During the X1 solar flare SOL2014-03-29T17:48, spectropolarimetry of the chromospheric Ca II 8542 Å line provided the first quantitative detection of stepwise changes of the chromospheric line-of-sight magnetic field: changes up to about 640 Mx cm−2^{-2}, stronger and more spatially extended than the simultaneously observed photospheric changes, and incoherent with them in timing, sign, magnitude, and spatial distribution (Kleint 2017). The physical process that produces flare-related magnetic-field changes in the lower solar atmosphere is not identified: the observed sign pattern rules out the simple contracting-loop geometry, while the remaining candidate scenarios (growing or untwisting loops; loops newly becoming visible; a genuine change of field magnitude) are compatible with the observed pattern but are not yet established from line-of-sight data; the changes show no one-to-one correspondence with white-light, hard X-ray, flare-ribbon, or sunquake signatures; and it remains unquantified how much of the chromospheric signal is true restructuring rather than flare-driven changes in line formation. Nonlinear force-free modeling of the same event fails at chromospheric heights, a second multi-height observation (the X1.6 flare SOL2014-10-22) confirms the incoherence, and the most recent review literature (2024) treats the question as open. This record states the problem: determine the mechanism of flare-driven lower-atmosphere magnetic restructuring and explain the photosphere-chromosphere incoherence, validated against multi-height, ideally vector-field, spectropolarimetric observations.

Background

Solar flares are sudden releases of stored magnetic energy in the Sun's corona, the hot outer atmosphere. A standard diagnostic of the magnetic field at the visible surface (the photosphere) is the magnetogram: a map of the line-of-sight magnetic flux density BLOSB_{\rm LOS}, obtained from the Zeeman splitting of a spectral line, i.e. from the circularly polarized (Stokes VV) component of the line profile; 1 Mx cm−2^{-2} equals 1 gauss (G). It has been established statistically that during many flares the photospheric BLOSB_{\rm LOS} changes abruptly and essentially permanently, in a stepwise fashion, typically by up to a few hundred Mx cm−2^{-2} (Sudol & Harvey 2005; Petrie & Sudol 2010). The leading interpretation is the coronal implosion picture of Hudson (2000): magnetic energy release requires the coronal field configuration to contract, making the field at the footpoints of coronal loops (arched magnetic structures anchored at both ends in the photosphere) more horizontal and thereby changing BLOSB_{\rm LOS} at the surface. Fisher et al. (2012) showed that such changes imply an abrupt change of the Lorentz force (the J×B\mathbf{J}\times\mathbf{B} force that electric currents and magnetic fields exert on the plasma) acting on the lower atmosphere, and downward Lorentz-force changes have been invoked as drivers of sunquakes — acoustic waves launched into the solar interior during some flares and detected as traveling ripples in Doppler maps.

Nearly all of this evidence concerns the photosphere, because measuring magnetic fields in the overlying chromosphere (roughly 500-2000 km above the photosphere, where flare energy is deposited) is much harder. The main tool is spectropolarimetry of chromospheric lines such as Ca II 8542 Å, and the simplest interpretation uses the weak-field approximation (WFA): when the Zeeman splitting is much smaller than the Doppler width of the line, the amplitude of Stokes VV is proportional to the line-of-sight flux density at the line's formation height. The more general and more demanding approach is non-local thermodynamic equilibrium (non-LTE) inversion: fitting the complete polarized line profiles with radiative-transfer models of the chromosphere, a layer whose gas is not in local thermodynamic equilibrium, to recover the magnetic field and thermodynamics. Kleint (2017) used the IBIS imaging spectropolarimeter (Interferometric Bidimensional Spectropolarimeter) at the Dunn Solar Telescope (DST) to obtain Ca II 8542 Å spectropolarimetry during the X1 flare SOL2014-03-29T17:48, together with photospheric magnetograms from the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO). This yielded the first quantitative detection of stepwise chromospheric BLOSB_{\rm LOS} changes during a flare. The chromospheric changes reached about 640 Mx cm−2^{-2} — stronger and more extended than the photospheric changes (below about 320 Mx cm−2^{-2}) — and in about 60% of the changed pixels ∣BLOS∣|B_{\rm LOS}| decreased, statistically compatible with the implosion picture at the event's heliocentric angle. But the two layers behaved incoherently: photospheric changes clustered near the polarity inversion line (PIL, the line separating opposite magnetic polarities), chromospheric changes near the footpoints of coronal loops, and the changes differed in timing, sign, and magnitude, with no one-to-one correspondence with white-light (continuum) kernels, hard X-ray (HXR) footpoint sources, flare ribbons (bright elongated bands where flare energy strikes the lower atmosphere), or the flare's small sunquake (Judge et al. 2014). A simple contracting-loop geometry predicted the wrong sign pattern at the footpoints, and since only line-of-sight fields were available the actual loop geometry could not be determined. Kleint (2017) therefore posed the problem stated below. Subsequent modeling (Kleint et al. 2018) and a second multi-height observation (Ferrente et al. 2023) have confirmed rather than resolved it, and chromospheric flare magnetometry is described as being in its infancy in the most recent review of the field (Fletcher 2024).

