High-resolution full-Stokes spectropolarimetry of the footpoints of two consecutive C-class solar flares (active region NOAA 13363, 2023 July 16), obtained with the GREGOR Infrared Spectrograph, revealed He I 10830 Å triplet profiles of extreme complexity: Doppler widths of 15–25 km s−1^{-1}, multiple velocity components with line-of-sight velocities approaching 90 km s−1^{-1}, and emission above the continuum, while all observed photospheric lines remained unchanged. Vashalomidze et al. (2026) reproduced only four representative intensity profiles by manual trial-and-error forward modelling with one to three HAZEL chromospheric slabs, using an empirical source-function enhancement factor β to shift absorption to emission. The open problem is that paper's explicitly deferred follow-up programme: a multi-step inversion of the complete full-Stokes maps of the three flare-covering scans delivering spatially resolved maps of the vector magnetic field, deprojected velocities, and slab parameters for each chromospheric component; the geometric deprojection of the supersonic flows at the near-limb viewing geometry (μ=0.57, the cosine of the angle between the line of sight and the local solar vertical); and the physical interpretation of the empirical enhancement factor β, needed to decide which energy-transport mechanisms produced the observed chromospheric response.

Background

A solar flare is a sudden release of magnetic energy in the solar corona, and a central question of flare physics is how this energy is transported into the lower atmosphere: by thermal conduction, by beams of non-thermal electrons accelerated during reconnection, or by magnetohydrodynamic waves. The chromosphere — the layer between the photosphere (the visible solar surface) and the corona — responds strongly, with line brightening, broadening, and Doppler shifts of several tens of km s−1^{-1}. Establishing which mechanism dominates at a given flare footpoint requires spatially resolved measurements of both the flows and the magnetic field in the flaring chromosphere itself.

The He I 10830 Å (1083.0 nm) infrared triplet is a prime diagnostic for this purpose. It forms in the upper chromosphere: coronal extreme-ultraviolet (EUV) illumination ionizes helium, and the subsequent recombination cascade overpopulates the metastable lower level of the triplet, so the line appears in absorption against the photospheric continuum; during flares it can swing into emission. Its polarization is governed by the Hanle effect (the modification of scattering-induced atomic polarization by weak magnetic fields) together with the Zeeman effect (magnetically induced line splitting), so the full Stokes vector — the four components (I,Q,U,V)(I,Q,U,V) of the polarized radiance — encodes both the strength and the orientation of the chromospheric magnetic field. The standard interpretation tool is HAZEL (HAnle and ZEeman Light; Asensio Ramos et al. 2008), which models the triplet with one or more constant-property chromospheric slabs — each characterized by the optical depth τ\tau of the red component of the triplet, a Doppler width, a line-of-sight (LOS) velocity, and a source function SS — lying above a standard semi-empirical quiet-Sun photosphere. To reproduce flare emission, HAZEL users must introduce an enhancement factor β\beta that multiplies the slab source function. Physically, β\beta is understood as an overpopulation of the upper 3P^3P level of the triplet, plausibly produced by flare-driven overionization followed by the recombination ladder, but HAZEL gives it no first-principles definition, and no quantitative calibration of β\beta in terms of EUV flux or particle-beam parameters exists.

