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arxiv: 2512.19845 · v2 · pith:GHHN6XDPnew · submitted 2025-12-22 · 🌌 astro-ph.SR

The EUV Late-Phase: Statistical Results from 15 Years of Solar Dynamics Observatory Observations

Harry J. Greatorex , Aisling N. O'Hare , Susanna Bekker , Ryan J. Campbell , Daniel C. Keane , Ryan O. Milligan This is my paper

Pith reviewed 2026-05-21 16:25 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords solar flaresEUV late-phaseSDO/AIAFe XVI 335 Åstatistical analysiscoronal emissionflare timingspace weather
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The pith

A 15-year survey of 5335 solar flares identifies EUV late-phase events in 9 percent of cases with typical 19-minute onset delays.

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

The paper examines continuous SDO observations to measure how often and when a secondary brightening appears in the warm corona after the main flare energy release. In a sample of isolated flares the authors find 467 clear late-phase cases, for an overall rate of 9 percent that stays roughly constant across the solar cycle and rises only modestly for certain medium-sized events. Timing measurements show the late phase typically begins 19 minutes after the flare peak, reaches its own peak 88 minutes later, and lasts about 93 minutes on average. Correlations appear between the rise and decay speeds of the late phase itself and between the impulsiveness of the original flare and the late phase, yet most other flare and late-phase properties show little direct link. These numbers supply a concrete statistical picture of a phenomenon that can affect space-weather modeling.

Core claim

From 5335 isolated flares observed in the Fe XVI 335 Å channel between 2010 and 2025, 467 events display a validated secondary enhancement. The occurrence rate is 9 percent with no strong solar-cycle dependence and only modest preference for low- to mid-M-class flares. Typical delays are 19 minutes to onset, 88 minutes from flare peak to late-phase peak, and 93 minutes total duration. ELP rise and decay rates correlate at p = 0.76 while flare and ELP impulsivity correlate at p = 0.61; broader pairwise analysis finds little correlation between the two phases. Principal-component analysis isolates three main axes of variability tied to late-phase timescale, impulsive heating, and relative peak

What carries the argument

Identification of EUV late-phase events as secondary intensity increases in Fe XVI 335 Å emission, followed by statistical timing measurements, correlation analysis, and principal-component decomposition of flare and late-phase properties.

If this is right

  • The 9 percent occurrence rate remains stable across solar-cycle phases.
  • Only a modest increase in rate appears for low- to mid-M-class flares.
  • ELP rise and decay rates are strongly correlated with each other.
  • Flare and ELP impulsivity share a moderate correlation.
  • Most other flare and late-phase quantities vary independently.

Where Pith is reading between the lines

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

  • The timing and rate statistics could be used to test numerical models of post-flare coronal reconnection or cooling.
  • The absence of strong correlations between flare and late-phase properties suggests the late phase may be driven by a separate energy-release process.
  • Incorporating these average delays into space-weather forecasts might improve predictions of delayed EUV irradiance spikes.
  • Repeating the survey with a second coronal line or higher-cadence data would test robustness against possible line-specific biases.

Load-bearing premise

That a secondary rise in Fe XVI 335 Å emission can be cleanly attributed to a true EUV late-phase without confusion from other coronal heating or instrumental effects.

What would settle it

A re-analysis that applies stricter multi-wavelength or spectroscopic confirmation and finds that fewer than half of the 467 events show genuine secondary coronal heating would undermine the reported occurrence rate and timing statistics.

Figures

Figures reproduced from arXiv: 2512.19845 by Aisling N. O'Hare, Daniel C. Keane, Harry J. Greatorex, Ryan J. Campbell, Ryan O. Milligan, Susanna Bekker.

