arxiv: 2605.06344 · v1 · submitted 2026-05-07 · 🌌 astro-ph.SR

Recognition: unknown

First time delay observation between two mid-infrared channels in solar flare footpoints

Miguel Rojas-Quesada , Lyndsay Fletcher , Hugh Hudson , Sargam M. Mulay , Paulo J. A. Simoes

Authors on Pith no claims yet

Pith reviewed 2026-05-08 05:11 UTC · model grok-4.3

classification 🌌 astro-ph.SR

keywords solar flaresmid-infrared emissiontime lagschromosphereflare footpointsenergy depositionopacity changes

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The pith

Mid-infrared emission at 8.2 micrometers peaks 0.3 to 0.45 seconds before 5.2 micrometers in solar flare footpoints.

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

The paper reports the first detection of a consistent time lag between two mid-infrared channels during a solar flare. Analysis of light curves from the flare footpoints using a local cross-correlation function shows that the 8.2 micrometer channel reaches maximum intensity ahead of the 5.2 micrometer channel. Radiation hydrodynamics simulations with the RADYN code reproduce lags in the observed range by tracking how flare-driven ionization changes opacity at each wavelength. The result indicates that longer-wavelength emission responds to energy input with a shorter delay relative to the intensity peak. This provides a new observational handle on the timing and depth of energy deposition in the chromosphere.

Core claim

Using a local cross-correlation function on time series from resolved flare footpoints, the 8.2 μm channel peaks 0.3–0.45 s before the 5.2 μm channel. RADYN simulations generate matching theoretical lags, with the difference arising because flare-induced ionization alters opacity such that longer wavelengths show a smaller time offset between the peak of energy injection and the peak of emitted intensity.

What carries the argument

Local cross-correlation function applied to paired 5.2 μm and 8.2 μm light curves, cross-checked against RADYN radiation-hydrodynamics calculations of wavelength-dependent opacity evolution in the chromosphere.

If this is right

Longer mid-IR wavelengths exhibit a smaller lag between energy-injection peak and intensity peak.
The measured lags act as a diagnostic for how quickly the chromosphere responds at different depths.
Higher-cadence mid-IR data could map the vertical structure of flare heating through these wavelength-dependent offsets.

Where Pith is reading between the lines

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

Repeated measurements across multiple flares could test whether the lag range is universal or depends on flare class and energy input.
Pairing the method with other chromospheric diagnostics might constrain the height at which energy is deposited.
The approach offers a direct observational test of how radiation-hydrodynamics codes handle prompt ionization and opacity changes.

Load-bearing premise

The local cross-correlation isolates genuine physical lags rather than artifacts from noise or instruments, and the RADYN code correctly models how ionization changes opacity at each wavelength.

What would settle it

A new flare observation in which the measured lag between the two mid-IR channels lies outside the range produced by RADYN runs for any plausible energy-deposition profile.

Figures

Figures reproduced from arXiv: 2605.06344 by Hugh Hudson, Lyndsay Fletcher, Miguel Rojas-Quesada, Paulo J. A. Simoes, Sargam M. Mulay.

**Figure 1.** Figure 1: Panel (a): the field-of-view of a sunspot within the active region 12172 captured by the McMath/Pierce East Auxiliary telescope in the 8.2 µm channel. The penumbral region and the quiet sun regions are shown by the yellow and black boxes, respectively. The flare footpoints are shown as blue and green boxed regions, labelled ‘flare A’ and ‘flare B’. The averaged intensities in these regions are used for fur… view at source ↗

**Figure 2.** Figure 2: Panel (a): Corrected intensity profiles after subtracting the mean quiet Sun intensity in each channel at the respective flare footpoints. The shaded area indicates the time interval (17:49:05-17:49:46 UT) used to calculate the time lags between the signals in the following panels. Panel (b): The two correlated signals for the footpoint A. Panel (c): Estimated time lag for the footpoint A between the signa… view at source ↗

**Figure 3.** Figure 3: For the flare analysed by Simões et al. (2024), the estimated value for the lag using the local CCF is 0.88 s (blue), instead of the reported 0.75 s (red). Considering the definition of the optical depth 𝜏𝜈 (𝑠) = ∫ 𝑠0 𝑠 𝑘𝑣 (𝑠 ′ ) d𝑠 ′ , (2) and the definition of the contribution function (CF) as: 𝐶𝐹(𝑠) = 𝑗𝜈 𝑒 −𝜏𝜈 (𝑠) , (3) then: 𝐼𝜈 (𝑠0) = ∫ 𝑠0 0 𝐶𝐹(𝑠)d𝑠. (4) This equation states that the observed intensity… view at source ↗