Vashalomidze et al. (2026) observed active region NOAA 13363 on 2023 July 16 with GRIS, the GREGOR Infrared Spectrograph at the 1.5-metre GREGOR telescope, at a heliocentric angle of 55° (μ=0.57\mu=0.57, with μ\mu the cosine of the angle between the line of sight and the local solar vertical). They obtained five full-Stokes slit scans containing the photospheric Si I 10827.1 Å, Ca I 10833.4 Å, Na I 10834.9 Å, and Ca I 10838.9 Å lines together with the He I triplet, with simultaneous Ca II H and TiO imaging; the first three scans cover the decay of one C-class flare (moderate on the GOES soft X-ray brightness scale, GOES being the Geostationary Operational Environmental Satellite programme that continuously monitors solar X-ray emission), the quiet interval, and the rise of a second C-class flare at the same location. Their analysis established that the photospheric lines show no measurable change, so the flare energy did not reach the lower photosphere, while the He I triplet becomes extremely broad (Doppler widths of 15–25 km s−1^{-1}, far above purely thermal widths at chromospheric temperatures), multiply structured, with LOS velocities of individual components reaching roughly 90 km s−1^{-1} in blueshifts and strong redshifts (modelled values from −88-88 to +25+25 km s−1^{-1}) — supersonic relative to the local sound speed of about 10 km s−1^{-1} — and sometimes in emission above the continuum. The apparent brightening of the Si I core was explained as overlapping, strongly blueshifted He I emission rather than photospheric heating. However, the modelling remained intensity-only forward fitting of four representative profiles by manual trial and error with one to three HAZEL slabs, with β\beta, τ\tau, and the velocities tuned by hand, no formal uncertainty analysis, and the weak linear-polarization signals left unexploited. The calibrated GRIS full-Stokes maps of these scans, together with the simultaneous HiFI+ Ca II H and TiO images, are publicly available from the GRIS data archive (sdc.leibniz-kis.de) under the observatory's open data policy, and the openly available Cassda GUI software removes the spectral spikes in the Stokes parameters. The full physical use of the dataset — inversion of the complete full-Stokes maps, deprojection of the velocities into the local reference frame, and a physical interpretation of β\beta — is the open problem.

Problem Statement

The open task, as posed by the source paper's own limitations and future-work statements, is to convert its intensity-only, manual forward modelling of the GRIS full-Stokes dataset of the 2023 July 16 C-class flares (active region NOAA 13363, μ=0.57) into a spatially resolved physical characterization of the flaring chromosphere at the footpoint. The common objective is the magnetic, dynamic, and excitation structure of the multiple chromospheric components seen in the He I 10830 Å triplet, and the task comprises three jointly required results:

(a) Multi-step full-Stokes inversion of the three flare-covering GRIS scans (Scans 1–3, 08:41–08:51 UT): classify the observed He I 10830 Å profiles into families of similar shape, invert the full Stokes vector (I,Q,U,V)(I,Q,U,V) across the entire maps with tailor-made multi-slab HAZEL configurations for each family, and deliver, for each chromospheric component and each scan, spatial maps of the vector magnetic field (strength, inclination, azimuth), line-of-sight velocity, optical depth, Doppler width, and source-function enhancement factor β\beta, with uncertainty estimates that honestly address the degeneracy of multi-slab fits and the weakness of the linear-polarization signals.

(b) Geometric deprojection: transform the inverted line-of-sight velocities into the local reference frame using the μ=0.57\mu=0.57 viewing geometry, so that true plasma velocities and flow orientations are obtained and the supersonic (up to roughly 90 km s−1^{-1}) components can be attributed to chromospheric evaporation (hot, beam- or conduction-driven upflows) or condensation (cool downflows).

(c) Energy-transport interpretation: use the inverted per-component parameters and the deprojected velocities to constrain which energy-transport mechanisms (thermal conduction, non-thermal electron beams, waves) produced the filamentary and diffuse chromospheric morphology across the flare phases captured by the dataset. Central to this interpretation is the physical meaning of the inferred values of the enhancement factor β\beta — empirically understood as an overpopulation factor of the upper 3P^3P level of the triplet — for which no quantitative calibration in terms of coronal EUV irradiation or non-thermal electron beams yet exists. For (c), a physically grounded interpretation that connects the inferred β\beta values to a dominant excitation mechanism, with its residual uncertainty stated, is sufficient; an independent first-principles recalibration of β\beta against a named EUV-irradiation or particle-beam model is not required, and, because (a)–(c) are jointly required, an interpretation of β\beta alone does not resolve the task.

The three results are linked: the inversions produce the per-component parameters whose velocities the deprojection must orient and whose excitation conditions, parametrized by β\beta, the interpretation must address, and together they support the single interpretive goal — deciding, quantitatively, how flare energy was transported from the corona into the lower atmosphere at this footpoint.

References

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