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Sulfur fractionation in coronal plumes as observed by Solar Orbiter/SPICE

T0 review · 2 major / 5 minor · reviewed 2026-08-02 · deepseek-v4-flash

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Pith's one-line read Coronal plumes enrich sulfur at their footpoints, Solar Orbiter data show

desk verdict A careful SPICE study reporting sulfur fractionation in two plumes, but the central detection rests on an unmeasured density upper bound that the plume footpoints may violate. read the letter →

arxiv 2602.23170 v1 pith:BMZIWHTI submitted 2026-02-26 astro-ph.SR

Slimane Mzerguat , Miho Janvier , Eric Buchlin , Deborah Baker , Andy S.H. To , David M. Long , Natalia Zambrana Prado This is my paper · ORCID
classification astro-ph.SR
keywords coronalplumesFIPbiassulfurfractionationintermediate-FIPelementsSolarOrbiterSPICEponderomotiveforcewindcomposition
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 evidence that sulfur, an element with intermediate first ionization potential, is fractionated in coronal plumes. Using Solar Orbiter/SPICE spectroscopy of two plumes in an equatorial coronal hole, the authors find a sulfur-to-nitrogen FIP bias of 1.1–1.5 (up to ~2.0 in individual pixels) at the plume footpoints, while the surrounding interplume plasma stays near 1.0. The fractionation is co-located with strong magnetic footpoints and remains constant within uncertainties over each plume's lifetime. The result matters because it shows that element separation in plumes affects mid-FIP elements, and it supports the ponderomotive force model in which Alfvén waves drive the fractionation.

What carries the argument

The central diagnostic is the sulfur-to-nitrogen FIP bias, measured with the Linear Combination Ratio (LCR) method on SPICE spectral lines: S IV 750.22 Å and S V 786.47 Å serve as the intermediate-FIP tracer, while N III 991.58 Å and N IV 765.15 Å provide the high-FIP reference. The N IV line is density-sensitive, so the analysis adopts a fixed electron density of 10^10 cm^-3—an upper bound for coronal-hole transition regions—to ensure the inferred bias is a conservative lower limit. The physical mechanism invoked to explain the enhancement is the ponderomotive force model, in which Alfvén waves refracted in the chromosphere push low- and mid-FIP ions upward; deep-penetrating torsional waves

What would settle it

Observe the same plumes with a full density-diagnostic line pair (for instance, the O V 760/761 Å ratio that SPICE's window only partially covers, or another instrument's density-sensitive lines) to measure n_e at the footpoints, then recompute the S/N FIP bias with the measured density; if the bias drops to ~1.0, sulfur is not fractionated.

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Extended reading notes

Core claim

The paper claims to have found the first unambiguous evidence that sulfur—an element with intermediate first ionization potential (10.36 eV)—is fractionated in coronal plumes. Using Solar Orbiter/SPICE spectra of two plumes in an equatorial coronal hole, the authors derive sulfur-to-nitrogen ratios at the plume footpoints and convert them to a FIP bias relative to photospheric abundances. In both plumes the bias is 1.1–1.5, with individual pixels up to ~2.0, while the adjoining interplume coronal-hole plasma stays at ~1.0. The fractionated plasma is cospatial with the strongest magnetic flux concentrations, and the bias remains constant within uncertainties over each plume's observed lifetim

Load-bearing premise

The central load-bearing premise is that the transition-region electron density in the plume footpoints is no higher than 10^10 cm^-3; if the true density exceeds that, the N IV line's sensitivity would lower the inferred FIP bias toward the photospheric value and the fractionation signal would vanish.

Editorial extensions

If this is right

  • If sulfur is fractionated in plumes, then fractionation in these structures affects intermediate-FIP elements, not just low-FIP ones, so plume composition provides a fuller probe of the fractionation process.
  • The co-location of fractionation with strong magnetic footpoints supports the idea that plumelets—small-scale reconnection events—drive the wave activity that fractionates the plasma.
  • The constant FIP bias over each plume's lifetime indicates that composition is set early, possibly within hours of plume formation, and then remains locked.
  • Reinterpreting earlier Si/S plume measurements with sulfur itself enriched implies silicon FIP bias could be up to ~1.7, resolving part of the contradiction between older plume studies.
  • The observed values are consistent with the ponderomotive force model's prediction for sulfur when Alfvén waves penetrate to the mid-chromosphere.

