Solar Cycle Variation of Sustained Gamma Ray Emission from the Sun

G. Sindhuja; Hong Xie; Nat Gopalswamy; Pertti M\"akel\"a; Sachiko Akiyama; Seiji Yashiro

arxiv: 2512.04360 · v3 · submitted 2025-12-04 · 🌌 astro-ph.HE · astro-ph.SR

Solar Cycle Variation of Sustained Gamma Ray Emission from the Sun

Nat Gopalswamy , Pertti M\"akel\"a , Seiji Yashiro , Sachiko Akiyama , Hong Xie , G. Sindhuja This is my paper

Pith reviewed 2026-05-17 02:04 UTC · model grok-4.3

classification 🌌 astro-ph.HE astro-ph.SR

keywords sustained gamma ray emissionsolar cycle 25solar cycle 24Fermi LATcoronal mass ejectionstype II radio burstshard X-ray bursts

0 comments

The pith

Sustained gamma-ray emission events occur more often in solar cycle 25 than in cycle 24, indicating greater solar activity now.

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

The authors track sustained gamma-ray emission events detected by the Fermi LAT instrument across two solar cycles. Direct observations show only 16 events in the first 61 months of cycle 25 compared with 27 in the matching period of cycle 24, but a spacecraft malfunction created large data gaps. Using the higher rates of fast wide coronal mass ejections and decameter-hectometric type II radio bursts observed in cycle 25, plus hard X-ray burst associations to fill gaps, they estimate a total of 43 to 50 events in cycle 25. This leads them to conclude that cycle 25 is stronger than cycle 24.

Core claim

The number of sustained gamma-ray emission events in solar cycle 25 is estimated at 43 to 50, exceeding the 27 events recorded in the corresponding epoch of solar cycle 24, after correcting for observational gaps with associations to fast wide CMEs, DH type II bursts, and long-duration hard X-ray bursts.

What carries the argument

Association rates between SGRE events and fast wide CMEs or DH type II bursts, supplemented by hard X-ray burst durations to identify likely events during LAT data gaps.

If this is right

Solar cycle strength can be compared using gamma-ray event counts in addition to sunspot numbers.
The current cycle should produce roughly 50 percent more SGRE events than the previous one when fully observed.
Higher SGRE numbers point to increased production of high-energy particles during cycle 25.
Future cycles can be monitored for activity level using the same combination of radio, X-ray, and gamma-ray signatures.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The method offers a way to assess solar activity even when direct gamma-ray observations are interrupted.
Similar gap-filling techniques could be applied to other high-energy solar phenomena in upcoming cycles.
The result suggests that particle acceleration efficiency may rise with overall cycle strength.

Load-bearing premise

The fraction of fast wide CMEs and DH type II bursts that produce SGRE events stays the same from cycle 24 to cycle 25, and hard X-ray bursts can accurately stand in for missing gamma-ray data.

What would settle it

A complete count of SGRE events throughout solar cycle 25 once the data gaps are resolved, or an independent measurement showing that the association rates have changed.

Figures

Figures reproduced from arXiv: 2512.04360 by G. Sindhuja, Hong Xie, Nat Gopalswamy, Pertti M\"akel\"a, Sachiko Akiyama, Seiji Yashiro.

**Figure 1.** Figure 1: Time-latitude plot of the solar sources of large SEP events and SGRE events in solar cycles 24 and 25. SEP events are marked by black circles. Filled circles represent ground level enhancement (GLE) events in SEPs. SEP events with SGRE are shown by red x symbols within the black circles. GLEs with SGRE are marked with white x. The blue x symbols denote SGRE events not associated with SEPs. The data points … view at source ↗

