REVIEW 4 major objections 5 minor 1 cited by
Properties of slow magneto-acoustic waves observed simultaneously using Hi-C 2.1 and AIA
T0 review · 4 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Damping lengths of slow magneto-acoustic waves in the same coronal loop agree between SDO/AIA and Hi-C 2.1 within uncertainties, indicating that a recently reported instrument-dependent difference does not generalize.
desk verdict First Hi-C 2.1 slow-wave detection shows damping lengths consistent with AIA, but the comparison is weakened by different time windows and a very uncertain AIA phase-tracking fit. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The quantitative engine is the damping length $L_d$, the distance over which the wave amplitude falls by a factor of $e$, estimated by two independent fitting procedures on time-distance maps built along the same loop. The Phase Tracking Method takes the spatial intensity profile at one chosen time step and fits it to an exponentially damped sinusoid, $I(x)=A_0 e^{-x/L_d}\sin(2\pi x/\lambda+\phi)+B_0+B_1 x$. The Amplitude Tracking Method converts the temporal standard deviation at each position into an amplitude through $A=\sqrt{2}\sigma$ and fits $A(x)=A_0 e^{-x/L_d}+C$, with $C$ fixed to the amplitude at the last usable position. Strict coalignment of the Hi-C frames to AIA (roll-angle correction, upscaling, and cross-correlation shifts, following a published procedure) ensures both instruments sample the same loop pixels, and it is the comparison of $L_d$ across instruments—rather than any single absolute value—that carries the argument.
What would settle it
Re-run the amplitude-tracking analysis using only the AIA data that overlap the Hi-C window, with the same flat background and the same 5-minute series length used for Hi-C: if the resulting $L_d$ leaves the quoted $3.4\pm1.0$ Mm uncertainty, the cross-instrument agreement depends on series length or background choice; if it stays near 3.7 Mm, the null result holds.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is a null comparison: for the single loop in which slow magneto-acoustic waves are clearly visible in both datasets, the damping length inferred from Hi-C 2.1 equals that inferred from SDO/AIA within the measurement uncertainties. Phase tracking yields $L_d = 4.0\pm2.1$ Mm for AIA and $4.1\pm0.3$ Mm for Hi-C; amplitude tracking yields $3.4\pm1.0$ Mm and $3.7\pm0.1$ Mm, respectively, so the two instruments also agree with each method. The oscillations share a period near 2.7–2.8 min and propagation speeds of $46.0\pm1.7$ km s$^{-1}$ and $48.1\pm0.6$ km s$^{-1}$, and they fade beyond about 7 Mm from the loop footpoint in both datasets. The paper also reports the first detection of propagating slow waves in Hi-C 2.1 data and uses the agreement to argue that instrument choice alone need not alter measured damping lengths.
Load-bearing premise
The comparison stands on the assumption that the 30-minute AIA series with its 5-minute smoothed background and the 5-minute Hi-C series with its flat full-duration background are measuring the same, effectively stationary wave amplitude.
Editorial extensions
If this is right
- The AIA and Hi-C damping lengths agree within uncertainties in both methods, so the large EUI-versus-AIA discrepancy reported elsewhere does not appear to be a universal property of cross-instrument slow-wave measurements.
- The two fitting methods also agree with each other within each dataset, indicating the measured decay is not an artefact unique to one fitting procedure.
- Because the damping length ($\sim4$ Mm) is comparable to the fitted wavelength ($\sim4.7$ Mm) and the oscillations vanish within about 7 Mm, these observations reinforce the picture of rapid, wavelength-scale damping of slow waves in sunspot-rooted loops.
- The first identification of propagating slow waves in Hi-C 2.1 data means short-duration, high-resolution sounding-rocket observations can contribute to wave studies when guided by cotemporal AIA data.
Reading between the lines
- Editorial inference: the null result shifts the burden of explanation for the EUI/AIA discrepancy onto that specific setup—e.g., the marginal difference in passband temperature response or the 19° viewing-angle separation—but this paper's data cannot distinguish those possibilities.
- Editorial inference: the amplitude-tracking comparison is the weakest link in the chain, because AIA's amplitude profile comes from a 30-minute series with a 5-minute smoothed background while Hi-C's comes from a 5-minute series with a flat average; a longer Hi-C observation or a matched-window AIA analysis would test whether this procedural mismatch is masking a real difference.