Reading between the lines

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

  • Because the analysis deliberately assumes a high density, a direct density diagnostic at plume footpoints would likely push the inferred bias higher (toward ~1.4), making sulfur fractionation easier, not harder, to detect.
  • The rapid onset of fractionation in Plume 2—within six hours of its appearance—suggests that composition changes on timescales comparable to plume growth; future high-cadence observations could test whether the bias ramps up gradually or jumps at formation.
  • If sulfur follows the same enrichment pattern in other coronal holes, then in-situ measurements of sulfur in the fast solar wind could be used as a remote fingerprint of plume plasma, linking the two datasets.
  • The sulfur-to-nitrogen ratio could serve as a diagnostic of mid-chromospheric wave fields, offering a way to infer Alfvén wave properties from composition maps alone.
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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

2 major / 5 minor

Summary. The paper analyzes Solar Orbiter/SPICE observations of two coronal plumes in an equatorial coronal hole during March–April 2024. Using the LCR method with S IV, S V, N III, and N IV lines, the authors construct S/N FIP bias maps and report enhanced sulfur fractionation (R≈1.1–1.5, up to ~2.0 in individual pixels) at the plume footpoints, while the surrounding interplume plasma remains near the photospheric value. The enhancement is co-located with strong magnetic flux and appears only when the plume is present. The authors interpret the result within the ponderomotive force model and conclude that these observations provide evidence for sulfur fractionation in plumes.

Significance. If robust, this would be the first direct evidence of intermediate-FIP sulfur fractionation in coronal plumes, extending earlier low-FIP studies and linking plume composition to wave-driven fractionation models. The paper is transparent about its methodology: SAFFRON is open-source, the spectral fitting and error propagation are described in detail, and the density sensitivity is explicitly examined. However, the central detection depends on an assumed electron density and on a permissive statistical threshold, so the significance of the finding is currently conditional.

major comments (2)
  1. [Sec. 3; App. B (Figs. 8–9)] The central detection rests on the assumption n_e = 10^10 cm^-3 as an upper bound for the TR density at plume footpoints. This is not established for plumes, which are denser and cooler than ambient CH plasma and are rooted in 50–220 G flux concentrations. App. B shows the inferred FIP bias decreases monotonically with n_e; at n_e > 10^10 cm^-3 the enhancement could fall below the detection threshold. Fig. 9 explores only 10^9–10^10 cm^-3 and does not test this. The 'conservative lower limit' argument is therefore only as strong as the unverified upper bound. The authors should either measure or observationally justify the density at the footpoints, show that the enhancement survives at n_e = 10^11 cm^-3, or soften the central claim.
  2. [Sec. 4.2; Fig. 6a] The criterion R − ΔR/2 > 1 used to define 'confidently fractionated' pixels is a half-sigma threshold, not a confident detection: a pixel with R=1.1 and ΔR=0.2 would satisfy it. This weak threshold calls into question the 'clear population of fractionated pixels' described in Sec. 4.2. To support the claim of genuine sulfur enrichment, the authors should report the number of pixels with R − ΔR > 1 (or a higher confidence threshold) and the significance of the weighted mean FIP bias in Fig. 6b. Without this, the visual impression is not quantitatively supported.
minor comments (5)
  1. [Sec. 4.1] The text refers to 'N iii 765.152 Å', but Table 1 and the rest of the paper identify this line as N IV. Please correct the ion label.
  2. [Sec. 3] The upper-bound density value is attributed to Doschek et al. (1997), a conference abstract. Given that this assumption is load-bearing, a peer-reviewed reference or additional justification is needed.
  3. [Abstract and Sec. 5] The phrasing 'These results provide the evidence for sulfur fractionation in plumes' is stronger than what the data support given the density and threshold caveats. Consider 'provide evidence consistent with' or 'suggest'.
  4. [Eq. (3)] The term 'low-FIP' is used for the sulfur lines even though sulfur is an intermediate-FIP element; this could confuse readers. Consider calling the two groups 'target' and 'reference' elements instead.
  5. [Sec. 4.3] The sentence describing the temporal behavior of P2 is a run-on and difficult to parse; please split for clarity.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the FIP-bias measurement is an empirical line-ratio diagnostic with independent calibration, and the density assumption is a stated systematic caveat, not a construction that forces the result.

