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REVIEW 3 major objections 6 minor 50 references

Challenges in identifying the coronal hole wind

T0 review · 3 major / 6 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read The paper argues that a standard charge-state classifier's fixed O7+/O6+ threshold mistakes slow solar wind for coronal hole wind during solar activity minima, especially 2009.

desk verdict Solid evidence that the Zhao et al. (2009) O7+/O6+ threshold overclassifies slow wind as coronal hole wind near solar minimum; worth refereeing. read the letter →

arxiv 2501.02935 v1 pith:AMNWPUXC submitted 2025-01-06 astro-ph.SR physics.space-ph

Verena Heidrich-Meisner , Sophie Teichmann , Lars Berger , Robert F. Wimmer-Schweingruber This is my paper · ORCID
classification astro-ph.SRphysics.space-ph
keywords solarwindclassificationcoronalholecharge-statecompositionO7+/O6+ratioactivitycyclek-meansclusteringACEdata
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

Solar wind from coronal holes is usually identified by two established schemes, one based on the O7+/O6+ charge-state ratio and one based on proton plasma properties. This paper argues that the charge-state scheme, which applies a fixed threshold of 0.145, mistakes slow solar wind for coronal hole wind during the solar activity minimum because the O7+/O6+ ratio of all solar wind falls systematically at minimum. The evidence is that the proton-plasma scheme and an unsupervised 7-means classification both find almost no coronal hole wind at ACE in 2009, while the charge-state scheme finds almost all wind to be coronal hole wind, and that very few low-latitude coronal holes were present in 2009. If correct, this means established origin-based solar wind classifications need to be revisited and should take the phase of the solar activity cycle into account.

What carries the argument

The central object is the O7+/O6+ charge-state ratio threshold of 0.145 used by the Zhao et al. (2009) scheme. The paper compares this with the proton-plasma decision boundaries of Xu & Borovsky (2015) and with a 7-means clustering trained on ACE data that produces two coronal hole types. SHAP values are used to show which parameters drive each classifier over time, and the O7+/O6+ ratio's systematic solar-cycle variation is the mechanism that causes the fixed threshold to fail.

What would settle it

Map the solar wind that the composition scheme labels as coronal hole wind in 2009 (O7+/O6+ below 0.145) back to its solar source using coronal hole and streamer-belt observations; if a substantial fraction traces to streamer-belt plasma, the misidentification claim is confirmed, while if it traces to faint low-latitude coronal holes, the claim fails.

Watch

Extended reading notes

Core claim

The paper's central claim is that the Zhao et al. (2009) composition scheme misidentifies slow solar wind as coronal hole wind in solar activity minimum, particularly 2009. It grounds this in the systematic drop of O7+/O6+ for all solar wind during the minimum, so the fixed threshold is crossed by all wind. It uses the Xu & Borovsky (2015) proton-plasma scheme and a k-means clustering with two coronal hole types as references; both give low coronal hole wind fractions in 2009, consistent with the scarcity of low-latitude coronal holes. The paper also finds that the k-means classification yields two coronal hole types, which differ in charge states and in their occurrence over the activity cycle, and that SHAP explainability values show the oxygen ratio is essentially the only parameter driving the composition scheme, so the threshold's drift explains the inflated coronal hole wind fraction.

Load-bearing premise

The load-bearing premise is that the 7-means classification, trained on the same ACE data with k=7, is a valid reference for what coronal hole wind actually is; the paper concedes that no ground truth exists, so if the clusters are wrong the misidentification conclusion weakens.

Editorial extensions

If this is right

  • The widely cited conclusion that 2009 solar wind was almost entirely coronal hole wind, based on the composition scheme, should be treated as an artifact of a fixed threshold rather than a physical fact.
  • Studies that use the composition scheme across solar minima will have coronal hole wind samples contaminated by slow solar wind, so their statistics on coronal hole wind properties need re-examination.
  • Solar wind classification schemes should include the phase of the solar activity cycle explicitly, for example by using cycle-dependent decision boundaries.
  • The k-means result provides a data-driven way to separate two coronal hole wind populations, one associated with weaker footpoint fields and one with stronger footpoint fields, which could be used in future classifications.
  • The near-zero coronal hole wind fraction at ACE in 2009 is physically plausible because very few low-latitude coronal holes were observed then, not necessarily a sign that the proton-plasma scheme is broken.