**Figure 4.** Figure 4: Variation of the contribution function at three instants during the flare. Panel (a): the initial conditions; Panel (b): the conditions at the time of maximum emission in the 8 µm channel, since the variation in intensity is with respect to the initial values. The initial CF is shown in shaded form. Panel (c): the conditions at the time when the RADYN simulation stops injecting energy. The animation associ… view at source ↗

**Figure 5.** Figure 5: Intensity variations in both channels for different scenarios. The first column contains cases in which the spectral index varies; the second column contains cases in which the cut-off energy values vary; and the third column contains cases in which the energy fluence values vary. The lag (in seconds) between the maximum emission peaks taken directly from the RADYN model results is shown in the legend, as … view at source ↗

**Figure 6.** Figure 6: Interpolated values of 𝑡max (𝜆) for different spectral indices and cut-off energies, for a fixed fluence of ℱ = 1 × 1011 erg cm−2 . Each surface corresponds to a wavelength channel, illustrating how the time required to reach the emission peak varies with the beam parameters; the z-axis indicates the time required to reach maximum emission from the moment the energy injection begins view at source ↗

**Figure 7.** Figure 7: Excess brightness curves for each mid-IR channel in the case of ℱ = 1 × 1011 erg cm−2 . The colour scale represents the number of electrons at 50 keV (purple for the largest, yellow for the smallest). The shaded area denotes the temporal profile of energy input. The 5.2 µm emission generally shows a longer delay, while for larger populations of energetic electrons the 8.2 µm emission more closely follows t… view at source ↗

read the original abstract

The strong correlation between energy injection and mid-infrared (mid-IR) emission observed during solar flares can be used to probe energy deposition throughout the chromosphere, since the IR tracks prompt flare-induced changes in electron density. Despite its diagnostic value, solar mid-IR observations are relatively recent, with sporadic campaigns over the last decade resulting in only a few recorded flares. Earlier studies found time lags between mid-IR emissions from spatially resolved footpoints, offering clues about flare energy transport. Building on this, we analyse the time lags between emissions at two wavelengths (5.2 micrometers and 8.2 micrometers) for each footpoint. Using a local cross-correlation function, we show for the first time that the 8.2 micrometers emission channel peaks 0.3 s-0.45 s before the 5.2 micrometer channel. We investigate the origin of this lag, obtaining infrared emission estimates using results from the RADYN radiation hydrodynamics code. The theoretical lag values fall within the range of the observed ones. Variations in opacity-primarily due to flare-induced ionization-explain the wavelength-dependent temporal shift between emission maxima. In particular, longer wavelengths exhibit a smaller lag between the peak of energy injection and peak of intensity. These results contribute to a better understanding of how energy deposition during a flare affects the chromospheric layers of the atmosphere. Future observations with higher temporal resolution could exploit measurements of these time lags to more fully characterize the dynamics of energy deposition during solar flares, opening a new avenue for studying heating and energy transport processes in the solar atmosphere.

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 reports the first claimed time lag between 5.2 and 8.2 μm mid-IR channels in resolved flare footpoints, with 8.2 μm leading by 0.3-0.45 s and RADYN models reproducing the range via opacity shifts from ionization.

read the letter

The main point is that this work measures a consistent lead of the 8.2 μm channel over the 5.2 μm one in solar flare footpoints for the first time, using local cross-correlation on existing data, and shows that RADYN radiation hydrodynamics runs produce lags in the same ballpark through wavelength-dependent opacity changes driven by flare ionization. Longer wavelengths respond with a shorter delay to the energy input peak, which matches the observations and gives a physical account rather than just a timing number. They extend earlier mid-IR flare studies by resolving footpoints separately and tying the result to chromospheric conditions. The RADYN comparison is the stronger part because it comes from an independent code rather than being tuned directly to the new data. The observed lag range is stated clearly enough to be testable. The soft spots sit mainly in the timing extraction. Sub-second lags from cross-correlation are vulnerable to detector timing offsets, flat-field issues, or noise, and the abstract gives no error bars, selection criteria, or tests for channel-specific systematics. If the full paper does not show explicit checks against those effects or alternative methods like peak fitting, the result stays provisional even if the model match holds. The stress-test concern about artifacts is reasonable at these timescales and should be addressed directly. This is aimed at solar physicists who work on flare energy transport and chromospheric diagnostics. Readers who follow multi-wavelength timing or radiation hydro models will see a concrete new observable and a way to use it. It has enough fresh observational content plus a model link to deserve peer review, though likely with requests for more on the data robustness and any instrumental corrections. I would send it to referees with that focus.

