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REVIEW 3 major objections 3 minor 2 references

The Extreme Space Weather Event of 1872 February: Sunspots, Magnetic Disturbance, and Auroral Displays

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

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Pith's one-line read The 1872 February storm belongs with the Carrington event and the 1921 storm as one of the three largest magnetic storms ever recorded.

desk verdict Solid archival synthesis; the 1872 storm was certainly extreme, but the top-three ranking leans on a single-station Dst proxy that the paper does not fully bound. read the letter →

arxiv 2501.00176 v1 pith:OT2FHPSF submitted 2024-12-30 astro-ph.SR astro-ph.EPphysics.geo-phphysics.plasm-phphysics.space-ph

Hisashi Hayakawa , Edward W. Cliver , Frédéric Clette , Yusuke Ebihara , Shin Toriumi , Ilaria Ermolli , Theodosios Chatzistergos , Kentaro Hattori
show 14 more authors
Delores J. Knipp Séan P. Blake Gianna Cauzzi Kevin Reardon Philippe-A. Bourdin Dorothea Just Mikhail Vokhmyanin Keitaro Matsumoto Yoshizumi Miyoshi José R. Ribeiro Ana P. Correia David M. Willis Matthew N. Wild Sam M. Silverman
This is my paper · ORCID
classification astro-ph.SRastro-ph.EPphysics.geo-phphysics.plasm-phphysics.space-ph
keywords solaractiveregionsflarescoronalmassejectionsauroraegeomagneticfieldsmagneticstormshistoricalspaceweather
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

The paper sets out to establish that the magnetic storm of 1872 February 4 is one of the three largest ever observed, comparable to the Carrington storm of 1859 September and the storm of 1921 May. It assembles a full case from contemporary sunspot drawings, magnetograms at Colaba, Greenwich, Tiflis, and Havana, and a worldwide collection of auroral reports. The storm began with a sudden commencement at about 14:27 UT, and the Colaba magnetogram yields a conservative minimum Dst* of at most -834 nT. Overhead aurorae at Jacobabad and Shanghai place the equatorward auroral boundary near 24.2 degrees invariant latitude, and 13 locations below 20 degrees magnetic latitude reported aurorae, versus one each for the 1859 and 1921 storms. If correct, the event becomes a benchmark for extreme space weather and shows that moderately sized sunspot groups can drive Carrington-class storms.

What carries the argument

The argument is carried by two linked quantities: the single-station Dist H value from the Colaba (Bombay) magnetogram, used in place of the four-station global Dst index, and the equatorward boundary of the overhead aurora expressed as invariant latitude, inferred from reported elevation angles under the assumption that auroral emission reaches about 400 km along field lines. The Colaba trace gives a conservative minimum of Dst* <= -834 nT, while the overhead aurora at Jacobabad and Shanghai puts the oval at about 24.2 degrees ILAT. A secondary mechanism is the sunspot-drawing record, which identifies a complex sunspot group near disk center and yields a roughly 29-hour transit time from the inferred eruption to the sudden commencement. These two independent strands, magnetic intensity and auroral extension, are then compared against the same quantities for the 1859 September and 1921 May storms.

What would settle it

A multi-station Dst* reconstruction for 1872 February 4-5, or a second intact low-latitude magnetogram from that night, that yielded a minimum above about -800 nT would overturn the central intensity claim; likewise, showing that the Jacobabad and Shanghai overhead aurorae were low-altitude features or mislocated would remove the 24.2 degrees ILAT boundary and weaken the comparison with 1859 and 1921.

Watch

Extended reading notes

Core claim

The paper's central discovery is that the Chapman-Silverman storm of 1872 February 4 belongs in the top tier of recorded magnetic storms. From the Colaba (Bombay) magnetogram the paper derives a conservative minimum Dst* of at most -834 nT, with a 13-minute data gap at the likely minimum caused by inserting a deflector magnet, so the true value is probably deeper; the Tiflis trace suggests an even larger disturbance. Overhead aurorae reported at Jacobabad and Shanghai imply an auroral oval reaching about 24.2 degrees invariant latitude, lower than the 25.1 and 27.1 degrees estimated for the 1859 and 1921 storms. The storm also produced the largest set of low-latitude auroral sightings, 13 sites below 20 degrees magnetic latitude including a credible report from Bombay at 10.0 degrees MLAT, and intense earth currents at Bombay and Khartoum. The paper ties the storm to a complex, moderate-area sunspot group (Group #29 in the contemporary drawings) located near S19 E05, and infers the responsible eruption at 9-10 UT on February 3, about 29 hours before the sudden commencement.