- Editorial inference: a natural numerical extension is to feed synthetic time-distance maps with known input $L_d$ and the two different series lengths through the PTM and ATM pipelines, which would quantify how much of the agreement (and of the EUI/AIA contrast) is caused by series length and background choice rather than by the waves.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper analyzes cotemporal observations of a fan loop in NOAA AR12712 obtained with Hi-C 2.1 and SDO/AIA, searching for propagating slow magneto-acoustic waves. The authors detect oscillations in both instruments, measure period, propagation speed, and damping length using two methods (PTM and ATM), and report that the damping lengths are consistent within uncertainties: 4.0±2.1 Mm versus 4.1±0.3 Mm from PTM and 3.4±1.0 Mm versus 3.7±0.1 Mm from ATM for AIA and Hi-C, respectively. They conclude that there is no notable instrument-dependent difference in damping lengths, in contrast to the recent result of Meadowcroft et al. (2024). The paper is based on a single loop and a single 5-minute Hi-C time series.
Significance. If the null result holds up, the paper is a valuable data point suggesting that the instrument-dependent damping lengths reported by Meadowcroft et al. (2024) may not be a universal feature of slow-wave observations, and it would support the idea that some of the discrepancy could be due to passband or viewing-angle effects in that particular pair. The paper also appears to be the first to report slow magneto-acoustic waves in Hi-C 2.1 data, and it gives a careful treatment of photon-noise and readout-noise error propagation in Appendix A, and it explicitly discusses limitations such as the short Hi-C series and localized brightenings. However, the central comparison is weakened by the fact that the AIA and Hi-C amplitude profiles are constructed from very different time intervals and background definitions, and the large AIA uncertainties limit the power of the null test. With a corrected AIA analysis over the common 5-minute window, the result could become a solid contribution; without that, the 'no difference' claim is not yet fully supported.
major comments (4)
- [Section 3, background subtraction; Section 3.3.2] The AIA and Hi-C damping lengths are not estimated from comparable quantities. The AIA background is a 5-minute running mean over a 30-minute series, while the Hi-C background is an average over the full ~5-minute duration; likewise, the ATM standard deviation is computed over ~29 minutes for AIA and over ~4.25 minutes for Hi-C. If the wave amplitude or background is not stationary over 30 minutes, the AIA amplitude profile used in Eq. (2) and the AIA phase-tracking profile are time-averaged quantities that need not represent the instantaneous oscillation state during the simultaneous 5-minute window. The reported consistency (4.0±2.1 vs 4.1±0.3 Mm from PTM; 3.4±1.0 vs 3.7±0.1 Mm from ATM) could therefore reflect different effective time averaging rather than a genuine absence of instrument-dependent damping. The authors should recompute the AIA analysis using only the 5-minute overlap period and the same background construction as Hi-C, and report whether the damping lengths remain consistent.
- [Section 3.3.2, Eq. (2)] The ATM damping length depends on the offset constant C, which is fixed as the amplitude at the last spatial position at about 12 Mm, a region where the oscillations are not visible and the signal is essentially noise. The fit itself is restricted to distances up to 7 Mm, so C essentially sets the asymptotic noise floor. The authors do not explore the sensitivity of Ld to this choice, and the quoted errors do not include the uncertainty in C. A different but plausible choice of C (for instance, the mean noise level or a free parameter) could shift Ld by an amount comparable to the quoted uncertainties. I ask the authors to vary C within a reasonable range and show how Ld changes, or to propagate the uncertainty in C into the reported errors.
- [Section 3.3.1 and Table 1] The Hi-C PTM and ATM errors are surprisingly small (0.3 Mm and 0.1 Mm) given that the Hi-C time series covers only about two wave cycles (304 s with a period of approximately 2.8 min). These formal fit errors do not include systematic contributions from the short time series, the manual selection of the ridge in the phase-tracking method, the choice of the excluded first 45 s, or the different background subtraction discussed above. The statement that the damping lengths agree is therefore driven largely by the much larger AIA uncertainties (2.1 Mm in PTM, 1.0 Mm in ATM), which makes the comparison low in statistical power. The authors should provide a systematic error budget for the Hi-C values or at least clearly caution against interpreting the small formal errors as evidence for a tight constraint.
