Stokes I±VI\pm V spectrograms of the X17.2/4B solar flare of 2003 October 28, taken with the Kyiv echelle spectrograph with the slit fortuitously crossing one of the flare's sunquake ('seismic') sources, show a complex circular-polarization (Stokes V) pattern in the far wings of the He I D3 line: a broad, sign-changing signal reaching 4% of the continuum over roughly −5-5 to +10+10 Å from line center, plus narrow (~0.6 Å) peaks at +4.0, +6.3 and +8.5 Å and a negative feature at +23.5 Å. Interpreted as Zeeman splitting, the wide features would imply magnetic fields up to the 10510^5 G range; interpreted as Doppler shifts, the narrow peaks would imply plasma descents of ‘∼‘`\sim`320–400 km s−1^{-1}, and up to ‘∼‘`\sim`1200 km s−1^{-1} if the +23.5 Å feature belongs to D3. Neither interpretation is secure: slab modeling by the same group shows that a few-kG multi-component atmosphere and a ∼105\sim10^5 G simpler atmosphere fit the same profiles equally well, and no independent analysis exists. The open problem is to determine the physical origin of this far-wing polarization — the magnetic fields, line-of-sight velocities, and polarization mechanism of the emitting and absorbing components at the seismic source — through polarized radiative-transfer modeling with finite optical depth and coupled Zeeman/Doppler effects, with blend and instrumental control, and to state what the result implies for the excitation of the sunquake.

Background

A 'sunquake' is a seismic wave excited in the solar interior by a solar flare, observed as expanding ripples in photospheric Doppler and continuum maps. The X17.2/4B flare of 2003 October 28 in active region NOAA 0486 (X17.2 is its GOES soft X-ray class and 4B its optical importance; one of the strongest flares of solar cycle 23) produced several sunquakes, and the mechanism by which flare energy reaches the photosphere to drive them (Lorentz-force reconfiguration of the coronal field, pressure impact of chromospheric condensations, or precipitating particle beams) is still debated; knowing the magnetic field and flows at a seismic source is therefore key evidence (Kosovichev 2006; Zharkova & Zharkov 2007). The He I D3 line of neutral helium at 5875.6 Å is a triplet formed in the upper chromosphere and in the transition region (the thin layer separating the chromosphere from the million-degree corona), and is a classic chromospheric magnetic-field diagnostic: in spectropolarimetry, Stokes VV is the difference between right- and left-circularly polarized intensities, and the Zeeman effect splits the line into circularly polarized σ\sigma components whose separation grows with field strength. When the splitting exceeds the line width (the 'strong-field regime'), the field strength can be read directly from the separation; for a multiplet line like D3, fields in the kilogauss range additionally mix the fine-structure levels, so the component pattern must be computed in the intermediate Paschen–Back regime rather than pure Zeeman splitting. In the standard diagnostic use of D3 (e.g., Libbrecht et al. 2019 with the Swedish 1-m Solar Telescope (SST) and the HAZEL code of Asensio Ramos et al. 2008, a publicly available inversion code that solves polarized radiative transfer for He I lines in a constant-property slab atmosphere), flare footpoints show chromospheric fields of order 1–2.5 kG and condensation velocities of tens of km s−1^{-1}. Against this backdrop, Lozitska, Yakovkin & Lozitsky (2023) reported a unique measurement: during the 2003 October 28 flare, the echelle spectrograph (a high-dispersion grating spectrograph) of the Kyiv horizontal solar telescope recorded simultaneous I+VI+V and I−VI-V spectra on adjacent bands of photographic plates (a technique that yields Stokes VV as half the difference of the two bands) over a 160 Å interval around D3, with the entrance slit crossing one of the flare's seismic sources. They found secondary Stokes VV peaks far from the D3 core — amplitudes up to 4% of the continuum, roughly an order of magnitude larger than previously detected in other flares — with the sign of circular polarization reversing across the line center and narrow peculiarities at +4.0, +6.3 and +8.5 Å. Taken at face value the wide splitting corresponds to fields up to the 10510^5 G range and the offsets to descending flows exceeding 400 km s−1^{-1} (about 1200 km s−1^{-1} for the +23.5 Å feature if it is D3-related); the unequal areas of the positive and negative peaks indicate the D3 emission is not optically thin, so neither reading is secure. A first modeling attempt with constant-property slab atmospheres (Yakovkin, Lozitska & Lozitsky 2024) demonstrated the core difficulty: a four-slab model with fields of a few kG and a three-component model with ∼105\sim10^5 G fields reproduce the observed profiles equally well, and the authors state that no preferred model can be chosen without additional criteria. Related far-wing peculiarities have since been reported even in non-flaring sunspots (Lozitsky 2025), so the phenomenon and its interpretation both remain unexplained.

