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Elemental Composition Evolution during the 2024 September 30 Solar Eruption: A Comparison of Hot and Cool Plasma Components with Solar Orbiter/SPICE, Hinode/EIS, and Chandrayaan-2/XSM

T0 review · 4 major / 6 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read This paper claims that the long-standing disagreement between X-ray and EUV measurements of flare composition arises because the two spectral ranges sample different plasma: the hot X-ray component becomes photospheric during the…

desk verdict The cleanest quadrature EUV/X-ray abundance comparison for a single flare to date, but the XSM FIP-bias trend rests on a two-temperature fit that never lets the hot and cool components have different abundances, so the headline contrast is partly model-dependent. read the letter →

arxiv 2608.12881 v1 pith:YYTLXMEK submitted 2026-08-13 astro-ph.SR

Momchil Molnar , Joseph Plowman , Amir Caspi , Ritesh Patel , Arpit Shrivastav , Tania Varesano , Don Hassler , Ryan French
show 2 more authors
Biswajit Mondal L. P. Chitta
This is my paper · ORCID
classification astro-ph.SR
keywords FIPbiassolarflareselementalabundanceschromosphericevaporationOrbiterSPICEChandrayaan-2XSMdifferentialemissionmeasurecoronalloops
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports that during the M7.6 solar flare of 2024 September 30, the elemental composition of the flaring plasma changed on timescales of minutes, and the hot and cool plasma components evolved in opposite directions. Using Solar Orbiter/SPICE EUV spectra, Hinode/EIS, and disk-integrated X-ray spectra from Chandrayaan-2/XSM, the authors find that the hot X-ray-emitting plasma's FIP bias—the coronal over-abundance of low-ionization-potential elements such as Fe, Si, and Mg—decreased from coronal toward photospheric values during the impulsive phase, while the bright tops of the cooler post-flare loops observed by SPICE retained coronal FIP bias. The paper argues that the apparent conflict between X-ray and EUV abundance measurements is not a calibration artifact: the two diagnostics sample different regions. STIX imaging places the X-ray source away from the EUV loops, consistent with X-rays seeing chromospherically evaporated material with photospheric composition, while the EUV loop tops receive coronal-composition material transported by downflows. If right, this resolves a long-standing discrepancy and shows that rapid FIP-bias changes can be spatially and temporally tracked with combined multi-thermal observations.

What carries the argument

The load-bearing diagnostic is the FIP bias, defined as the abundance ratio of low- to high-first-ionization-potential elements relative to photospheric values, tracked separately in hot and cool plasma. The machinery has four parts: (1) a combined DEM+abundance inversion on SPICE lines ($10^5$–$10^6$ K) that varies a single parameter interpolating between photospheric and coronal abundance models; (2) two-line FIP-bias ratios (S V 786 Å/N IV 765 Å and Mg IX 706 Å/Ne VIII 770 Å) that corroborate the inversion using only the inferred thermal structure; (3) two-temperature spectral fitting of XSM soft X-ray spectra (1–15 keV) with temperature, emission measure, and elemental abundances as free parameters, the same abundances applied to both thermal components; and (4) a linear force-free field reconstruction with loop filling that gives the 3D orientation of post-flare loops, turning ambiguous Doppler shifts into upflow/downflow assignments. STIX imaging in the 6–10 keV band locates the X-ray source relative to the EUV loops, tying the abundance signals to distinct spatial regions.

What would settle it

Refit the 1-minute XSM spectra with the two thermal components allowed to have independent elemental abundances. If the hot component's FIP bias is constant at photospheric values while its emission measure grows, the apparent decrease is a weighted-average artifact; if the hot component's FIP bias itself falls, the paper's evaporation interpretation is supported. Separately, compare the onset of the XSM FIP-bias drop with the STIX 6–10 keV footpoint light curves: evaporation predicts the composition change begins when footpoint heating begins.