Problem Statement

Kleint (2017) asks what physical process causes flare-related stepwise magnetic-field changes in the lower solar atmosphere, and why the chromospheric and photospheric responses are incoherent. Specifically, an answer must determine:

  1. Mechanism and locus. Which process produces the abrupt, permanent changes of the chromospheric and photospheric magnetic field during flares — contraction of coronal loops, growth or stretching of loops, untwisting of loops, loops newly becoming visible, or a genuine change of field magnitude — and why do the changes occur only at specific locations (photosphere: near the polarity inversion line; chromosphere: near loop footpoints), with no one-to-one correspondence to white-light kernels, hard X-ray sources, flare ribbons, or sunquake sites?

  2. Layer-to-layer incoherence. Why are the chromospheric and photospheric changes independent of each other in timing, sign, magnitude, and spatial distribution, although both layers are threaded by the same magnetic structure? In the source event the chromospheric changes were stronger (below about 640 Mx cm−2^{-2}, versus below about 320 Mx cm−2^{-2}) and more extended than the photospheric ones, and a nonlinear force-free model of the same flare reproduces the photospheric changes only moderately and the chromospheric changes not at all (Kleint et al. 2018).

  3. True restructuring versus line-formation effects. How much of the apparent chromospheric BLOSB_{\rm LOS} change is genuine magnetic restructuring, and how much is caused by flare-driven changes in line formation — optical-depth shifts that make different atmospheric layers, with different field strengths, contribute to the observed line profile — or by evolving chromospheric structures entering the line of sight? The permanence of the measured changes argues against a purely intensity-driven artifact, but this argument has not been made quantitative.

A complete answer provides a quantitative physical account — a single model that includes the non-force-free physics of the low atmosphere (where gas pressure and gravity are dynamically important, so magnetic stress does not dominate the force balance as it does in the corona) — validated against high-cadence, multi-height, ideally vector-field spectropolarimetric observations of at least one well-observed flare, reproducing all four properties (the sign, magnitude, timing, and spatial distribution) of the stepwise changes at both layers with a single model. Such an account must also indicate where flare energy is dissipated when chromospheric changes occur without measurable photospheric counterparts, how the Lorentz-force change (previously derived only for the photosphere) is modified when chromospheric restructuring is included, and why the magnetic-change sites in the source flare showed an unusually early response in the 211 Å passband of the Atmospheric Imaging Assembly (AIA, SDO's extreme-ultraviolet imager of the hot corona) and only a tiny sunquake despite substantial magnetic changes.

References

  1. Kleint, L. (2017). First Detection of Chromospheric Magnetic Field Changes During an X1-Flare. The Astrophysical Journal 834, 26. DOI: 10.3847/1538-4357/834/1/26. https://doi.org/10.3847/1538-4357/834/1/26 (arXiv:1608.02552, https://arxiv.org/abs/1608.02552)
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