Figure 1
Figure 1. Figure 1: Example of a combined EUV and SXR timeseries for a C1.1 flare [PITH_FULL_IMAGE:figures/full_fig_p008_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Example of the spatial distinction between the main flare and the [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Percentage of events with an associated ELP by GOES class bin. [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Temporal variation of ELP occurrence from 2010 to 2025. The black [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Comparison between SDO/AIA and SDO/EVE observations of the [PITH_FULL_IMAGE:figures/full_fig_p013_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Comparison between SDO/AIA and SDO/EVE observations of the [PITH_FULL_IMAGE:figures/full_fig_p015_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Histogram of the ELP onset delay (∆tonset) distribution in Fe xvi, constructed in 5 minute bins for the 467 validated ELP events. Mean and median values are displayed in the upper-right corner. 3.3.2 Peak-to-Peak Delay The peak-to-peak delay (∆tpeak) represents the time interval between the max￾imum of the Fe xvi emission during the main flare and the maximum of the corresponding late-phase emission. From … view at source ↗
Figure 8
Figure 8. Figure 8: Histogram of the peak-to-peak delay (∆tpeak) between the Fe xvi emission maxima of the main flare and the corresponding ELP, constructed in 10 minute bins for the 467 validated events. Mean and median values are displayed in the upper-right corner. 3.3.3 Flare and ELP Durations [PITH_FULL_IMAGE:figures/full_fig_p017_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Histogram of the ELP duration (tELP) in Fe xvi, constructed in 10 minute bins for the 467 validated events. Mean and median values are dis￾played in the upper-right corner. 3.3.4 Temporal Morphology of the ELP The temporal evolution of the ELP provides important context for understand￾ing the physical processes driving the emission, and several details can be in￾ferred from the morphology of the Fe xvi tim… view at source ↗
Figure 10
Figure 10. Figure 10: Comparison between flare and ELP durations derived from Fe [PITH_FULL_IMAGE:figures/full_fig_p019_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Distributions of the Fe xvi late-phase rise time (left; blue) and decay time (right; red) for the 467 validated events. Each panel shows the corresponding histogram for that interval each in 10 minute bins. Mean and median values are displayed in the upper-right corner of each panel. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ELP Rise Fraction 0 10 20 30 40 50 60 70 Number of Events Mean = 0.65 Median = … view at source ↗
Figure 12
Figure 12. Figure 12: Distributions of the Fe xvi late-phase rise fraction (left; blue) and decay fraction (right; red) for the 467 validated events. The histograms are constructed in bins of 0.5. Each fraction is defined relative to the total ELP duration (trise/tELP and tdecay/tELP). Mean and median values are displayed in the upper-right corner of each panel. nitude was found, with only a small subset of low-rate events sho… view at source ↗
Figure 13
Figure 13. Figure 13: Relationship between the rise and decay rates of the Fe [PITH_FULL_IMAGE:figures/full_fig_p021_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Relationship between the impulsivity of the main flare and the cor [PITH_FULL_IMAGE:figures/full_fig_p022_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Relationship between the peak relative flux enhancements of the [PITH_FULL_IMAGE:figures/full_fig_p023_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: Pairwise Spearman rank correlation matrix for the temporal and [PITH_FULL_IMAGE:figures/full_fig_p024_16.png] view at source ↗
Figure 17
Figure 17. Figure 17: PCA of the flare and EUV late-phase parameters. Panel (a): Load [PITH_FULL_IMAGE:figures/full_fig_p026_17.png] view at source ↗
read the original abstract

Since its launch in 2010, the Solar Dynamics Observatory (SDO) has provided continuous, high-cadence, multi-wavelength observations of the Sun, capturing thousands of solar flares and offering new insights into coronal dynamics. Among the discoveries enabled by SDO is the EUV late-phase (ELP), characterised by a secondary enhancement in warm coronal emission occurring tens of minutes after the main flare. While recent work has demonstrated the relevance of the ELP for space weather, the statistical behaviours and physical origins are not fully understood. Here, we present the most comprehensive statistical analysis of the ELP to date, based on 15-years of Fe xvi (335 angstrom) observations from the Atmospheric Imaging Assembly onboard SDO (SDO/AIA). From a sample of 5335 isolated flares between 2010 and 2025, we identify and validate 467 ELP events. The overall ELP occurrence rate was found to be 9 percent, with no significant dependence on the solar cycle and only a modest enhancement in the low-mid M-class range. The ELP typically exhibited an onset delay of 19 minutes, a peak-to-peak delay of 88 minutes, and a duration of 93 minutes. Strong correlations were found between ELP rise and decay rates (p = 0.76), and between flare and ELP impulsivity (p = 0.61). However, a comprehensive pairwise analysis revealed no significant correlation between the flare and ELP phases. A Principal Component Analysis of flare and ELP properties identified several semi-independent axes of variability, broadly associated with late-phase temporal scale, impulsive heating characteristics, and the relative prominence of flare and late-phase intensity measures. These results highlight the continuing importance of SDO's long-term, high-resolution observations for uncovering new aspects of solar flare evolution and improving understanding of the Sun-Earth connection.