Watch

Extended reading notes

Core claim

The central discovery is that the hot and cool components of a single flare sample different plasma reservoirs with different FIP-bias evolution. In the impulsive phase, XSM's two-temperature fits to 1–15 keV spectra show the abundances of the low-FIP elements Mg, Si, Ca, and Fe falling toward photospheric values over about half an hour, while the high-FIP element Ar stays flat; this is interpreted as chromospheric evaporation injecting photospheric-composition plasma into the hottest loops. At the same time, SPICE's DEM+abundance inversion and independent two-line ratios (S V/N IV and Mg IX/Ne VIII) show the bright post-flare loop tops gaining coronal FIP bias within about ten minutes. Doppler redshifts above the loop tops, interpreted through a 3D magnetic reconstruction, indicate downflows bringing coronal-composition material to the loop tops, while blueshifts at the footpoints mark evaporation. The authors conclude that the X-ray/EUV abundance discrepancy is not instrumental but geometric: disk-integrated X-rays see the evaporating hot component, while the EUV sees older cooling loops whose tops are loaded from above.

Load-bearing premise

The X-ray result rests on a spectral model that forces the hot and cool plasma components to share the same elemental abundances, so the measured drop in composition contrast could be a weighted-average artifact of a growing hot component rather than a change in the hot plasma's own make-up.

Editorial extensions

If this is right

  • If the interpretation is right, X-ray and EUV flare abundance measurements should not be expected to agree; each diagnostic reports the composition of the plasma it is most sensitive to, so multi-wavelength campaigns are needed to separate evaporation from reconnection signatures.
  • The observed minutes-timescale FIP-bias changes imply that flare plasma can be transported and fractionated quickly enough to alter composition during a single impulsive phase, placing a timing constraint on models of FIP fractionation and flare loop dynamics.
  • The co-spatial mismatch between 6–10 keV X-ray sources and EUV loop tops means disk-integrated X-ray abundance curves during flares should be read as hot-component diagnostics, not as the composition of the whole active region.
  • The looptop FIP-bias enhancement seen by SPICE, corroborated by EIS Si X/S X ratios, supports a picture in which post-flare loops are loaded from above by reconnection outflows or condensations rather than solely by footpoint evaporation.
  • The paper's method—simultaneous DEM and abundance inversion with SPICE—demonstrates that spatially resolved composition evolution can be tracked on raster timescales, enabling future statistical studies of FIP-bias dynamics in flares.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Because the XSM fits force one abundance set on both thermal components, the measured FIP-bias decrease may in part be an emission-measure-weighted average of a coronal-abundance cool component and a photospheric-abundance hot component; testing a two-abundance model, or a DEM-resolved abundance fit, would show whether the intrinsic hot-plasma FIP bias actually changes.
  • The scenario implies a spatial abundance gradient inside cooling flare loops: photospheric-composition plasma at the footpoints and coronal-composition plasma at the tops. High-cadence spectral maps of a single loop system during the decay phase could directly test this prediction.
  • If chromospheric evaporation drives the X-ray FIP-bias drop, the timing of the drop should track the hard X-ray footpoint flux from STIX or GOES; a mismatch in onset times would favor a different mechanism, such as reconnection outflow mixing.
  • The 'mass loading from above' picture predicts that the looptop coronal FIP bias should decay once reconnection outflow ceases; following the same arcade into the late decay phase should show the looptop bias relaxing back toward the surrounding plasma composition.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 6 minor

Summary. The paper reports on multi-instrument observations of the 2024 September 30 M7.6 solar flare, combining Solar Orbiter/SPICE and EUI, Hinode/EIS, Chandrayaan-2/XSM, and Solar Orbiter/STIX. The authors infer FIP-bias evolution in cool and hot plasma components: SPICE shows coronal (elevated) FIP bias at post-flare loop tops, while XSM shows a decrease in low-FIP abundances during the impulsive phase, interpreted as chromospheric evaporation feeding hot reconnection outflows. The study uses a simultaneous DEM+abundance inversion on SPICE lines, two-line ratio checks, a CROBAR 3D reconstruction to interpret Doppler shifts, and STIX imaging to argue that the X-ray and EUV diagnostics sample spatially distinct plasma.