read the original abstract

We investigated the occurrence rate of the sustained gamma ray emission (SGRE) events from the Sun using data obtained by Fermi Large Area Telescope (LAT) since its launch in 2008. Only 16 SGRE events were observed during the first 61 months of solar cycle (SC) 25, likely due to the solar array drive assembly's malfunction in 2018; 27 SGRE events were observed in SC 24 over the corresponding epoch. The average sunspot number (SSN) increased from 56.9 in SC 24 to 79.0 in SC 25. Fast and wide (FW) CMEs and decameter-hectometric (DH) type II bursts increased significantly in SC 25 by 29% and 33%, respectively when normalized to SSN. Therefore, we expect a higher number of SGREs in SC 25. We estimated the number of SGREs in SC 25 using three methods. (i) If the SGRE number varies commensurate with SSN, we should have 38 SGRE events in SC 25. However, FW CMEs and DH type II bursts in SC 25 were overabundant by 29% and 33%, so the number SGRE events should be 48 or 50. (ii) In SC 24, ~18% of FW CMEs and 27% of DH type II bursts were associated with SGRE events. At this rate SC 25 should have 48 and 49 SGRE events. (iii) Since SGRE events are invariably associated with >100 keV hard X-ray (HXR) bursts, we identified DH type II bursts associated with >100 keV HXR bursts from Fermi's Gamma ray Burst Monitor (GBM) during LAT data gaps. Almost all SGRE events in SCs 24 and 25, and 27 of the 79 LAT-gap type IIs were associated with HXR bursts of duration > ~5 min. These DH type II bursts are indicative of SGRE, bringing the total number of SGRE events to 43 (16 + 27). Thus, the three methods provide similar estimates of the number of SGRE events in SC 25. We, therefore, conclude that SC 25 is stronger than SC 24 based on the estimated number SGRE events.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper estimates 43-50 SGRE events for early SC25 versus 27 in SC24 and concludes the cycle is stronger, but this rests on carrying over association rates from the prior cycle and an unvalidated gap proxy.

read the letter

Hi, the main thing to know is that this paper estimates 43 to 50 sustained gamma-ray events in the first 61 months of solar cycle 25, against 27 in the matching stretch of cycle 24, and uses that to argue SC25 is the stronger cycle. They observed only 16 events directly, blaming the solar array malfunction for LAT gaps, then filled in the rest with three scaling methods. One ties the count to the rise in sunspot number. The second applies the 18 percent SGRE association rate with fast wide CMEs and the 27 percent rate with DH type II bursts that held in SC24. The third treats 27 of the 79 gap-period DH type II bursts that also show long hard X-ray bursts as likely SGRE events, bringing the total to 43. The three approaches land close together, which is a reasonable internal check, and the authors correctly note that normalized CME and type II rates are also up in SC25. That part is straightforward and uses the new early-cycle data. The soft spots sit in the assumptions. The association fractions come straight from SC24 and get applied without any test of whether the underlying conditions changed. On the gap proxy, the text says almost all known SGRE events had those long HXR bursts, but it gives no reverse efficiency: what share of DH type II plus long-HXR events actually produce detectable SGRE when LAT data are available. If that fraction is well below one, the added 27 events are inflated and the numerical edge for SC25 shrinks. The abstract also skips error bars and details on gap handling. This work is mainly for solar physicists who track cycle strength through gamma-ray and radio proxies or who need updated counts for space-weather modeling. A reader already following SGRE studies or multi-indicator cycle comparisons will get usable numbers from it, though the advance is incremental rather than foundational. I would send it to peer review. The new observations and the cross-method comparison are worth referee time, but the authors need to add uncertainty estimates and check the proxy false-positive rate before the central claim can be taken as firm.

Referee Report

3 major / 2 minor

Summary. The paper analyzes Fermi LAT observations of sustained gamma-ray emission (SGRE) events from the Sun, reporting 16 events in the first 61 months of solar cycle 25 (affected by a 2018 solar array malfunction) versus 27 in the corresponding SC 24 epoch. It notes increases in fast-wide CMEs (29%) and DH type II bursts (33%) normalized to sunspot number in SC 25, then applies three estimation methods—scaling with SSN, using SC 24 association fractions (18% for FW CMEs, 27% for DH type IIs), and proxy identification via long-duration HXR bursts during LAT gaps—to arrive at 43–50 SGRE events in SC 25, concluding that SC 25 is stronger than SC 24.

Significance. If the estimates hold, the result would indicate that SGRE production tracks the overabundance of FW CMEs and DH type II bursts relative to SSN in SC 25, offering a diagnostic for cycle-to-cycle differences in high-energy particle acceleration. A strength is the convergence of the three methods on similar totals despite different inputs; this internal consistency is explicitly noted and provides some robustness under the stated assumptions.

major comments (3)