Referee Report

2 major / 2 minor

Summary. The paper reports the first observation of a time delay between mid-infrared emissions at 5.2 μm and 8.2 μm in solar flare footpoints. Applying a local cross-correlation function to the data, the authors find that the 8.2 μm channel peaks 0.3–0.45 s before the 5.2 μm channel. They compare these lags to predictions from the RADYN radiation hydrodynamics code and attribute the wavelength dependence to differences in opacity evolution driven by flare-induced ionization, noting that longer wavelengths exhibit smaller lags relative to the peak of energy injection.

Significance. If robust, the result offers a new observational diagnostic for probing the vertical structure of energy deposition in the chromosphere during solar flares. Mid-IR emission tracks prompt electron-density changes, so measured lags between channels could constrain the timing of heating at different depths. The comparison to independent RADYN simulations (rather than a fit to the data) is a methodological strength that supports the opacity-based interpretation.

major comments (2)

[Cross-correlation analysis] The headline claim rests on the local cross-correlation function returning a physical lag of 0.3–0.45 s. The manuscript provides no quantitative assessment of how noise, detector response differences, flat-field residuals, or wavelength-dependent atmospheric transmission affect the correlation peak at these sub-second timescales. Without Monte Carlo tests or explicit robustness checks, it is unclear whether the reported lag range is free of instrumental artifacts.
[Theoretical modeling and comparison] The comparison to RADYN is described only as “theoretical lag values fall within the range of the observed ones.” No specific simulated lag values, their uncertainties, or a quantitative metric of agreement (e.g., overlap fraction or reduced chi-squared) are given. This leaves the match qualitative and weakens the support for the claimed opacity-evolution mechanism.

minor comments (2)

[Abstract] The abstract states the lag range but omits any mention of uncertainties, the specific flare event(s) analyzed, or the number of footpoints examined. Adding these details would improve clarity.
Figure captions and text should explicitly state the temporal resolution of the observations and the exact implementation parameters of the local cross-correlation function (window length, lag range searched, etc.).

Simulated Author's Rebuttal

2 responses · 0 unresolved

We are grateful to the referee for the detailed and constructive feedback on our manuscript. The comments highlight important areas for improvement in the presentation of our analysis and modeling. We have addressed each major comment below and made corresponding revisions to the manuscript.

read point-by-point responses

Referee: [Cross-correlation analysis] The headline claim rests on the local cross-correlation function returning a physical lag of 0.3–0.45 s. The manuscript provides no quantitative assessment of how noise, detector response differences, flat-field residuals, or wavelength-dependent atmospheric transmission affect the correlation peak at these sub-second timescales. Without Monte Carlo tests or explicit robustness checks, it is unclear whether the reported lag range is free of instrumental artifacts.

Authors: We agree that the manuscript would benefit from explicit robustness checks for the cross-correlation analysis. In the revised version, we will include Monte Carlo tests to evaluate the effects of noise on the determined lags, as well as a discussion addressing potential contributions from detector response differences, flat-field residuals, and atmospheric transmission. These additions will provide a quantitative assessment of the reliability of the reported 0.3-0.45 s lag. revision: yes
Referee: [Theoretical modeling and comparison] The comparison to RADYN is described only as “theoretical lag values fall within the range of the observed ones.” No specific simulated lag values, their uncertainties, or a quantitative metric of agreement (e.g., overlap fraction or reduced chi-squared) are given. This leaves the match qualitative and weakens the support for the claimed opacity-evolution mechanism.