Significance. If the central claim holds, the paper provides a rare, directly comparative view of FIP-bias evolution in flare plasma across a very wide temperature range, using a favorable quadrature geometry. The strengths include the multi-instrument dataset, the explicit SPICE PSF correction, the use of an S V/N IV line ratio that is genuinely independent of the DEM+abundance inversion, and the attempt to use 3D geometry to interpret Doppler signals. However, the paper's headline X-ray abundance trend rests on a two-temperature spectral model in which both components share a single abundance set; this is precisely the configuration in which a decreasing FIP bias can arise as a weighted-average artifact of a growing hot-component emission measure. This issue, together with missing pixel-wise uncertainties in the SPICE abundance maps, must be resolved before the main conclusion is secure.

major comments (4)
  1. [Section 3.3 and Figure 11] The XSM analysis fits a two-temperature model in which both thermal components share the same elemental abundances, yet the paper's interpretation requires the hot component to be photospheric and the cool component to be more coronal. With a single abundance set, the observed decrease in low-FIP abundances during the impulsive phase could be a weighted-average artifact of the growing hot-component emission measure rather than a real change in either component's composition. Please test this explicitly by (a) allowing the two thermal components to have independent abundance sets, or (b) fixing the hot component at photospheric abundances and the cool component at coronal abundances while varying only their emission measures, and showing that the observed FIP-bias trend is reproduced without changing any abundances. Without such a test, the central claim that XSM sees FIP bias decreasing from coronal to photospheric is not established.
  2. [Section 3.5 and Figure 7] The SPICE abundance maps and the two-line FIP-bias maps are presented without pixel-wise uncertainties, and Section 3.5 itself notes that the abundance recovery becomes less reliable near the temperature boundaries of the line set. Since the main spatially resolved result is the contrast between coronal-composition loop tops and photospheric-composition surroundings, the absence of uncertainty maps makes it impossible to assess whether this contrast is significant. Please provide uncertainty maps (e.g., from Monte Carlo perturbations of the line intensities or from the inversion's covariance structure) and quantify the reliability of the looptop/footpoint difference.
  3. [Section 4.2] The claim that the two-line ratios independently corroborate the DEM+abundance inversion is only partially correct. The S V/N IV ratio is a genuinely independent check because S V was deliberately excluded from the DEM inversion in Section 3.5. However, the Mg IX/Ne VIII ratio is not independent: both of those lines are used in the DEM fit, and the ratio is computed using the DEM derived from those same lines. The authors should state this distinction explicitly and base the robustness argument on the S V/N IV cross-check, or demonstrate that the Mg IX/Ne VIII result is insensitive to the DEM used.
  4. [Section 4.3 and Figure 8] The CROBAR-based interpretation of Doppler shifts as evaporation at footpoints and downflows above loop tops is load-bearing for the proposed transport mechanism (reconnection-driven downflows of coronal material). However, the linear force-free field twist parameter alpha is a free parameter chosen by a minimal chi2 metric with no quoted uncertainty, and the resulting line-of-sight orientations of the reconstructed loops are presented without error bars. A different alpha or magnetogram choice could change which features are identified as upflows versus downflows. Please quantify how sensitive the inferred footpoint/looptop orientations are to alpha and to the HMI magnetogram selection, or soften the dynamical interpretation accordingly.
minor comments (6)
  1. [Abstract] The phrase 'FIP-bias decreasing from coronal to a hybrid' is unclear; please specify what 'hybrid' means (e.g., a value between photospheric and coronal) and use consistent terminology throughout.
  2. [Figure 7] The color scales for the SPICE abundance maps and the two-line FIP-bias maps are not defined in the caption; please add colorbar labels and units, and state whether the two-line ratios are in absolute FIP-bias units or normalized.
  3. [Section 2] The ~40 arcsec helioprojective longitude shift applied to the SPICE data during alignment with EUI is quoted without an uncertainty; please provide at least a rough estimate of the alignment precision.
  4. [Section 3.5] The contribution functions are computed assuming an electron density of 1e8 cm^-3, but flaring loop densities may be higher; the paper notes agreement between CHIANTI v8 and v11, but the density sensitivity of the abundance inference should be checked or at least discussed.
  5. [Section 4.4] The EIS Si X/S X ratio map is used as an important corroborating FIP-bias proxy, but no uncertainty map is shown and the statement that EIS has 'lower spatial and temporal resolution' could be quantified (e.g., raster duration versus SPICE raster cadence).
  6. [Section 4.4] The sentence comparing EIS and SPICE resolutions would benefit from concrete numbers (EIS raster scan time versus the SPICE 2 min 58 s cadence) to make the limitation quantitative.

