Inferring chromospheric velocities in an M3.2 flare using He

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 <ns0:sc>I</ns0:sc> 10830 Å triplet and Ca <ns0:sc>II</ns0:sc> 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 <ns0:sc>I</ns0:sc> and Ca <ns0:sc>II</ns0:sc> lines to recover the atmospheric parameters of the emitting plasma. Different inversion configurations were tested for Ca <ns0:sc>II</ns0:sc>, 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 <ns0:sc>I</ns0:sc> 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 <ns0:sc>I</ns0:sc>, with velocities of up to −10 km s−1. In addition, at the front we found the coexistence of multiple He <ns0:sc>I</ns0:sc> profiles within one pixel. The central part of the ribbon is dominated by He <ns0:sc>I</ns0:sc> and Ca <ns0:sc>II</ns0:sc> redshifted emission profiles. A flare-loop system, visible only in He <ns0:sc>I</ns0:sc> absorption and not in Ca <ns0:sc>II</ns0:sc>, 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 <ns0:sc>II</ns0:sc> 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 <ns0:sc>I</ns0:sc> triplet in the analysis helped resolve the ambiguity between two possible solutions for the plasma velocities detected in the Ca <ns0:sc>II</ns0:sc> line. This highlights the importance of combining multiple chromospheric spectral lines to achieve a more comprehensive understanding of flare dynamics.