{"id":"ae22cea9-fe76-425a-8038-f237dbedc979","arxiv_id":"1908.07509","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"HAWC observations from 2016 to 2018 show the 10-200 TeV cosmic-ray Sun Shadow shrinking during the solar cycle 24 decline, with a linear dependence on the median photospheric magnetic field below 8 G.","lead":"Sky maps from the HAWC observatory show that the Sun's shadow, a deficit of ultra-high-energy cosmic rays, weakened from 2016 to 2018 as the solar cycle declined. The size of this shadow appears to track the Sun's photospheric magnetic field, which may allow TeV cosmic rays to be used as a remote probe of solar magnetism.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The solar origin of the Sun-shadow decline is not secured without a same-pipeline Moon-shadow control; detector/atmospheric drift could mimic the trend.","rationale":"The reader's weakest assumption is exactly the most load-bearing weak point: the claim that A_RI declines and correlates with the photospheric field requires that the analysis system be stable. I agree. There is no Moon-shadow control in the paper, and HAWC's \"standard procedures\" (Sec. 2) are not a substitute for a same-pipeline reference source. The paper does provide positive evidence: long integration gives high signal-to-noise, the fitted width and centroid cuts are applied, and the simulation in Sec. 4 is a parameter-light illustration; these support, but do not secure, the solar origin. Secondary concerns include the lack of uncertainties on the quoted slopes and the post-hoc 8 G validity threshold, but these would matter less if the Moon-shadow test passes. If the Moon shadow is stable, the linear correlations with the photospheric field, although still needing error bars and a named magnetogram product, become plausible; if the Moon shadow drifts, the headline claim falls. Thus the verdict remains CONDITIONAL pending this one control.","tokens_in":5355,"tokens_out":5704,"duration_ms":578067,"concrete_test":"Using the same event selection, map integration (one Carrington rotation and yearly), and Gaussian fitting used for the Sun, compute the Moon Shadow A_RI for 2016-2018 from HAWC data. If the Moon A_RI decreases at a rate comparable to the Sun-shadow rate of -0.013 to -0.015 yr^-1, the Sun-shadow trend is dominated by detector or atmospheric systematics and the magnetic correlations are spurious; if the Moon A_RI is consistent with a constant over the same period, the solar origin of the trend is supported.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that the Sun-shadow amplitude A_RI decreased from 2016 to 2018 and that this decrease is linearly tied to the photospheric magnetic field. The least secure link is the assumption that the year-to-year and Carrington-rotation-to-Carrington-rotation changes in A_RI are solar rather than instrumental. The analysis uses \"standard HAWC procedures\" (Sec. 2) and applies cuts on fitted width and centroid, but no reference source is analyzed with the same pipeline. HAWC is an air-shower array whose angular reconstruction, energy response, and atmospheric background can drift on annual timescales; because the Sun-shadow deficit is a small relative-intensity modulation, a slow change in detector acceptance, shower reconstruction quality, or the cosmic-ray background model could produce a spurious decreasing amplitude. The Moon produces the same type of cosmic-ray deficit without a solar magnetic field, so it is the natural control; its absence means the observed correlations with the photospheric field (slopes 1.46e-2 and -1.19e-2 G^-1, Sec. 3) could be correlations of the magnetogram with time, not with the actual shadow amplitude. Thus the paper does not yet exclude the null hypothesis of a stable solar shadow plus drifting detector.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents three years (2016–2018) of HAWC observations of the Sun Shadow (SS), the deficit of 10–200 TeV cosmic rays from the solar direction. It reports a decreasing trend in the relative intensity of the deficit (SS_RI) over time and claims a linear relationship between SS_RI and the median photospheric magnetic field in the active-region belt, with the opposite sign for the polar fields, valid only when the median field is below 8 G. A simple simulation of cosmic-ray deflection in a radial coronal magnetic field is used to argue that particles in the HAWC energy range are deflected by a few degrees.","tokens_in":5670,"tokens_out":4087,"duration_ms":43054,"significance":"If