Load-bearing premise

The ranking of the 1872 storm as Carrington-class rests on treating the single-station Colaba H-disturbance, computed with a pre-storm baseline and a monthly quiet-day variation, as a proxy for the global Dst index, despite a 13-minute gap at the likely minimum.

Editorial extensions

If this is right

  • Extreme storm scenarios should include 1872 alongside 1859 and 1921 as a reference event for grid impact and satellite risk.
  • Moderate-size active regions, not just very large sunspot groups, must be treated as capable of producing Carrington-class storms.
  • Low-latitude auroral reports below 20 degrees magnetic latitude can serve as a sparse but global intensity indicator for historical storms.
  • The earth-current reports at Bombay and Khartoum imply that storms of this class can drive geomagnetically induced currents at magnetic latitudes near 10-13 degrees, far below typical GIC latitudes.
  • Because the 1872 storm occurred during the declining phase of a large solar cycle, the paper supports the empirical association between declining phases and extreme storms.

Reading between the lines

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

  • If three Carrington-class storms appear in less than a century of instrumental records, the long-run occurrence rate of such events may be higher than estimates drawn from the space age alone.
  • The Colaba gap and the deeper Tiflis Dist H suggest the true global minimum could be well below -834 nT; a dedicated digitization of other contemporary low-latitude stations could test this.
  • The same method, combining single-station Dst proxies with auroral ILAT boundaries, could systematically re-rank other nineteenth-century storms for which comparable archives exist.
  • Aden at 8.3 degrees MLAT remains a single-source report; independent confirmation would push the auroral boundary even further equatorward, while refutation would test the credibility chain connecting Bombay, Khartoum, and the other low-latitude sightings.
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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 / 3 minor

Summary. Hayakawa et al. re-examine the extreme geomagnetic storm of 1872 February 4 using newly digitized archival magnetograms from Greenwich, Colaba, Tiflis and Havana, together with extensive auroral reports from Europe, Asia, Africa and the Americas. They identify the likely solar source (Secchi Group #29), infer a ~29 hr transit time for the associated ICME, estimate a minimum Dst* of ≤ -834 nT from the Colaba Dist H time series, triangulate the equatorward auroral boundary to about 24.2° ILAT, and assemble 13 low-latitude (<|20°| MLAT) auroral reports. They conclude that the 1872 storm ranks among the three largest magnetic storms in observational history, comparable to 1859 September and 1921 May.

Significance. The paper's strengths are its exhaustive archival work, the careful source-by-source evaluation of auroral reports (including new evidence for Aden, Khartoum, Mecca and Bombay), the cross-checking of Colaba and Tiflis magnetograms, and the explicit caveats about the data gap and single-station limitation. The reconstructed auroral extension and the collection of low-latitude reports are important contributions in their own right, independent of the intensity ranking. If the central Dst* estimate can be made robust to a quantified sensitivity analysis, the paper would provide a valuable benchmark for extreme space weather climatology.