Reading between the lines

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

  • The paper does not build an adjusted classifier, but its results imply that making the O7+/O6+ threshold track the solar cycle would shrink the composition scheme's minimum-period coronal hole wind fraction.
  • The two k-means coronal hole types are interpreted as polar/weak-footpoint and equatorial/strong-footpoint populations only indirectly; a direct source-mapping comparison could test that identification.
  • The same fixed-threshold-versus-cycle-drift tension could affect other composition-based solar wind categories, such as Fe charge-state bins, which the paper does not examine.
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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

3 major / 6 minor

Summary. This paper addresses the discrepancy between three solar wind categorization schemes in the fraction of solar wind identified as coronal hole wind in ACE data during 2001–2010, focusing on the deep solar minimum around 2009. The schemes are the charge-state based Zhao et al. (2009) threshold on O7+/O6+, the proton-plasma based Xu & Borovsky (2015) scheme, and a 7-means clustering trained on the same ACE data. The authors show that in 2009 the Zhao scheme classifies nearly all solar wind as coronal hole wind, while the other two schemes identify very little. They argue that the Zhao threshold is fixed while the O7+/O6+ ratio for all solar wind drops systematically during solar minimum, so that slow solar wind falls below the threshold and is misclassified. They also identify two k-means coronal hole types, CH1 and CH2, with different charge-state and speed properties, and use SHAP values to show which parameters drive each classification. They conclude that origin-oriented solar wind classification should take the solar cycle phase into account.

Significance. The paper identifies a concrete, physically plausible mechanism—solar-cycle modulation of the O charge-state baseline interacting with a fixed threshold—and supports it with multi-year histograms. If correct, the result has practical impact on solar wind classification and on the interpretation of composition-based solar wind statistics during solar minima. Strengths include the explicit acknowledgment that no ground truth exists (Section 1), the use of a data-driven reference in addition to two established schemes, the availability of data (Berger et al. 2023) and code (Teichmann et al. 2023), and the falsifiable recommendation to include the solar cycle phase in classification. The main weaknesses are the reliance on unvalidated reference classifications for the conclusion and the lack of quantitative uncertainties and sensitivity checks.