Authors: We acknowledge that the RADYN comparison was presented in a qualitative manner. We will revise the manuscript to report the specific lag values from the RADYN simulations, including any associated uncertainties, and provide a quantitative measure of agreement with the observations to more firmly support the proposed opacity-evolution mechanism. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper's core result is an observational detection of 0.3-0.45 s lags between 8.2 µm and 5.2 µm channels via local cross-correlation applied to flare footpoint data. This measurement is independent of the subsequent RADYN radiation-hydrodynamics modeling, which supplies separate theoretical lag estimates based on flare-driven ionization and wavelength-dependent opacity evolution. The statement that theoretical values fall within the observed range is a consistency check, not a derivation that reduces the data product to a fitted parameter or self-citation. No equations, ansatzes, or uniqueness claims in the provided text loop back to the inputs by construction, and no load-bearing self-citations are invoked.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The claim rests on the accuracy of cross-correlation for lag extraction and the fidelity of RADYN in modeling flare-driven changes in chromospheric opacity and electron density at mid-IR wavelengths.

axioms (1)

domain assumption RADYN radiation hydrodynamics code accurately captures flare-induced ionization and resulting wavelength-dependent opacity variations in the chromosphere
Invoked to generate theoretical lag values that are compared to observations

pith-pipeline@v0.9.0 · 8846 in / 1181 out tokens · 80208 ms · 2026-05-08T05:11:54.432169+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

30 extracted references · 27 canonical work pages

[1]

Infrared Technology and Applications XXXII , year = 2006, editor =

Two-color quantum well infrared photodetector focal plane arrays. Infrared Technology and Applications XXXII , year = 2006, editor =. doi:10.1117/12.665803 , adsurl =

work page doi:10.1117/12.665803 2006
[2]

, keywords =

Comparison of 30 THz impulsive burst time development to microwaves, H , EUV, and GOES soft X-rays. , keywords =. doi:10.1051/0004-6361/201425520 , archivePrefix =. 1512.01763 , primaryClass =

work page doi:10.1051/0004-6361/201425520
[3]

, keywords =

HATS: A Ground-Based Telescope to Explore the THz Domain. , keywords =. doi:10.1007/s11207-020-01621-3 , archivePrefix =. 2003.12907 , primaryClass =

work page doi:10.1007/s11207-020-01621-3 2003
[4]

, keywords =

The New 30 THz Solar Telescope in S \ a o Paulo, Brazil. , keywords =. doi:10.1007/s11207-015-0749-1 , adsurl =

work page doi:10.1007/s11207-015-0749-1
[5]

Ground-based and Airborne Telescopes II , year = 2008, editor =

New telescopes for ground-based solar observations at submillimeter and mid-infrared. Ground-based and Airborne Telescopes II , year = 2008, editor =. doi:10.1117/12.788889 , adsurl =

work page doi:10.1117/12.788889 2008
[7]

Space Weather , year = 2018, month = sep, volume =

The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid-IR. Space Weather , year = 2018, month = sep, volume =. doi:10.1029/2018SW001969 , adsurl =

work page doi:10.1029/2018sw001969 2017
[8]

, keywords =

A solar flare driven by thermal conduction observed in mid-infrared. , keywords =. doi:10.1051/0004-6361/202141967 , archivePrefix =. 2110.15751 , primaryClass =

work page doi:10.1051/0004-6361/202141967
[9]

, keywords =

High-resolution Observations of an X6.4 Solar Flare in the Mid-infrared. , keywords =. doi:10.3847/2041-8213/adee95 , adsurl =

work page doi:10.3847/2041-8213/adee95 2041
[10]

doi:10.5281/zenodo.840393 , publisher =

ColourBlind: A Collection of Colour-blind-friendly Colour Tables. doi:10.5281/zenodo.840393 , publisher =

work page doi:10.5281/zenodo.840393
[11]

, year = 2010, month = may, volume =

Solar Observation Target Identification Convention for use in Solar Physics. , year = 2010, month = may, volume =. doi:10.1007/s11207-010-9553-0 , adsurl =

work page doi:10.1007/s11207-010-9553-0 2010
[12]

Journal of Double Star Observations , year =

Harshaw, Richard and others , title =. Journal of Double Star Observations , year =
[13]

, year = 1964, month = dec, volume =

The McMath solar telescope of Kitt Peak National Observatory. , year = 1964, month = dec, volume =. doi:10.1364/AO.3.001337 , adsurl =

work page doi:10.1364/ao.3.001337 1964
[14]

, keywords =

Solar Flare Spectroscopy. , keywords =. doi:10.1146/annurev-astro-052920-010547 , adsurl =

work page doi:10.1146/annurev-astro-052920-010547
[15]

S., Barnes, W

The SunPy Project: Open Source Development and Status of the Version 1.0 Core Package , journal =. doi:10.3847/1538-4357/ab4f7a , url =

work page doi:10.3847/1538-4357/ab4f7a
[16]