the empirical relationships hold, the paper would introduce a new remote-sensing diagnostic of solar magnetic fields using TeV cosmic-ray shadows, with the attractive feature of opposite-sign slopes for the toroidal (low-latitude) and poloidal (polar) field components. The use of Carrington-rotation integration to track the shadow on monthly timescales and the explicit separation of low- and high-latitude photospheric fields are valuable steps. However, the quantitative claims currently lack uncertainty estimates, goodness-of-fit information, and any control for instrument or atmospheric stability, so the significance is conditional on addressing these issues.","major_comments":[{"comment":"The central decreasing trend of SS_RI over 2016–2018 is presented without a control for detector or atmospheric stability. HAWC's angular reconstruction, acceptance, and background model can vary on annual timescales, and the Sun-shadow deficit is a small relative-intensity modulation. Because the Moon produces the same type of cosmic-ray deficit without a solar magnetic field, a same-pipeline Moon-shadow analysis is the natural control; its absence leaves open the possibility that the observed decline—and therefore the correlations in Sec. 3—are dominated by instrumental drift rather than solar physics. Please add such a control or otherwise demonstrate stability of the relevant HAWC quantities over the period.","section":"Sec. 2, Fig. 5"},{"comment":"The quoted linear rates, 1.46 × 10^-2 G^-1 for the active-region belt and -1.19 × 10^-2 G^-1 for the polar caps, are given without uncertainties, correlation coefficients, p-values, or goodness-of-fit statistics. These quantities are load-bearing for the paper's main claim, and the reader cannot assess whether the linearity is statistically significant or whether the slopes are determined to even one significant figure. Please report the full regression output, including confidence intervals and a chi-squared or similar measure, for both fits.","section":"Sec. 3, Fig. 8"},{"comment":"The validity range 'B < 8 G' is defined by excluding Carrington rotations 2179–2181, which the authors state depart from the linear relation. This is a post hoc selection that can artificially create a linear trend. Please show that the linear model is a good fit within the remaining range without the excluded points, and discuss whether the threshold is robust to small variations. For example, test whether the slopes and their significance change if the excluded set is expanded or contracted by one rotation.","section":"Sec. 3, Fig. 8"},{"comment":"The active-region latitude band is defined inconsistently: the abstract states -40° ≤ lat ≤ 40°, while Sec. 3 and the Summary use -30° to 30° (with the Summary also containing a typo '−30° ≥ Lat ≥ 30°'). Since the median field and the derived slope depend on the chosen band, please specify the exact band used for the fits and use it consistently throughout the paper.","section":"Abstract vs. Sec. 3"}],"minor_comments":[{"comment":"The uncertainty expression, ε = sqrt(ε_RI² + A_RI cos(9/2 θ_az + π)²), is unclear: the text below it refers to A0 and ε_fit as the fitted amplitude and its error, but these symbols do not appear in the formula. Please define all symbols and verify that the formula is written correctly.","section":"Sec. 2, uncertainty formula"},{"comment":"The statement that the SS_RI decreases at rates of -0.013 and -0.015 year^-1 is quoted without uncertainties or a measure of scatter. Please provide fit errors and, if possible, the reduced chi-squared for these linear fits.","section":"Sec. 2.1"},{"comment":"Figure 8 does not show the fitted lines or confidence bands for the linear relationships. Adding the best-fit lines and, where feasible, the excluded Carr_Rots (2179–2181) in a distinct color would make the claimed linearity and the 8 G threshold much easier to evaluate.","section":"Fig. 8"},{"comment":"The Larmor radius formula uses B in µG, but the simulation input Bphot is described as being in the range 0.1 to 20 G. Please clarify the unit conversion used in the numerical integration.","section":"Sec. 4, Eq. (4.1)"},{"comment":"There are minor language issues, such as 'form 2016 to 2018' in the abstract and 'order quantify' in Sec. 2. A careful proofread would improve readability.","section":"Abstract and Sec. 1"}],"recommendation":"major_revision","confidential_remarks":"The paper is an ICRC2019 proceedings contribution and is relatively brief. The main technical gap is the missing Moon-shadow control; this is a standard and feasible addition for a cosmic-ray observatory and would substantially strengthen the case that the observed decline is solar. The missing error bars on the slopes are also straightforward to add. If the authors can supply these, the paper could become acceptable; without them, the central quantitative claims are not yet supported."