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

TL;DR

This study investigates chromospheric line-of-sight velocities during an M3.2 solar flare using He I 1083.0 nm and Ca II 854.2 nm spectral data, applying inversion techniques to understand flare dynamics and resolve velocity ambiguities.

Contribution

The paper introduces combined spectroscopic inversion analysis of He I and Ca II lines to distinguish between condensation and evaporation scenarios in solar flares.

Findings

Detected short-lived blueshifted layers at flare ribbon fronts.

Observed strong downflows at flare footpoints up to 39 km/s.

Resolved velocity ambiguity using He I triplet data.

Abstract

We study the chromospheric LOS velocities during the GOES M3.2 flare (SOL2013-05-17T08:43) using simultaneous spectroscopic data of the He I 1083.0 nm triplet and Ca II 854.2 nm line. A filament was present in the flaring area. The observational data were acquired with the 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. 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. 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. 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. In the flare, the Ca II line represents lower heights compared to the quiet Sun, with peak sensitivity shifting from $\log \tau \simeq -5.2$ to $\log \tau \simeq -3.5$. The inferred LOS velocities support a cool-upflow scenario at the leading edge of the flare. The solar filament in the region remained stable. 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.