Circularity Check

1 steps flagged · score 2.0 of 10

Minor circularity: the MgIX/NeVIII two-line 'independent' check reuses the DEM fitted with those lines; the central SPICE/XSM abundance comparison remains a direct data fit.

  1. fitted input called prediction [Section 4.2 (two-line FIP-bias check), Eq. (1); Section 3.5 (joint DEM+abundance inversion)]
    "When we computed the FIP bias for the MgIX706 Å to NeVIII770 Å, and SV786 Å to NIV765 Å lines using two-line ratios, we found similar results, corroborating the previous analysis. ... This estimate is independent from the one presented in Section 4.1, where we have utilized the resulting DEM from the analysis in Section 3.5 only."

    The MgIX/NeVIII ratio is evaluated with Eq. (1) using the DEM produced by the Section 3.5 joint inversion, which included MgIX 706 Å and NeVIII 770 Å among its constraining lines. If that fit reproduces the two intensities, Eq. (1) algebraically returns the same abundance parameter the inversion already fit, so the two-line result is a consistency check rather than an independent estimate. The S V/N IV pair is partially independent because S V was deliberately omitted from the DEM fit, but the paper's blanket 'corroborating' and 'independent' language overstates the MgIX/NeVIII support.

full rationale

The paper's main results are direct spectral fits against external CHIANTI atomic data: SPICE line intensities constrain a joint DEM+abundance inversion, and XSM spectra are fit with a two-temperature chisoth model with free elemental abundances. No target conclusion is baked into those fits, and the looptop coronal FIP-bias versus XSM decreasing FIP-bias contrast is a data-driven comparison. One supporting step is partially circular: the MgIX/NeVIII two-line 'independent' check uses the very DEM that was jointly fitted with those lines, so for that pair it recovers the fitted abundance by construction; the S V/N IV ratio and the Hinode/EIS SiX/SX ratio provide more independent corroboration. A separate limitation, not a circularity, is that the XSM model applies one abundance set to both thermal components (Section 3.3), so the paper cannot separately measure the hot component's FIP bias; the interpretation of the drop as photospheric-abundance chromospheric evaporation in the hot component is underdetermined and should be checked with per-component abundances. Self-citations to SPICE PSF reduction, DEM tools, CROBAR, and prior XSM analyses are method/tool citations and are not used to forbid alternatives. Overall, the central claim has independent observational content; score 2 reflects the overstated independence of the MgIX/NeVIII corroboration.

Assumptions & free parameters 5 free parameters · 6 assumptions · 0 invented entities

The central claim rests on spectral fitting models (two-temperature XSM with common abundances, SPICE DEM with one-dimensional abundance interpolation), an assumed electron density for atomic data, and a linear force-free field model whose twist parameter is hand-tuned. No new physical entities are introduced; the data are public mission observations.