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this ICRC proceeding reports a three-year decrease in the HAWC Sun Shadow deficit and opposite-sign linear correlations with photospheric field medians in two latitude bands—the first time I've seen that quantified. The underlying dataset and the basic analysis are real, but the paper does not include a Moon-shadow control or any uncertainty on the fitted slopes, so the solar interpretation is plausible rather than proven.\n\nWhat's new and good: The Carrington-rotation time series from 2016-2018 shows a clear downward trend in the relative-intensity deficit, with rates of about -0.013 to -0.015 per year. Comparing to median photospheric field values is a sensible choice given how inhomogeneous active-region fields are. The opposite signs of the two correlations—positive with the active-region belt, negative with polar caps—make physical sense in the declining phase and are a genuinely new observational result if they hold. The simple trajectory simulation showing few-degree deflections provides a rough plausibility check.\n\nThe soft spots are real, though not fatal. First, the linear slopes (1.46e-2 and -1.19e-2 G^-1) are quoted without error bars, correlation coefficients, or goodness-of-fit values. Second, the 8 G validity threshold is defined by the same data that show the deviation from linearity, so it's not an independent test. Third, and most importantly, there is no reference source like the Moon shadow analyzed with the same pipeline to rule out a slow drift in HAWC's angular reconstruction, acceptance, or atmospheric background. Because the shadow deficit is a small relative-intensity modulation, a detector-related decrease over three years could masquerade as a solar signal. The paper's use of 'standard HAWC procedures' doesn't answer this.\n\nThese are addressable concerns, and the paper's own text doesn't claim to have addressed them. As a conference proceeding, the level of detail is typical, but as a journal claim about solar magnetic fields, the missing control matters. I'd encourage a referee to ask for the Moon-shadow analysis and full statistical reporting before accepting the solar origin.\n\nWho should read this: solar and heliospheric physicists interested in TeV cosmic-ray diagnostics, and HAWC collaborators. It's a useful dataset and an interesting empirical result, but it's not yet a settled measurement. I'd take it to a reading group as an example of an exciting preliminary claim held up by a simple control.\n\nFor peer review: I'd send it out rather than desk reject. The claim is clear and falsifiable, the dataset is real, and the missing control can be added. It deserves serious referee time.","headline":"A useful but undercontrolled dataset: the HAWC Sun Shadow shows a decreasing trend and opposite-sign field correlations, but without a Moon-shadow control or fit uncertainties the solar origin is not yet secured.","tokens_in":6165,"tokens_out":2912,"would_cite":false,"duration_ms":30344,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The Sun's cosmic-ray shadow weakened steadily from 2016 to 2018 and tracked the photospheric magnetic field linearly, with opposite signs between the active-region belt and polar caps.","keywords":["cosmic rays","Sun Shadow","solar cycle 24","photospheric magnetic field","HAWC observatory","TeV cosmic rays","solar magnetic field","cosmic ray deflection"],"falsifier":"Compare the Sun Shadow amplitude with the Moon Shadow amplitude measured with the same HAWC maps over the same time windows. The Moon Shadow is produced by the same detector-level cosmic-ray deficit, but the Moon has no magnetic field. If the Moon Shadow amplitude also declines at a similar rate, the reported Sun Shadow decrease and its field correlations are detector or atmospheric artifacts.","tokens_in":5099,"feed_emoji":"☀️","tokens_out":4899,"duration_ms":42647,"temperature":0.7,"pith_summary":"This paper uses three years of HAWC data (2016-2018) to measure the Sun Shadow, the deficit of 10-200 TeV cosmic rays arriving from the Sun's direction. It reports that the