[Abstract] Abstract / Estimation methods (i) and (ii): The prediction of 48–49 SGRE events applies the SC 24 association rates (~18% of FW CMEs and 27% of DH type II bursts) directly to SC 25 counts. This step assumes the association fractions are cycle-independent, but the manuscript provides no test, discussion, or justification for invariance between SC 24 and SC 25; the assumption is load-bearing for the claim of a higher SGRE rate in SC 25.
[Abstract] Abstract / Method (iii): The addition of 27 indicative events (from 79 LAT-gap DH type II bursts matched to >100 keV HXR bursts lasting >~5 min) to reach a total of 43 relies on the statement that “almost all SGRE events … were associated with HXR bursts of duration > ~5 min.” No converse efficiency or false-positive rate is reported (i.e., the fraction of such DH type II + long-HXR events that actually produce detectable SGRE when LAT coverage is present). If this fraction is appreciably below unity, the added count is inflated and the numerical excess used to declare SC 25 stronger is correspondingly weaker.
[Abstract] Abstract: The final estimates (43–50 events) and the direct comparison to SC 24’s 27 events are presented without uncertainty ranges, error propagation on the association percentages, or sensitivity tests for data-gap handling. This omission makes it difficult to evaluate whether the reported excess is statistically significant.

minor comments (2)

[Abstract] Abstract: Typographical issue—“the number SGRE events” should read “the number of SGRE events.”
[Abstract] Abstract: The exact start and end dates defining the “first 61 months” epochs for SC 24 and SC 25 are not stated, complicating direct comparison of the raw counts (16 vs. 27).

Simulated Author's Rebuttal

3 responses · 1 unresolved

We thank the referee for the constructive and detailed comments. We address each major comment below, indicating revisions where the manuscript will be updated to improve clarity and robustness.

read point-by-point responses

Referee: [Abstract] Abstract / Estimation methods (i) and (ii): The prediction of 48–49 SGRE events applies the SC 24 association rates (~18% of FW CMEs and 27% of DH type II bursts) directly to SC 25 counts. This step assumes the association fractions are cycle-independent, but the manuscript provides no test, discussion, or justification for invariance between SC 24 and SC 25; the assumption is load-bearing for the claim of a higher SGRE rate in SC 25.

Authors: We acknowledge that methods (i) and (ii) rely on applying the SC 24 association fractions to SC 25 without an explicit test of cycle-to-cycle invariance. Because SC 25 remains incomplete, a direct empirical test is not yet possible with the current dataset. The assumption is motivated by the physical linkage between SGRE and the same FW CMEs and DH type II bursts whose normalized rates are observed to be higher in SC 25. We will revise the text to state this assumption explicitly, discuss its implications for the estimated excess, and note that full-cycle data will eventually allow a test. We will also add a brief sensitivity analysis showing how plausible variations in the association fraction affect the final range. revision: yes
Referee: [Abstract] Abstract / Method (iii): The addition of 27 indicative events (from 79 LAT-gap DH type II bursts matched to >100 keV HXR bursts lasting >~5 min) to reach a total of 43 relies on the statement that “almost all SGRE events … were associated with HXR bursts of duration > ~5 min.” No converse efficiency or false-positive rate is reported (i.e., the fraction of such DH type II + long-HXR events that actually produce detectable SGRE when LAT coverage is present). If this fraction is appreciably below unity, the added count is inflated and the numerical excess used to declare SC 25 stronger is correspondingly weaker.

Authors: We agree that the manuscript reports only the forward association (nearly all observed SGRE events coincide with long-duration HXR bursts) and does not quantify the converse efficiency during LAT gaps. Because the proxy is applied precisely where LAT coverage is absent, a direct measurement of how many such DH type II + long-HXR events would have produced detectable SGRE is not available. In the revised version we will clarify this limitation, report the high association rate observed in the LAT-covered intervals, and discuss the possible impact on the added count of 27 events. The convergence of this estimate with the two independent scaling methods provides some cross-check, but we will note that a lower efficiency would reduce the reported excess. revision: partial
Referee: [Abstract] Abstract: The final estimates (43–50 events) and the direct comparison to SC 24’s 27 events are presented without uncertainty ranges, error propagation on the association percentages, or sensitivity tests for data-gap handling. This omission makes it difficult to evaluate whether the reported excess is statistically significant.

Authors: We accept that the absence of uncertainty ranges and sensitivity tests limits quantitative assessment of the excess. In the revised manuscript we will propagate uncertainties on the association fractions (using binomial or Poisson statistics on the underlying counts), attach ranges to the 43–50 estimate, and include sensitivity tests for the HXR duration threshold and data-gap assumptions. These additions will allow readers to judge the statistical significance of the difference relative to the 27 events in the corresponding SC 24 epoch. revision: yes

standing simulated objections not resolved

The converse efficiency (false-positive rate) for the proxy used in method (iii) cannot be directly measured because it concerns intervals without LAT coverage.