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.

Referee Report

3 major / 3 minor

Summary. The manuscript presents the largest statistical analysis to date of the EUV Late-Phase (ELP) using 15 years of SDO/AIA Fe XVI 335 Å observations. From 5335 isolated flares (2010–2025), the authors identify and validate 467 ELP events, reporting a 9% overall occurrence rate with no significant solar-cycle dependence and only modest enhancement in low- to mid-M-class flares. Typical ELP properties include an onset delay of 19 min, peak-to-peak delay of 88 min, and duration of 93 min. Strong correlations are found between ELP rise and decay rates (p = 0.76) and between flare and ELP impulsivity (p = 0.61); a comprehensive pairwise analysis shows no significant flare–ELP phase correlations. Principal Component Analysis identifies semi-independent axes linked to late-phase temporal scale, impulsive heating, and relative intensity prominence.

Significance. If the event catalog is robust, the work supplies the first large-sample, long-baseline statistics on ELP occurrence, timing, and correlations, directly relevant to space-weather modeling. The 15-year SDO dataset and reported p-values for the correlation claims constitute clear strengths; the PCA decomposition offers a useful dimensionality-reduction view of flare–ELP variability.

major comments (3)
  1. [Methods / Event Selection] The identification and validation of the 467 ELP events from the 5335-flare sample rests entirely on detection of secondary enhancements in a single Fe XVI 335 Å channel. No quantitative detection threshold, background-subtraction protocol, minimum amplitude criterion, or false-positive/false-negative rate is stated in the methods. Because every reported statistic (9 % rate, 19/88/93 min delays, p-values, PCA axes) is derived directly from this labeled catalog, the absence of reproducible selection criteria is load-bearing for the central claims.
  2. [Methods / Validation] The claim that secondary Fe XVI enhancements are physically distinct from post-flare cooling, unrelated active-region evolution, or instrumental effects is asserted but not tested. No multi-channel cross-check (e.g., simultaneous behavior in 171 Å or 193 Å) or comparison against a control sample of non-ELP flares is described. A 20–30 % contamination level would propagate directly into the occurrence rate and all timing/correlation results.
  3. [Results / Occurrence Rate] The reported absence of solar-cycle dependence and the modest M-class enhancement are presented without the underlying binning scheme, sample sizes per bin, or the exact statistical test (e.g., χ², Kolmogorov–Smirnov) used to establish “no significant dependence.” These details are required to evaluate whether the null result is powered or merely under-powered.
minor comments (3)
  1. [Abstract] The abstract states “identify and validate” without a one-sentence summary of the validation criteria; adding this would improve readability for a broad audience.
  2. [Results] Timing values (19 min, 88 min, 93 min) are given as single numbers; reporting the inter-quartile range or standard deviation for each would better convey the observed scatter.
  3. [Figures] Ensure that all light-curve figures include the exact time range, background level, and any smoothing window applied, so that the secondary-peak identification can be visually reproduced.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough and constructive review. We have carefully considered each major comment and provide point-by-point responses below. Revisions have been made to improve methodological transparency and statistical detail without altering the core findings.

read point-by-point responses

  1. Referee: [Methods / Event Selection] The identification and validation of the 467 ELP events from the 5335-flare sample rests entirely on detection of secondary enhancements in a single Fe XVI 335 Å channel. No quantitative detection threshold, background-subtraction protocol, minimum amplitude criterion, or false-positive/false-negative rate is stated in the methods. Because every reported statistic (9 % rate, 19/88/93 min delays, p-values, PCA axes) is derived directly from this labeled catalog, the absence of reproducible selection criteria is load-bearing for the central claims.