free parameters (5)
  • CROBAR LFFF twist parameter alpha = -7.5 turns/Gm
    Chosen as the constant-alpha value that best matches the observed AR geometry (Section 4.3); it sets the 3D loop orientations used to interpret Doppler shifts as downflows.
  • Assumed electron density for CHIANTI contribution functions = 10^8 cm^-3
    Used to precompute SPICE line contribution functions (Section 3.5); abundance and DEM results can shift if the true density differs.
  • XSM two-temperature component temperatures and emission measures = fitted per 1-minute spectrum, time series in Figure 11
    Free parameters of the XSPEC model (Section 3.3); the composition trend is inferred together with these.
  • XSM elemental abundances for Mg, Si, S, Ar, Ca, Fe = fitted per 1-minute spectrum, time series in Figure 11
    Free parameters of the XSPEC model (Section 3.3); these are the central measured quantities of the X-ray analysis.
  • SPICE single abundance interpolation parameter = per-pixel, per-timestep maps in Figure 7
    The DEM+abundance inversion varies this one scalar that linearly interpolates between photospheric and coronal abundance models (Section 3.5); it is the measured quantity, but the 1D interpolation constraint is a modeling assumption.
assumptions (6)
  • ad hoc to paper XSM spectra are fit with two thermal components that share a single elemental abundance set (Section 3.3).
    The model cannot represent composition differences between the hot and cool components, yet the paper's interpretation attributes the abundance trend to the hot component alone.
  • domain assumption The SPICE abundance analysis represents all element abundances as a linear interpolation between the photospheric and coronal abundance models, controlled by one scalar (Section 3.5).
    If the true fractionation is element-specific or violates this 1D sequence, the inferred FIP-bias maps could be biased.
  • domain assumption The SPICE lines are optically thin and collisionally excited; radiative excitation is negligible (Section 3.5).
    Standard for the low corona and transition region, but can fail at higher altitudes where radiative excitation becomes significant.
  • domain assumption CROBAR linear force-free field extrapolation with constant alpha approximates the post-flare AR magnetic field (Section 4.3).
    Used to convert observed Doppler shifts into upflows and downflows; if the field geometry is wrong, the downflow interpretation is unsupported.
  • domain assumption The STIX 6-10 keV source location is representative of the XSM-emitting plasma volume (Section 4.4).
    XSM is disk-integrated; STIX is used to ascribe the X-ray abundance drop to evaporation upflows. The spatial correspondence is approximate.
  • standard math CHIANTI v8.0.7 atomic data and ionization equilibria are used for contribution functions (Section 3.5).
    Standard atomic database; the text notes comparison with v11 showed no significant difference.

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Cite this review

Pith. "Pith review of Elemental Composition Evolution during the 2024 September 30 Solar Eruption: A Comparison of Hot and Cool Plasma Components with Solar Orbiter/SPICE, Hinode/EIS, and Chandrayaan-2/XSM." pith.science (2026). https://pith.science/paper/YYTLXMEK

@misc{pith2026260812881,
  author       = {Pith},
  title        = {Pith review of: Elemental Composition Evolution during the 2024 September 30 Solar Eruption: A Comparison of Hot and Cool Plasma Components with Solar Orbiter/SPICE, Hinode/EIS, and Chandrayaan-2/XSM},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/YYTLXMEK}},
  note         = {Machine review of arXiv:2608.12881}
}
read the original abstract

Solar plasma composition differs between the photosphere and corona over a range of timescales, with preferential enhancement of elements with low first ionization potential (FIP). However, the physical origin of the FIP fractionation remains incompletely understood. Furthermore, during flares, the FIP bias also exhibits rapid changes, associated with fast transport of material with different FIP biases. We present novel observations from Solar Orbiter SPICE and EUI, Hinode/EIS, and Chandrayaan-2 XSM instruments, finding rapid abundance changes in the emitting plasma, on timescales of minutes, during the eruptive M7.6-class solar flare observed on 2024 Sept 30. These instruments have wide temperature coverage and find contrasting abundance-evolution patterns between the hotter and cooler plasma components. 3D reconstruction of the active region and additional observations from the Solar Orbiter STIX X-ray telescope show how the hot and cool plasma components, emitting in different spectral regions and observed with the various instruments, sample the plasma composition evolution in distinct locations within the observed flaring plasma. The bright post-flare loop tops observed by SPICE show coronal FIP bias, while the hot plasma observed with XSM exhibits FIP-bias decreasing from coronal to photospheric during the impulsive phase. We interpret these observations as evidence of the X-ray diagnostics seeing hot coronal reconnection outflows mixing with chromospheric plasma as flare loops sequentially energize and relax, explaining why the FIP bias decreases from coronal to a hybrid; and the cool loop tops seen with SPICE show coronal abundances due to coronal material deposited near the looptops.