shadow's relative intensity decreases steadily during the declining phase of solar cycle 24. The amplitude of the deficit correlates linearly with the median photospheric magnetic field in the active-region belt (−40° to 40° latitude) and anti-correlates linearly with the polar field (|lat| ≥ 60°), with slopes +1.46 × 10⁻² G⁻¹ and −1.19 × 10⁻² G⁻¹, valid only when the median field is below 8 G. A simple simulation argues that such high-energy cosmic rays are bent by only a few degrees in the coronal field, so the observed shadow is a direct probe of the near-Sun magnetic environment.","feed_headline":"Sun shadow shrinks as solar cycle fades, HAWC finds","feed_subtitle":"As the Sun calms, its cosmic-ray shadow fades in lockstep with the changing magnetic field.","key_machinery":"The central object is the Sun Shadow relative intensity $SS_{RI}$, obtained by fitting a circular 2D gaussian $F(x,y) = A_0 + A_{RI} \\exp(-(((x-C_x)/W_x)^2 + ((y-C_y)/W_y)^2)/2)$ to HAWC maps of cosmic-ray deficit in the solar direction. $A_{RI}$, the gaussian amplitude, is the time series analyzed. The comparison variable is the median photospheric magnetic field, computed per Carrington rotation in two latitude bands: the active-region belt (−40° to 40°) and polar caps (|lat| ≥ 60°). The argument also relies on the Larmor-radius formula $R_L = (3.3\\times 10^{12}\\ \\mathrm{cm}) \\times E(\\mathrm{GeV})/B(\\mu\\mathrm{G})$ with the coronal field modeled as $B_r = B_{\\mathrm{phot}}/(r - r_{\\mathrm{phot}})^2$ to compute deflection angles. These ingredients together convert a detector-level deficit map into a measurement of solar magnetic field evolution.","core_discovery":"The paper's central claim is that the relative intensity of the Sun Shadow, measured by the amplitude of a 2D gaussian fitted to HAWC sky maps, falls approximately linearly with time from 2016 to 2018 at a rate of about −0.013 to −0.015 per year. Comparing this amplitude with median photospheric magnetic field values per Carrington rotation, the paper finds a positive linear relationship with the active-region belt field and a negative linear relationship with the polar field, reflecting the transition from a multipolar to a dipolar heliospheric field topology. The authors state that these relationships hold only when the median field is below 8 G; during higher-activity rotations the linear correlation breaks. They also present trajectory simulations showing that 10-200 TeV protons are deflected by less than about 2 degrees in a spherically symmetric coronal magnetic field, which they interpret as the mechanism that produces the observed shadow.","pith_inferences":["The authors do not compare the Sun Shadow with the Moon Shadow; doing so would test whether the declining trend is truly solar, since the Moon Shadow carries the same detector-level signature without a magnetic field.","The reported slopes imply a sensitivity of roughly 1.5% shadow-amplitude change per gauss of active-region field and 1.2% per gauss of polar field; these numbers could be turned into a quantitative test of coronal magnetic-field models by predicting the shadow depth from magnetogram data.","A natural extension is to continue the same analysis through the cycle 24/25 minimum and into the next rising phase; the paper's 8 G threshold predicts that the linear relations should reappear only when the median field drops back below 8 G and then break again as activity rises.","Because the analysis uses only the gaussian amplitude, the fitted width and centroid (which the paper describes but does not use) may carry additional information about the angular structure of the deflection; reanalyzing those parameters in the same framework could sharpen the physical interpretation."],"forward_implications":["If the linear relation holds, the Sun Shadow amplitude measured per Carrington rotation can be used as a remote, continuous monitor of the median photospheric magnetic field during low-activity phases, providing a solar probe at TeV energies.","The opposite signs of the two slopes imply that the active-region belt and polar fields affect the cosmic-ray deficit in opposite ways, consistent with the claimed shift from a multipolar to a dipolar heliospheric field topology as the cycle declines.","The 8 G validity threshold means the linear calibration applies only in the declining and minimum phases; using it to infer field strengths during solar maximum or during active rotations would require a different model.","If 10-200 TeV cosmic rays are