major comments (3)
  1. [Section 3.2] The central ranking claim rests on the identification of the Colaba Dist H value (≤ -834 nT) with the global Dst* minimum. Because Dst is designed to remove local-time-dependent partial ring current, DP2, and ionospheric contributions by averaging four stations, a single-station estimate at 18:30 UT near local midnight is not automatically conservative. The paper invokes DP2 to explain the Greenwich trace but applies no analogous correction to Colaba, and the Tiflis Dist H (-1402 ± 62 nT) is set aside as affected by overhead aurora, so the two-station information is not used to bound the asymmetric contribution. The authors should provide a sensitivity analysis varying the pre-storm baseline, the Sq subtraction (Moos 1910 monthly variation), and a plausible range of non-ring-current contamination, and quote the resulting range for the true Dst* before the 'comparable to 1859/1921' statement can be considered established.
  2. [Section 5 and Table 2] The '13 versus one' low-latitude auroral count is presented as evidence of storm intensity, but the paper does not control for uneven archival coverage among the three epochs. The 1859 and 1921 surveys have historically been concentrated in different regions and source types (e.g., ship logs, colonial observatories), and the number of surviving reports depends on population, scientific infrastructure, and prior compilation effort. The statement in Section 6 that the 13 reports 'provide additional evidence' therefore needs a caveat that the raw counts are not completeness-normalized; at most, the count indicates that the 1872 storm was well observed in densely populated regions, which is not a direct intensity measure.
  3. [Section 6 and Summary] The paper's use of the Yokoyama et al. (1998) auroral-boundary–Dst relation to infer about -1250 nT should not be blended with the measured Colaba estimate. The text already notes the three-point limitation, but the Summary and Section 7 state that the measured Dst* places the event in the 'minimum Dst* ≥ -800 nT' category; this wording is defensible, yet the near-equality with the 1859 and 1921 estimates should be presented as a lower bound with a quantified uncertainty from the single-station analysis, not as a point estimate.
minor comments (3)
  1. [Abstract and Section 3.2] The less-than-or-equal symbols appear as '£' and '¤' in several places (e.g., Abstract, Section 3.2, Section 6); these should be typeset as '≤' in the published version.
  2. [Section 3.2] The text says 'we have further followed Dst calculation procedures to quantify the storm magnitude and time series in the Dst estimate (Dst*)', but the Dst formula from Sugiura (1964) is not written out; a brief equation or an explicit statement of the latitudinal weighting used would help readers understand the exact scaling applied to the Colaba Dist H.
  3. [Table 2] Table 2 lists multiple entries for Bombay (seven separate references) and single entries for other sites; the table caption should clarify whether the rows represent independent reports or multiple records for the same site, as this affects the interpretation of the '13 sites' count.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: the Dst* lower bound and auroral ILAT are independent observational reconstructions; self-citations are archival cross-references, not load-bearing feed-back loops.

full rationale

The paper's central quantitative claim is a lower bound on storm intensity from the Colaba magnetogram (Dist H <= -834 nT) and an equatorward auroral boundary (24.2 ILAT) from elevation-angle reports at Shanghai and Jacobabad. These are two independent observational streams; neither is defined in terms of the other. The paper openly states that Dst* is approximated by single-station Dist H ('we approximate the Dst estimate (Dst*) with Dist H, as it is extremely difficult to locate four magnetograms from mid/low-latitude regions for this storm'), which is a data-limitation caveat, not a circular reduction: the -834 nT value comes from a digitized historical trace with baseline and Sq subtraction, not from the 1859/1921 Dst values it is compared against. The comparison values -949 +/- 31 nT and -907 +/- 132 nT are taken from prior publications that include some overlapping authors, but those are standalone archival reconstructions of other storms, and this paper adds no fitted parameter that forces the 1872 ranking. The aurora-Dst empirical relation from Cliver et al. (2022a) is invoked only as a bracketing cross-check, and the paper itself cautions that it has only three extreme-storm data points. Similarly, the ILAT used to test visibility at Aden and Khartoum is derived from Shanghai and Jacobabad and is used only for plausibility, not as an independent confirmation of the boundary. No equation in the paper reduces a prediction to its input; no fitted quantity is renamed as a prediction. The self-citations are archival results (e.g., Hayakawa et al. 2018, 2022) and do not carry the load of the central claim. Hence no significant circularity.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The central ranking claim rests on no new physical entities, but it depends on two numerical modeling choices (400 km emission altitude and the GUFM1 field model), a single-station Dst approximation, and qualitative credibility judgments about historical reports. None of these are fitted to the target storm, which keeps the analysis non-circular, but each carries real uncertainty.