full rationale

The central claim that sulfur is fractionated in the observed plumes is derived from Eq. 3, a fixed linear combination of measured SPICE line intensities normalised by photospheric abundances (Grevesse et al. 2007) and CHIANTI atomic data. The LCR coefficients α_i and β_j were optimised in prior published work on standard DEMs (Zambrana Prado & Buchlin 2019); they are not fitted to the plume pixels, so the plume/interplume contrast is an empirical difference in the data, not an output forced by the calibration. The fixed electron density of 10^10 cm^-3 is an externally sourced, explicitly stated assumption, and App. B quantifies its effect; the paper deliberately labels the derived values as conservative lower limits. That is a systematic uncertainty about the magnitude of the effect, not a definitional or fitted construction that makes R>1 by construction. The self-citations to the LCR method and the companion SPICE study are independent published methodological benchmarks; no uniqueness theorem from the same authors is invoked to forbid alternatives, and no known empirical pattern is merely renamed. The plume footpoint regions were selected from EUV imaging and magnetograms, not from the FIP-bias maps, so there is no selection on the dependent variable. Overall, the derivation chain is self-contained with respect to the measurement itself, and the identified weaknesses are physical/caveat concerns rather than circularity.

Assumptions & free parameters 2 free parameters · 6 assumptions · 0 invented entities

No new physical entities are introduced. The interpretation invokes the established ponderomotive-force model and Alfvén-wave dynamics without postulating new particles, forces, or conserved quantities. The main burdens are the fixed electron density and the LCR calibration inherited from prior work.

free parameters (2)
  • Assumed electron density n_e = 1e10 cm^-3 (fixed; upper bound for CH transition region)
    The density-sensitive N IV 765.15 Å line requires an electron density for emissivity calculation. The SPICE data lack reliable density diagnostics, so 1e10 cm^-3 was chosen as a conservative high value. If true plume-footpoint densities exceed this, the inferred FIP bias falls and the fractionation signal weakens.
  • LCR coefficients alpha_i, beta_j = Not quoted in paper; inherited from Zambrana Prado & Buchlin (2019), optimized on CHIANTI DEMs (AR, CH, QS)
    The FIP-bias scale in Eq. (3) depends on coefficients optimized to make the ratio insensitive to DEM and normalized to photospheric abundances. The plume enhancement is not fit by these coefficients, but systematic off-sets in the calibration would shift all reported FIP-bias values.
assumptions (6)
  • domain assumption CHIANTI v11.0.2 collisional-radiative model accurately predicts S IV, S V, N III, and N IV emissivities in the plume-footpoint temperature range.
    All line intensities are converted to abundances through CHIANTI contribution functions; errors in atomic data propagate directly into the FIP-bias values. See Sec. 3 and Table 1.
  • domain assumption Photospheric abundances from Grevesse et al. (2007) define the R=1 reference level.
    The LCR normalization uses these photospheric abundances; systematic errors in the reference composition shift the inferred FIP bias uniformly. See Eq. (3).
  • domain assumption The selected emission lines, especially N III 991.577 Å, are optically thin in plume footpoints.
    Optical-thinness is cited from Jordan et al. (2001) and Varesano et al. (2024); if opacity is significant, the S/N ratio would be biased. See Sec. 3.
  • domain assumption The LCR method remains sufficiently DEM-insensitive for the SPICE S/N line set in plumes.
    Coefficients were optimized on typical AR/CH/QS DEMs and the insensitivity is inherited from Zambrana Prado & Buchlin (2019); no plume-specific DEM verification is performed in this paper. See Sec. 3.
  • ad hoc to paper n_e = 1e10 cm^-3 is an upper bound for the transition-region density at plume footpoints.
    Chosen because density diagnostics are unavailable; this is the load-bearing systematic assumption for the central detection claim. See Sec. 3 and App. B.
  • domain assumption Visual identification of P1 and P2 from EUI/FSI 174 images and light curves correctly identifies the plume footpoint regions.
    Plume presence and spatial regions are defined by visual inspection and fixed Carrington coordinates, not by an automated segmentation algorithm. See Sec. 2.3.