major comments (3)
  1. [Section 4.1, Fig. 3] The central claim that the Zhao scheme 'misidentifies some slow solar wind' is supported only qualitatively. The authors show that the O7+/O6+ distribution drops below the Zhao threshold in 2008–2009 and that the Zhao coronal hole subset in 2009 contains low vp and Tp, but they do not quantify the fraction of Zhao-identified coronal hole wind with slow-wind-like properties or compare it to the fraction in years of good agreement (e.g., 2003). A quantitative decomposition, such as the fraction of Zhao coronal hole wind with vp below a slow-wind threshold or a two-dimensional density plot of vp versus O7+/O6+ in 2009, would make the misclassification claim testable. As written, 'likely misidentifies' rests on visual inspection.
  2. [Section 3 and Section 1] The conclusion that the Zhao scheme overestimates coronal hole wind in 2009 treats the Xu & Borovsky (2015) and 7-means classifications as references, yet the paper states that no ground truth exists (Section 1) and that the 7-means classifier was trained on the same 2001–2010 ACE data used in the comparison (Section 3, 'The 7-means classification was trained on the ACE data from 2001-2010'). The Xu & Borovsky scheme is also a fixed-threshold scheme and is admitted to be 'better adjusted for solar activity minimum than maximum conditions' (abstract). The disagreement alone therefore does not identify which scheme is correct. The authors should either validate the reference classifications against an external benchmark (e.g., coronal hole source mapping or the Fujiki et al. coronal hole areas) or explicitly frame the conclusion as conditional on the data-driven reference. The current phrasing overstates the evidence.
  3. [Figure 1 and Section 3] The fractions and counts (e.g., 153594 vs 74188 vs 33702/31070 data points) are reported without uncertainties or significance tests. The 10-Fe-ion threshold (Section 2.1) and SWICS counting statistics introduce selection effects and sampling noise; the authors even note that the threshold biases against dilute polar coronal hole wind. Without bootstrap or Poisson uncertainties on the per-Carrington-rotation fractions, the reader cannot assess whether the 2009 differences are statistically significant or dominated by sampling. This is important because the entire paper is about a difference in fractions.
minor comments (6)
  1. [Abstract] The phrase 'identification of the coronal wind' should be 'identification of the coronal hole wind' to match the rest of the paper.
  2. [Section 2.2] There is a typo: 'Shapeley values' should be 'Shapley values', and in the footnote 'loosing' should be 'losing'.
  3. [Section 3] The word 'collosional' should be 'collisional' in the description of the proton-proton collisional age.
  4. [Section 4.1, last paragraph] The description of which 7-means type dominates during solar maximum/minimum is internally inconsistent with Section 4 and the abstract. Section 4 states 'CH1 is dominant during the solar activity minimum whereas CH2 is more frequent during the solar activity maximum,' but the last paragraph of Section 4.1 states 'CH1 represents properties of smaller coronal holes dominant during the solar activity maximum and CH2 represents properties of larger coronal holes that are dominant during the solar activity minimum.' This contradiction should be corrected.
  5. [Section 4.3] The statement that 'ACE is probably connected more frequently to equatorial coronal holes in 2003' is plausible but no connectivity analysis is shown; consider adding a reference or a simple Parker-spiral connection estimate to support this interpretation.
  6. [Section 2.1] The sentence 'The MAG and SWEPAM data is binned to the native 12-min time resolution of SWICS' has a subject-verb agreement error; it should be 'data are binned'.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the central claim rests on a physical comparison and external coronal-hole data, not on a fitted input or a self-citation chain.

full rationale

The paper's central claim is a comparative classification assessment, not a derived prediction from a fitted input. It investigates why two published schemes disagree and compares them with a third, k-means, as an explicitly labeled 'data-driven reference classification.' The key supporting evidence for the claim that Zhao et al. (2009) misidentifies slow solar wind as coronal hole wind during the solar minimum is physical: the systematic drop of the O7+/O6+ ratio below Zhao's fixed threshold for essentially all solar wind in 2008-2009, together with the low-speed, low-temperature properties of the extra 'Zhao coronal hole wind' population. This evidence does not reduce by construction to the k-means labels. The k-means classifier is trained on the same ACE 2001-2010 interval, so its in-sample fractions are descriptive rather than predictive, but the paper does not present k-means as a ground-truth derivation; it explicitly states that a ground truth for solar wind categorization is still not available. The choice k=7 is justified by a prior paper from the same group, but the main conclusion does not hinge on that specific k: the combined CH1+CH2 coronal hole wind agrees with Xu & Borovsky (2015), and the external Fujiki et al. (2016) coronal-hole-area observations independently support the low 2009 fraction. The SHAP result that O-ratio importance drops in 2009 is acknowledged by the authors as a consequence of almost all data lying below the Zhao threshold; although weak as evidence, it is presented as an explanatory consequence rather than as the load-bearing derivation. No step satisfies the required standard of exhibiting an equation that equals its input by construction or a fitted parameter renamed as a prediction. The absence of a ground truth is a correctness and external-validity limitation, not a circularity.