deflected by only a few degrees, as the simulation indicates, then the Sun Shadow is created very close to the Sun and is insensitive to interplanetary magnetic structures, distinguishing it from lower-energy cosmic-ray modulation signals."],"supporting_citations":[{"why":"Supplies the HAWC detector description and sensitivity that define the 10-200 TeV energy range.","marker":"[1]"},{"why":"Establishes the standard HAWC analysis procedures used to construct the sky maps.","marker":"[2]"},{"why":"Provides the physical picture of high-energy cosmic rays interacting with the low solar atmosphere and strong near-photospheric fields.","marker":"[6]"},{"why":"Describes the HAWC anisotropy analysis techniques used for the Sun Shadow maps.","marker":"[7]"},{"why":"Supplies the HEALPix framework used for high-resolution discretization and analysis of the sky maps.","marker":"[8]"},{"why":"Presents the earlier HAWC Sun Shadow analysis that this work extends to the 2016-2018 declining phase.","marker":"[9]"}],"fun_headline_variants":["HAWC sees sun shadow fade as solar cycle 24 winds down","Sun's cosmic-ray shadow shrinks with solar magnetic field","Cosmic ray sun shadow tracks solar magnetic field changes","HAWC tracks sun shadow decline with solar cycle 24","Sun shadow's linear drop tied to photospheric field"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The analysis takes the year-to-year decrease in the Sun Shadow amplitude at face value, assuming HAWC's angular reconstruction, acceptance, and atmospheric background model stayed stable from 2016 to 2018, without using a reference source such as the Moon Shadow to separate solar changes from detector drift.","fun_headline_variants_meta":{"raw":{"variants":["HAWC sees sun shadow fade as solar cycle 24 winds down","Sun's cosmic-ray shadow shrinks with solar magnetic field","Cosmic ray sun shadow tracks solar magnetic field changes","HAWC tracks sun shadow decline with solar cycle 24","Sun shadow's linear drop tied to photospheric field"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000577,"raw_usage":{"total_tokens":2768,"prompt_tokens":1037,"completion_tokens":1731,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":653,"completion_tokens_details":{"reasoning_tokens":1650}},"tokens_in":653,"tokens_out":1731,"duration_ms":11776,"temperature":1.0,"reasoning_tokens":1650,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T12:07:45.003721+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Compare the Sun Shadow amplitude with the Moon Shadow amplitude measured with the same HAWC maps over the same time windows. The Moon Shadow is produced by the same detector-level cosmic-ray deficit, but the Moon has no magnetic field. If the Moon Shadow amplitude also declines at a similar rate, the reported Sun Shadow decrease and its field correlations are detector or atmospheric artifacts.","supporting_citations":[{"cited_title":"U., and 104 colleagues 2014.\\ Sensitivity of HAWC to high-mass dark matter annihilations.\\ Physical Review D 190 , 122002","cited_arxiv_id":null,"evidence_quote":"Supplies the HAWC detector description and sensitivity that define the 10-200 TeV energy range."},{"cited_title":"U., and 106 colleagues 2017.\\ Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory.\\ The Astrophysical Journal 843 , 39","cited_arxiv_id":null,"evidence_quote":"Establishes the standard HAWC analysis procedures used to construct the sky maps."},{"cited_title":"K.\\ 1992.\\ Cosmic ray albedo -rays from the quiet Sun ..\\ NASA Conference Publication 542","cited_arxiv_id":null,"evidence_quote":"Provides the physical picture of high-energy cosmic rays interacting with the low solar atmosphere and strong near-photospheric fields."},{"cited_title":"U., and 102 colleagues 2018.\\ Observation of Anisotropy of TeV Cosmic Rays with Two Years of HAWC.\\ The Astrophysical Journal 865 , 57","cited_arxiv_id":null,"evidence_quote":"Describes the HAWC anisotropy analysis techniques used for the Sun Shadow maps."},{"cited_title":"M., and 6 colleagues 2005.\\ HEALPix: A Framework for High-Resolution Discretization and Fast Analysis of Data Distributed on the Sphere.\\ The Astrophysical Journal 622 , 759","cited_arxiv_id":null,"evidence_quote":"Supplies the HEALPix framework used for high-resolution discretization and analysis of the sky maps."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Presents the earlier HAWC Sun Shadow analysis that this work extends to the 2016-2018 declining phase."}],"review_version":1}