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Cite this review

Pith. "Pith review of Sulfur fractionation in coronal plumes as observed by Solar Orbiter/SPICE." pith.science (2026). https://pith.science/paper/BMZIWHTI

@misc{pith2026260223170,
  author       = {Pith},
  title        = {Pith review of: Sulfur fractionation in coronal plumes as observed by Solar Orbiter/SPICE},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/BMZIWHTI}},
  note         = {Machine review of arXiv:2602.23170}
}
read the original abstract

Coronal plumes are bright, narrow structures rooted in coronal holes that contribute to the solar wind. Their composition, particularly elemental fractionation as a function of first ionization potential (FIP), provides diagnostics of plasma properties and magnetic connectivity. Earlier plume studies of fractionation using low-FIP elements reached conflicting conclusions. Intermediate-FIP elements may provide additional diagnostic insight, since their fractionation is thought to involve processes beyond those affecting low-FIP species. We investigate sulfur (intermediate-FIP element) in plumes to assess the presence of fractionation, its evolution, and its relation to wave activity. We analyzed Solar Orbiter observations of two plumes in an equatorial coronal hole during March--April 2024, using Spectral Imaging of the Coronal Environment (SPICE) to derive the sulfur-to-nitrogen ratio. EUV imaging and magnetograms provided additional context. Data were processed with the open-source Python tool Spectral Analysis Fitting Framework and Reduction of Noise (SAFFRON). Both plumes showed sulfur fractionation that remained constant within uncertainties. The fractionated plasma was co-located with strong magnetic footpoints, in contrast with the surrounding interplume plasma. These results provide the evidence for sulfur fractionation in plumes and suggest, consistent with the ponderomotive force model, wave dynamics in the chromosphere as a driver.

Figures

Figures reproduced from arXiv: 2602.23170 by the authors.

Figure 1
Figure 1. Observation context and timeline. (a) Context images and point￾ing. (b) Solar Orbiter’s relative locations and instrument timeline [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. (a) Intensity evolution and plume context for Plume 1 and Plume 2. (b) Magnetic and intensity context of plume footpoints with flux density dis￾tributions. to them as “P1” and “P2”, the locations in which Plume 1 and Plume 2 formed. Plume 1 was [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Sample SPICE composition raster from 2024 March 31 at 09:05:32 UT. First and third columns show mean images for each of the six spectral windows (window 1 to window 6), averaged along wavelength dimen￾sion. Right to each image panel we show the corresponding average spectra per window across the spatial axis. To apply the LCR method to the SPICE data, we adopted a fixed electron density of 1010 cm−3 . In principle, … view at source ↗
Figures from the paper (7 more)
Figure 4
Figure 4. Figure 4: Contribution functions (in erg cm3 s −1 sr−1 ) for the SPICE com￾position lines used in this study, computed with CHIANTI v.11 (using the IDL package of CHIANTI) for three constant electron densities of 109 , 1010, and 1011 cm−3 . Each curve is labeled with the ion, wa…
Figure 5
Figure 5. Figure 5: Sample of SPICE observations from each observation set: the first set is shown in the first two columns, and the second set in the last two columns. Each column displays, from top to bottom, the S/N FIP bias maps and radiance maps for SPICE N iii (used in the FIP bias …
Figure 6
Figure 6. Figure 6: a, using Eq. 4. Each point is associated with the average scan time of the relevant sub-region. Bottom panel: Normalized mean intensity evo￾lution from EUI/FSI174 during the SPICE obser￾vation period. Lime bars indicate the time win￾dows of the SPICE rasters, as in Fig…
Figure 7
Figure 7. Figure 7: Sample SPICE radiance maps from March 31 at 09:05, shown for N iv 765.152 ˚A (one of the lines used for the composition diagnostics), Ne viii 770.428 ˚A, and Mg ix 706.060 ˚A. The top row displays the full-resolution maps, and the bottom row shows the corresponding bin…
Figure 8
Figure 8. Figure 8: Effect of electron density on sulfur FIP bias derived from SPICE data. Left column: FIP bias maps (top) and corresponding histograms (bottom) for two representative electron densities: ne = 109 cm−3 and ne = 1010 cm−3 , highlighting the impact of density assumptions on…
Figure 9
Figure 9. Figure 9: Same as Fig. 6b, but now also showing the FIP bias computed assuming an electron density of 109 cm−3 (filled markers above each original point). Additional vertical error bars (in orange for “P1” and in purple for “P2”) represent the uncertainty introduced by varying t…
Figure 10
Figure 10. Figure 10: Example light curve from region “P1” in EUI/FSI174. Orange: raw intensity. Red: corrected intensity after pointing jump normalization. Dashed vertical lines indicate detected pointing changes. coordinates in arcseconds), and CDELT1, CDELT2 (pixel scale in arcsec/pixel…

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