Assumptions & free parameters 4 free parameters · 4 assumptions · 0 invented entities

The central argument rests on the fixed Zhao threshold (0.145), the choice of k=7 for k-means, the unproven assumption that the k-means cluster labels correspond to true coronal hole wind, and the physical assumption that coronal hole wind has high speed and temperature. These are taken from prior literature or from the authors' own modeling choices, not derived in this paper.

free parameters (4)
  • Zhao et al. O7+/O6+ threshold = 0.145
    Fixed threshold from Zhao et al. (2009) that the paper argues is mismatched to solar minimum conditions; the central claim depends on this value being the actual threshold.
  • k (number of clusters in k-means) = 7
    Chosen based on prior work by the same group (Heidrich-Meisner & Wimmer-Schweingruber 2018); the two CH clusters and their interpretation depend on this choice.
  • Xu & Borovsky decision boundaries
    Fixed decision boundaries from Xu & Borovsky (2015) are taken as given; the comparison depends on their exact values, which are not restated in this paper.
  • Minimum Fe ion count = 10
    Threshold for including the Fe charge state average; paper states results are robust to this choice, but it still introduces a selection bias toward denser wind.
assumptions (4)
  • domain assumption Coronal hole wind is characterized by high proton speed, low density, high temperature, and low O7+/O6+ ratio.
    Used to label k-means clusters CH1/CH2 as coronal hole wind and to argue the Zhao et al. low-speed component is slow wind. Stated in Section 3 and Figure 2.
  • domain assumption The O7+/O6+ ratio varies systematically with the solar activity cycle.
    Cited to Zurbuchen et al. (2002); Zhao et al. (2009); Shearer et al. (2014). The core explanation relies on this variation.
  • domain assumption ACE near the ecliptic connects primarily to low-latitude coronal holes.
    Used in Section 4.3 to argue the low fraction in 2009 is plausible because few low-latitude coronal holes were observed (Fujiki et al. 2016).
  • ad hoc to paper The k-means classification is a valid reference for solar wind categorization.
    The paper uses its own k-means model as a data-driven reference; no ground truth exists (Section 1), so this is an assumption specific to the paper's argument.

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Cite this review

Pith. "Pith review of Challenges in identifying the coronal hole wind." pith.science (2026). https://pith.science/paper/AMNWPUXC

@misc{pith2026250102935,
  author       = {Pith},
  title        = {Pith review of: Challenges in identifying the coronal hole wind},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/AMNWPUXC}},
  note         = {Machine review of arXiv:2501.02935}
}
read the original abstract

Solar wind is frequently categorized based on its respective solar source region. Two well-established categorizations of the coronal hole wind, the scheme based on the charge-state composition, and the scheme based on proton plasma, identify a very different fraction of solar wind in the data from the Advanced Composition Explorer (ACE) as coronal hole wind during the solar activity minimum at the end of solar cycle 24. We investigate possible explanations for the different identifications of the coronal wind in 2009 in the scheme based on the charge-state composition (almost only coronal hole wind) and in the scheme based on the proton plasma (almost no coronal hole wind at the same time). We compared the properties of the respective coronal hole wind types and their changes with solar activity cycle in 2001- 2010. As a comparison reference, we included the coronal hole wind as identified by an unsupervised machine-learning approach, k-means, in our analysis. We find that the scheme based on charge-state composition likely misidentifies some slow solar wind as coronal hole wind during the solar activity minimum. The k-means classification we considered includes two types of coronal hole wind, the first of which is dominant during the solar activity maximum, whereas the second is dominant during the solar activity minimum. A low fraction of coronal hole wind from low-latitude coronal holes observed by ACE in 2009 is plausible because during this time period, a very small number of low-latitude coronal holes was observed. The results imply that the origin-oriented solar wind classification needs to be revisited, and they also suggest that an explicit inclusion of the phase of the solar activity cycle can be expected to improve the classification of the solar wind.

Figures

Figures reproduced from arXiv: 2501.02935 by the authors.

Figure 1
Figure 1. Fractions of solar wind types from three solar wind classifications per three Carrington rotations. From left to right: a) [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. 1-dimensional histograms of solar wind properties for five [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. 1-dimensional histograms of solar wind properties over time. Each 1-d histogram represents three consecutive Carrington rotations. [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Averaged SHAP values per three Carrington rotations over [PITH_FULL_IMAGE:figures/full_fig_p009_4.png]

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