Figures

Figures reproduced from arXiv: 2608.12881 by the authors.

Figure 1
Figure 1. [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. GOES X-ray light curve of the flare, where the right-hand vertical axis shows the flare classification. Over￾plotted are the exact times of observations from SO/SPICE and EUI instruments, as well as the Chandrayaan-2/XSM X-ray instrument observations used in this work. (EUI/FSI), showing the full Sun for context in panel (b). During this observing campaign, EUI/HRI 174 ˚A ob￾served with a 2-second cadence during 202… view at source ↗
Figure 4
Figure 4. The SPICE PSF correction mitigates the spurious velocity signals in the data. The top row shows the Doppler velocity derived from the uncorrected SPICE data, whereas in the bottom row we show the resulting Doppler velocities after the PSF artifact removal was applied (J. E. Plowman et al. 2026). 3.1. Reducing the SPICE data SPICE is the EUV imaging spectrometer aboard Solar Orbiter, designed to obtain observations r… view at source ↗
Figures from the paper (9 more)
Figure 3
Figure 3. Figure 3: Overview of the observed spectral line intensities during the flare on 2024 Sept 30 with SPICE. The three time instances in panels (a), (b), and (c) correspond to the pre-flare stage, impulsive phase including the acceleration of the ejecta, and post-flare loop contrac…
Figure 5
Figure 5. Figure 5: Overview of the spectral line parameters observed with SPICE during the flare. Each row represents a spectral line, noted on the right, where the panels show the temporal evolution with time to the right, as noted on the panel titles. SPICE observations cover well the …
Figure 6
Figure 6. Figure 6: Temperature contribution functions from CHIANTI for the SPICE spectral lines used in this study. The differently colored lines correspond to the different solar abundance models adopted in the contribution function computations. The spectral lines with an asterisk in t…
Figure 7
Figure 7. Figure 7: Evolution of the coronal plasma composition during the flare, for the pre-eruption (left column), impulsive phase (middle column), and gradual phase (right column). Each column corresponds to the same time instance. First row: EUI/HRI 174 ˚A observations of the flare p…
Figure 8
Figure 8. Figure 8: 3D Reconstruction of NOAA AR 13842 with the CROBAR modeling framework. Panel (a): SDO AIA 211 ˚A cutout image of the modeled region. The magenta (looptops) and cyan (footpoints) rectangles show the regions of the FOV used for the field tracing. Panel (b): Reprojected C…
Figure 9
Figure 9. Figure 9: Hinode/EIS observations of the erupting AR in a pair of spectral lines with close formation temperatures, with low and high FIP, respectively. These observations were taken before, during, and after the flare, where the corresponding scanning time of the raster is note…
Figure 10
Figure 10. Figure 10: Example XSM spectrum (blue) for time 23:52 UT fitted with two-temperature models from the chisoth package for the XSPEC spectral fitting software, where the contributing atomic spectral features are noted. The normalized difference between the model fit and the spectr…
Figure 11
Figure 11. Figure 11: Top panel: Inferred FIP bias (the ratio of the in￾ferred abundance over photospheric abundance model) from the X-ray XSM data for the different elements listed in the legend for the impulsive duration of the flare, for which XSM data is available (magenta curve). The …
Figure 12
Figure 12. Figure 12: Evolution of the STIX X-ray sources overlaid on top of the EUI/HRI 174 ˚A images for four timesteps during the impulsive phase of the flare. The corresponding X-ray energy bins are outlined as the different colored contours, outlining the 60% emission regions. We note…

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