, volume=

Speckle interferometry with adaptive optics corrected solar data , author=. , volume=. 2008 , publisher=

2008
[17]

2008 , journal =

Benz, Arnold O , pages =. 2008 , journal =

2008
[18]

and White, S

Kaufmann, P. and White, S. M. and Freeland, S. L. and Marcon, R. and Fernandes, L. O. T. and Kudaka, A. S. and de Souza, R. V. and Aballay, J. L. and Fernandez, G. and Godoy, R. and Marun, A. and Valio, A. and Raulin, J.-P. and Gim. 2013 , journal =. doi:10.1088/0004-637X/768/2/134 , issn =

work page doi:10.1088/0004-637x/768/2/134 2013
[19]

2016 , journal =

Penn, Matt and Krucker, Säm and Hudson, Hugh and Jhabvala, Murzy and Jennings, Don and Lunsford, Allen and Kaufmann, Pierre , number =. 2016 , journal =. doi:10.3847/2041-8205/819/2/L30 , issn =

work page doi:10.3847/2041-8205/819/2/l30 2016
[20]

2024 , journal =

Simões, Paulo J A and Fletcher, Lyndsay and Hudson, Hugh S and Kerr, Graham S and Penn, Matt and Lopez, Karla F , number =. 2024 , journal =. doi:10.1093/mnras/stae1511 , issn =

work page doi:10.1093/mnras/stae1511 2024
[21]

Simões, Paulo J. A. and Kerr, Graham S. and Fletcher, Lyndsay and Hudson, Hugh S. and Gim. 2017 , journal =. doi:10.1051/0004-6361/201730856 , issn =

work page doi:10.1051/0004-6361/201730856 2017
[22]

Kontar, E. P. and Ratcliffe, H. and Bian, N. H. , month =. 2012 , journal =. doi:10.1051/0004-6361/201118216 , issn =

work page doi:10.1051/0004-6361/201118216 2012
[23]

, number =

Welsh, W.F. , number =. 1999 , journal =. doi:10.1086/316457 , issn =

work page doi:10.1086/316457 1999
[24]

2021 , journal =

Gierens, Klaus and Eleftheratos, Kostas , number =. 2021 , journal =. doi:10.1127/metz/2020/0985 , issn =

work page doi:10.1127/metz/2020/0985 2021
[25]

and Avrett, E

Heinzel, P. and Avrett, E. H. , month =. 2012 , booktitle =. doi:10.1007/s11207-011-9823-5 , issn =

work page doi:10.1007/s11207-011-9823-5 2012
[26]

2023 , journal =

Carlsson, Mats and Fletcher, Lyndsay and Allred, Joel and Heinzel, Petr and Ka. 2023 , journal =. doi:10.1051/0004-6361/202346087 , issn =

work page doi:10.1051/0004-6361/202346087 2023
[27]

Lin, R. P. and Dennis, B. R. and Hurford, G. J. and Smith, D. M. and Zehnder, A. and Harvey, P. R. and Curtis, D. W. and Pankow, D. and Turin, P. and Bester, M. and Csillaghy, A. and Lewis, M. and Madden, N. and Van Beek, H. F. and Appleby, M. and Raudorf, T. and McTiernan, J. and Ramaty, R. and Schmahl, E. and Schwartz, R. and Krucker, S. and Abiad, R. a...

work page doi:10.1023/a:1022428818870/metrics 2002
[28]

, month =

Neupert, Werner M. , month =. 1968 , journal =. doi:10.1086/180220 , issn =

work page doi:10.1086/180220 1968
[29]

2020 , journal =

Steyn, Ruhann and Strauss, Du Toit and Effenberger, Frederic and Pacheco, Daniel , volume =. 2020 , journal =. doi:10.1051/swsc/2020067 , issn =

work page doi:10.1051/swsc/2020067 2020
[30]

van Hoof, P. A.M. and Williams, R. J.R. and Volk, K. and Chatzikos, M. and Ferland, G. J. and Lykins, M. and Porter, R. L. and Wang, Y. , number =. 2014 , journal =. doi:10.1093/mnras/stu1438 , issn =

work page doi:10.1093/mnras/stu1438 2014
[31]

, keywords =

Probing Progression of Heating Through the Lower Flare Atmosphere via High-cadence IRIS Spectroscopy. , keywords =. doi:10.3847/1538-4357/adccc8 , archivePrefix =. 2504.10619 , primaryClass =

work page doi:10.3847/1538-4357/adccc8