free parameters (2)
  • Auroral emission altitude = 400 km
    Assumed for converting elevation-angle reports to ILAT (Section 4.1, following Roach et al. 1960 and Silverman 1998). Not fitted in this paper; lower or higher altitudes shift the inferred equatorward boundary.
  • Pre-storm baseline and monthly Sq variation = Pre-storm level and Moos (1910) monthly H diurnal curve
    Used to compute Dist H at Colaba and Tiflis (Section 3.2); a different baseline or Sq choice would change the Dst* estimate.
assumptions (5)
  • domain assumption Auroral emissions extend up to 400 km altitude along magnetic field lines for converting elevation reports to invariant latitude
    Invoked in Section 4.1, following Roach et al. (1960) and Silverman (1998); the ILAT boundary of 24.2 degrees depends on this height.
  • domain assumption The GUFM1 geomagnetic field model gives reliable MLAT and ILAT coordinates for 1872
    Used for all site magnetic coordinates (footnote 25, Table 1); an incorrect historical field model would shift the auroral boundaries.
  • domain assumption Single-station Dist H at Colaba can approximate the Dst index for this storm
    Section 3.2 approximates Dst* with Colaba Dist H because four-station data are unavailable; local current contamination or an incorrect Sq baseline would change the central intensity estimate.
  • domain assumption Historical auroral reports can be assigned credibility from descriptive detail, corroboration, and geometry
    Sections 4 and 5 select credible reports by these criteria; this selection determines the low-latitude site count and the ILAT boundary.
  • domain assumption Limb prominence activity on 1872 February 3 was related to the eruptive flare from Secchi Group 29 that produced the storm
    Section 2.3 explicitly says there is no direct evidence of a flare; the association is inferred from location, timing, and transit time.

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Cite this review

Pith. "Pith review of The Extreme Space Weather Event of 1872 February: Sunspots, Magnetic Disturbance, and Auroral Displays." pith.science (2026). https://pith.science/paper/OT2FHPSF

@misc{pith2026250100176,
  author       = {Pith},
  title        = {Pith review of: The Extreme Space Weather Event of 1872 February: Sunspots, Magnetic Disturbance, and Auroral Displays},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/OT2FHPSF}},
  note         = {Machine review of arXiv:2501.00176}
}
read the original abstract

We review observations of solar activity, geomagnetic variation, and auroral visibility for the extreme geomagnetic storm on 1872 February 4. The extreme storm (referred to here as the Chapman-Silverman storm) apparently originated from a complex active region of moderate area (\approx 500 {\mu}sh) that was favorably situated near disk center (S19{\deg} E05{\deg}). There is circumstantial evidence for an eruption from this region at 9--10 UT on 1872 February 3, based on the location, complexity, and evolution of the region, and on reports of prominence activations, which yields a plausible transit time of \approx29 hr to Earth. Magnetograms show that the storm began with a sudden commencement at \approx14:27 UT and allow a minimum Dst estimate of {\pounds} -834 nT. Overhead aurorae were credibly reported at Jacobabad (British India) and Shanghai (China), both at 19{\deg}.9 in magnetic latitude (MLAT) and 24{\deg}. 2 in invariant latitude (ILAT). Auroral visibility was reported from 13 locations with MLAT below |20|{\deg} for the 1872 storm (ranging from |10{\deg}. 0|--|19{\deg}. 9| MLAT) versus one each for the 1859 storm (|17{\deg}. 3| MLAT) and the 1921 storm (|16.{\deg}2| MLAT). The auroral extension and conservative storm intensity indicate a magnetic storm of comparable strength to the extreme storms of 1859 September (25{\deg}.1 \pm 0{\deg}.5 ILAT and -949 \pm 31 nT) and 1921 May (27{\deg}.1 ILAT and -907 \pm 132 nT), which places the 1872 storm among the three largest magnetic storms yet observed.

Figures

Figures reproduced from arXiv: 2501.00176 by the authors.