Circularity Check

0 steps flagged

No significant circularity: estimates apply prior-cycle empirical rates to independent SC 25 counts

full rationale

The paper's derivation chain uses three estimation methods for SGRE events in SC 25. Method (i) scales an SSN-proportional baseline by observed overabundance factors in FW CMEs and DH type II bursts. Method (ii) multiplies SC 25 counts of FW CMEs and DH type II bursts by association percentages (~18% and ~27%) measured in SC 24. Method (iii) adds 27 proxy events identified via DH type II + long-duration HXR during LAT gaps, justified by the statement that almost all observed SGRE events show this association. None of these steps reduce the final estimated totals (43–50) to the input observations by algebraic identity or definitional equivalence; each incorporates an explicit assumption that the SC 24 association rates or scaling relations continue to hold. The conclusion that SC 25 is stronger follows from comparing these extrapolated totals against the SC 24 observed count of 27. No self-citation chain, ansatz smuggling, or renaming of known results is required for the load-bearing steps. The analysis is therefore self-contained against external benchmarks once the constancy assumption is granted.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The claim rests on empirical rates measured in the prior cycle and domain assumptions about event associations; no new entities are postulated.

free parameters (2)

SGRE association fraction for FW CMEs = 18%
18% rate taken from SC 24 and used to scale SC 25 counts
SGRE association fraction for DH type II bursts = 27%
27% rate taken from SC 24 and used to scale SC 25 counts

axioms (1)

domain assumption SGRE events are invariably associated with >100 keV HXR bursts of duration >~5 min
Invoked to identify candidate SGRE events during LAT data gaps

pith-pipeline@v0.9.0 · 5770 in / 1319 out tokens · 31458 ms · 2026-05-17T02:04:13.572292+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

We estimated the number of SGREs in SC 25 using three methods... conclude that SC 25 is stronger than SC 24 based on the estimated number SGRE events.
IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

Hard X-ray bursts at energies >100 keV when accompanied by DH type II bursts are good indicators of SGRE events

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Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages

[1]

G., Davila, J

Doi: 10.1007/s11214-012-9890-4 Gopalswamy, N., Xie, H., Akiyama, S., Yashiro, S., Usoskin, I. G., Davila, J. M.: 2013, The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights. Astrophys. J. Lett., 765, L30. Doi: 10.1088/2041-8205/765/2/L30 Gopalswamy N, Mäkelä P: 2014, Latitudinal conn...

work page doi:10.1007/s11214-012-9890-4 2013
[2]

L., Fichtel, C

doi:10.3847/1538-4357/abae5e Kanbach, G., Bertsch, D. L., Fichtel, C. E., et al.: 1993, Detection of a long-duration solar gamma-ray flare on June 11, 1991 with EGRET on COMPTON-GRO. Astr. Astrophys. Sup., 97, 349. Kiplinger, A. L.: 1995, Comparative Studies of Hard X-Ray Spectral Evolution in Solar Flares with High-Energy Proton Events Observed at Earth....

work page doi:10.3847/1538-4357/abae5e 1993

[1] [1]

G., Davila, J

Doi: 10.1007/s11214-012-9890-4 Gopalswamy, N., Xie, H., Akiyama, S., Yashiro, S., Usoskin, I. G., Davila, J. M.: 2013, The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights. Astrophys. J. Lett., 765, L30. Doi: 10.1088/2041-8205/765/2/L30 Gopalswamy N, Mäkelä P: 2014, Latitudinal conn...

work page doi:10.1007/s11214-012-9890-4 2013

[2] [2]

L., Fichtel, C

doi:10.3847/1538-4357/abae5e Kanbach, G., Bertsch, D. L., Fichtel, C. E., et al.: 1993, Detection of a long-duration solar gamma-ray flare on June 11, 1991 with EGRET on COMPTON-GRO. Astr. Astrophys. Sup., 97, 349. Kiplinger, A. L.: 1995, Comparative Studies of Hard X-Ray Spectral Evolution in Solar Flares with High-Energy Proton Events Observed at Earth....

work page doi:10.3847/1538-4357/abae5e 1993