclusions. The inferred LOS velocities support a cool-upflow scenario at the leading edge of the flare, with rapid transition from blueshifts to redshifts likely to occur within seconds to tens of seconds. Although the flare had a significant impact on the surrounding atmosphere, the solar filament in the region remained stable throughout all flare phases. The inclusion of the He I triplet in the analysis helped resolve the ambiguity between two possible solutions for the plasma velocities detected in the Ca II line. This highlights the importance of combining multiple chromospheric spectral lines to achieve a more comprehensive understanding of flare dynamics.