Figure 1
Figure 1. Solar activity around the time of the Chapman–Silverman storm: The top panel shows the daily international sunspot number (Version 2; Clette & Lefèvre 2016) time series maintained by the World Data Centre sunspot Index and Long-term Solar Observations (Source: World Data Centre SILSO). The other panels show counts of sunspot groups, individual spots, pores, faculae, and prominences, as well as corrected areas of sun… view at source ↗
Figure 2
Figure 2. ) had areas of 166 μsh and 111 μsh, respectively. The “large group” in Meldrum (1872a) was almost certainly Secchi’s Spot Group #29 in the southern hemisphere, and the “chain of sunspots” that Meldrum (1872a) mentioned corresponds to Spot Groups numbered 28, 33, 34, 27, 26, and 32 by Secchi in the northern hemisphere. Because the major magnetic storm occurred on February 4, the responsible flare and eruption likely … view at source ↗
Figure 3
Figure 3. Tacchini’s full-disk drawings made on 1872 February 1–2 (Source: Osservatorio Astrofisico di Catania, MS INAF 2, ff. 34 and 36). 24 This sunspot group and/or an associated coronal hole may have been recurrent for 3 solar rotations, as indicated from recurrent auroral activity from early 1872 January to early 1872 March (Bhaskar et al. 2020). 5 The Astrophysical Journal, 959:23 (20pp), 2023 December 10 Hayakawa et al… view at source ↗
Figures from the paper (10 more)
Figure 4
Figure 4. Figure 4: Bernaerts’ sunspot drawings on 1872 February 2–3 (Source: RAS MS Bernaerts, v.3, ff. 25–26; courtesy of the Royal Astronomical Society) [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]
Figure 5
Figure 5. Figure 5: Temporal variations of magnetic field strengths at Greenwich in horizontal (H) and vertical (Z) components for the magnetic disturbances on 1872 February 4–5, which are digitised from Sheets 2 and 3 of Royal Observatory Greenwich (1872). 6 The Astrophysical Journal, 95…
Figure 6
Figure 6. Figure 6: Digitization of the Colaba magnetogram on 1872 February 4–5 from Basu (1954, p. 50). For the blue curve showing the tabulated H data, the trace after the data gap was adjusted from that in Basu (1954, p. 50) to remove an offset that was caused by the insertion of a def…
Figure 7
Figure 7. Figure 7: These measurements are important because Tiflis is located approximately midway on a great circle line from Colaba to Greenwich. While the temporal evolution of the H￾disturbance in the Tiflis magnetogram is similar to that in the Colaba magnetogram, the maximum amplit…
Figure 9
Figure 9. Figure 9: in Hayakawa [PITH_FULL_IMAGE:figures/full_fig_p009_9.png]
Figure 8
Figure 8. Figure 8: Auroral visibility on 1872 February 4–6. Each red dot indicates a location for which auroral visibility was reported. The contour lines indicate the magnetic latitude (MLAT) with an interval of 10°, which are based on the magnetic poles for 1872 determined by the model…
Figure 9
Figure 9. Figure 9: MLAT of auroral visibility λ0, MLAT of the topside of aurora λ and ILAT of the aurora λi (reproduced from [PITH_FULL_IMAGE:figures/full_fig_p010_9.png]
Figure 10
Figure 10. Figure 10: A Japanese drawing showing an auroral display seen at Okazaki (N37°54, E137°10, 24°. 4 MLAT) for the 1872 February 4 storm, which is preserved in the Shounji Temple, as reproduced from Hayakawa et al. (2018), with permission of the Shounji Temple. 11 The Astrophysical…
Figure 11
Figure 11. Figure 11: shows the auroral visibility on 1872 February 4–5 as a function of magnetic latitude and time, with sites of auroral visibility ranging from 10° to 68°. The aurora was first noted by several observers near the SC time of ≈14:27 UT (as was the case for the 1989 March s…
Figure 12
Figure 12. Figure 12: Corona aurorae witnessed in Barnstaple (Hall 1872, Frontispiece) and Guildford (Capron 1879, Plate IV) on 1872 February 4. 26 Von Gumpach was a professor of mathematics and astronomy at Peking (Beijing) College from 1866 to 1868, and then moved to Shangai until his de…

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Works this paper leans on

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