Problem Statement

For the 2003 October 28 X17.2/4B flare observations (Kyiv horizontal-telescope echelle spectrograph, 11:15 UT, Stokes I±VI\pm V photographic spectrograms whose entrance slit crossed seismic source #2), determine the physical origin of the secondary Stokes VV peaks in the far wings of the He I D3 line — the broad sign-changing polarization reaching 4% of the continuum over approximately −5-5 to +10+10 Å and the narrow (~0.6 Å) peaks at +4.0, +6.3 and +8.5 Å — and thereby determine: (i) the magnetic field strength(s) of the solar components producing the pattern, deciding between the few-kG multi-component solution and the ∼105\sim10^5 G solution that current slab modeling cannot distinguish; (ii) the line-of-sight plasma velocities, including whether the narrow peaks imply descending transients of roughly 320–400320\text{–}400 km s−1^{-1} and whether the negative feature at +23.5 Å is D3-related and implies a descent of ∼1200\sim1200 km s−1^{-1}; and (iii) whether the polarization arises purely from the Zeeman and Paschen–Back effects or includes non-Zeeman contributions (for example strong-electric-field/Stark-related effects or optical-depth effects in an optically thick D3 source, as the unequal areas of the positive and negative peaks suggest). The determination requires polarized radiative-transfer modeling of the D3 multiplet with finite optical depth and coupled Zeeman/Doppler shifts, confronted with the published profiles, together with control of weak photospheric blends in the ≈5861–5905\approx5861\text{–}5905 Å window and of the instrumental polarization of the telescope/spectrograph. All three determinations (i)–(iii) are required, and the answer must also state what the resulting fields and flows imply for the excitation of the sunquake at this location.

References

  1. N. I. Lozitska, I. I. Yakovkin, V. G. Lozitsky, 'Unique spectral manifestations around the D3 line observed in the region close to the seismic source of a large solar flare', Monthly Notices of the Royal Astronomical Society: Letters 528, L1–L3 (2023), DOI 10.1093/mnrasl/slad163, https://doi.org/10.1093/mnrasl/slad163
  2. I. Yakovkin, N. Lozitska, V. Lozitsky, 'Altitude Heterogeneity of Magnetic Fields and Doppler Velocities in the Area of Seismic Source of a Strong Solar Flare from Data in Helium, Sodium, and Nickel Lines', Universe 10(6), 262 (2024), DOI 10.3390/universe10060262, https://doi.org/10.3390/universe10060262
  3. V. G. Lozitsky, I. I. Yakovkin, N. I. Lozitska, 'Comparison of magnetic fields and Doppler velocities in an X-class solar flare as measured by D1, D2, D3, Hα, and NiI 5892.9 lines', Advances in Space Research 75, 1343–1353 (2025), DOI 10.1016/j.asr.2024.08.006, https://doi.org/10.1016/j.asr.2024.08.006
  4. V. G. Lozitsky, 'Possible Signatures of Extremely Strong Magnetic Fields Observed in a Sunspot Based on Spectropolarimetry near the D3 Line', Kinematics and Physics of Celestial Bodies 41, 259–267 (2025), DOI 10.3103/S0884591325060054, https://doi.org/10.3103/S0884591325060054 (English translation of the Ukrainian original: V. G. Lozitsky, 'Possible evidences of extremely strong magnetic fields observed in a sunspot using spectro-polarimetry around the D3 line', Kinematika i Fizika Nebesnykh Tíl 41(6), 58–72 (2025), DOI 10.15407/kfnt2025.06.058, https://doi.org/10.15407/kfnt2025.06.058)
  5. T. Libbrecht, J. de la Cruz Rodríguez, S. Danilovic, J. Leenaarts, H. Pazira, 'Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D3 spectro-polarimetry', Astronomy & Astrophysics 621, A35 (2019), DOI 10.1051/0004-6361/201833610, arXiv:1806.06880, https://doi.org/10.1051/0004-6361/201833610
  6. I. I. Yakovkin, V. G. Lozitsky, 'Search for superstrong magnetic fields in active processes on the Sun using spectro-polarimetry within 15 Å around the D3 line', Monthly Notices of the Royal Astronomical Society 523, 5812–5822 (2023), DOI 10.1093/mnras/stad1816, https://doi.org/10.1093/mnras/stad1816
  7. A. Asensio Ramos, J. Trujillo Bueno, E. Landi Degl'Innocenti, 'Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects', The Astrophysical Journal 683, 542–565 (2008), DOI 10.1086/589433, https://doi.org/10.1086/589433 (the HAZEL code)
  8. A. G. Kosovichev, 'Properties of Flares-Generated Seismic Waves on the Sun', Solar Physics 238, 1–11 (2006), DOI 10.1007/s11207-006-0190-6, https://doi.org/10.1007/s11207-006-0190-6
  9. V. V. Zharkova, S. I. Zharkov, 'On the Origin of Three Seismic Sources in the Proton-rich Flare of 2003 October 28', The Astrophysical Journal 664, 573–585 (2007), DOI 10.1086/518731, https://doi.org/10.1086/518731