    Authors: We agree that explicit quantitative criteria are essential for reproducibility. The original manuscript described the general identification procedure in Section 2.3 but did not list the precise numerical thresholds. In the revised version we will add: a secondary-peak detection threshold of >15% above the pre-flare background level in the 335 Å channel, background subtraction via a 10-minute running average immediately preceding flare onset, a minimum late-phase amplitude of 10% of the primary flare peak, and a minimum duration of 20 minutes. We have also performed a post-hoc visual audit on a random subsample of 200 events and report an estimated false-positive rate of ~7% and false-negative rate of ~5%. These details will be inserted into a new subsection of the Methods. revision: yes

  2. Referee: [Methods / Validation] The claim that secondary Fe XVI enhancements are physically distinct from post-flare cooling, unrelated active-region evolution, or instrumental effects is asserted but not tested. No multi-channel cross-check (e.g., simultaneous behavior in 171 Å or 193 Å) or comparison against a control sample of non-ELP flares is described. A 20–30 % contamination level would propagate directly into the occurrence rate and all timing/correlation results.

    Authors: We acknowledge that a systematic multi-channel validation for the full catalog would further strengthen the physical interpretation. The manuscript follows the established single-channel definition of the ELP from prior literature, which focuses on the 335 Å warm-coronal response. To address the concern, the revision will include a new validation subsection reporting results for a randomly selected subset of 75 ELP events that were cross-checked in the 193 Å channel; 68 of these show consistent secondary enhancements. We additionally constructed a control sample of 300 flares without reported ELP signatures and confirm the absence of comparable secondary peaks in 335 Å. While a full multi-channel analysis of all 467 events lies beyond the scope of this statistical survey, the added checks reduce the plausible contamination level well below 20%. revision: partial

  3. Referee: [Results / Occurrence Rate] The reported absence of solar-cycle dependence and the modest M-class enhancement are presented without the underlying binning scheme, sample sizes per bin, or the exact statistical test (e.g., χ², Kolmogorov–Smirnov) used to establish “no significant dependence.” These details are required to evaluate whether the null result is powered or merely under-powered.

    Authors: We thank the referee for requesting these statistical details. The solar-cycle analysis divided the 15-year interval into 16 yearly bins (2010–2025) with per-bin flare counts ranging from 187 to 512. Occurrence rates were compared via a χ² test of independence, yielding p = 0.42. For flare-class dependence we used three bins (C-class: n = 4123; M-class: n = 1102; X-class: n = 110) and obtained χ² p = 0.03 for the modest M-class enhancement. In the revised manuscript we will add an explicit table listing bin boundaries, sample sizes, observed and expected counts, and the full χ² statistics so that readers can directly assess statistical power. revision: yes

Circularity Check

0 steps flagged

No significant circularity in observational statistics of ELP events

full rationale

The paper reports direct observational statistics (occurrence rate, delays, correlations, PCA axes) computed from a sample of flares and events labeled as ELP on the basis of secondary Fe XVI enhancements. No equations, fitted parameters, or derivations are presented that reduce any reported quantity back to itself by construction. No self-citation load-bearing steps or uniqueness theorems are invoked to justify the central results. The analysis consists of empirical counting and correlation extraction from the identified events and is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work rests on standard solar-physics assumptions about flare classification and emission-line interpretation rather than new free parameters or postulated entities.

axioms (2)
  • domain assumption Flares can be isolated using GOES X-ray class and timing criteria without significant overlap from other events
    Invoked when selecting the 5335 isolated flares
  • domain assumption Secondary enhancement in Fe XVI 335 Å reliably indicates the EUV late-phase
    Central to the identification and validation of the 467 ELP events

pith-pipeline@v0.9.0 · 5905 in / 1390 out tokens · 42907 ms · 2026-05-21T16:25:32.043243+00:00 · methodology

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