- [Section 4 (Discussion and Conclusions)] The conclusion that there is no notable difference in damping lengths is a single-loop, single-event null result. Given that the AIA PTM error is more than half of the measured value (4.0±2.1 Mm), the test has limited ability to detect a difference of the magnitude reported by Meadowcroft et al. (2024) (6.9 vs 12.8 Mm). The authors should explicitly state this low statistical power in the conclusions rather than presenting the null result as a robust finding. This limitation is acknowledged indirectly in the text, but it should be made prominent because it directly affects the interpretation of the comparison.
minor comments (5)
- [Section 3, paragraph 1] There are typographical errors: 'oscillatons' should be 'oscillations' and 'Addtionally' should be 'Additionally'; in Section 4, 'quantise' should be 'quantify'.
- [Section 2, paragraph 2] The statement that AIA data were calibrated using 'a robust pipeline developed by Rob Rutten' should include a reference or a footnote to the pipeline, since the reader cannot otherwise verify the calibration details.
- [Appendix A] The Hi-C gain is assumed to be unity because it is not provided in the instrument documentation; this assumption directly affects the error bars in Figures 4 and 5 and should be justified or tested, for example by checking how the derived damping lengths change if the gain is varied by a factor of two.
- [Figure 2 caption] Panel (b) is described as the AIA time-distance map in the duration overlapping Hi-C, but it is not clear whether this panel is simply a temporal crop of the full 30-minute detrended map (with a 5-minute running-mean background) or whether it was re-detrended using only the 5-minute interval. This distinction is central to the analysis and should be stated explicitly in the caption or in the text.
- [Section 3.2] The derivation of the 21-degree inclination angle from the ratio of the observed propagation speed to the nominal sound speed assumes that the loop is straight and that the wave propagates exactly along the loop axis; the projection correction should be described as an assumption, since loop curvature and off-axis propagation could affect the inferred angle.
Circularity Check
Independent fits to each instrument's own data; no fitted parameter is renamed as a prediction; the sole co-author method citation is non-load-bearing.
full rationale
The paper's central claim — that damping lengths from AIA and Hi-C 2.1 are mutually consistent — is a measured null result, not a construction. Each damping length is obtained by an independent chi-square fit of equation (1) (PTM) or equation (2) (ATM) to that instrument's own detrended time-distance map; the two fits share no fitted parameters, so consistency is not forced by construction. No parameter is fitted to one dataset and then 'predicted' for the other, and the comparison is not self-definitional: Ld is a free fit parameter, not defined in terms of the other instrument's Ld. The only author-overlapping citation is 'two different methods similar to that explained in Krishna Prasad et al. (2019)', but the PTM/ATM procedures and fitting functions are reproduced in full in Sections 3.3.1–3.3.2 (Eqs. 1–2) and Appendix A, so the citation is methodological provenance, not load-bearing support. The period and propagation speed estimates are likewise independent (spatially averaged FFT and ridge-slope linear fit) and are used only as consistency checks. The skeptic's concern — that AIA's 5-minute-smoothed background over a 30-minute series and 29-minute standard deviation may not represent the same oscillation state as Hi-C's 5-minute full-duration average — is a comparability and statistical-power risk (and plausibly explains the larger AIA uncertainties), but it is not a circularity: the AIA damping length remains an honest fit to the AIA detrended data, and it could in principle have disagreed with Hi-C's value. No derivation step reduces to its own inputs, and no fitted quantity is relabelled as a prediction.
Assumptions & free parameters
free parameters (4)
- ATM offset constant C =
amplitude at ~12 Mm (last spatial position)
- Background smoothing window (AIA) =
5 minutes
- Time-distance smoothing window =
~30 s
- Excluded initial time in ATM =
45 s
assumptions (5)
- domain assumption Photon noise and readout noise dominate the uncertainty (Eq. A1)
- standard math A = √2 σ for a sinusoidal signal
- domain assumption The intensity profile follows an exponentially decaying sinusoid (Eq. 1) and the amplitude decays exponentially (Eq. 2)
- domain assumption The target loop is a coherent 1D structure over which intensity can be averaged
- domain assumption The same loop is observed by both instruments after coalignment
Cite this review
Pith. "Pith review of Properties of slow magneto-acoustic waves observed simultaneously using Hi-C 2.1 and AIA." pith.science (2026). https://pith.science/paper/ETHX65KO
@misc{pith2026250606126,
author = {Pith},
title = {Pith review of: Properties of slow magneto-acoustic waves observed simultaneously using Hi-C 2.1 and AIA},
year = {2026},
howpublished = {\url{https://pith.science/paper/ETHX65KO}},
note = {Machine review of arXiv:2506.06126}
}
abstract
Propagating slow magneto-acoustic waves are commonly observed in different coronal structures but are most prominent in active region fan loops. Their rapid damping with damping lengths of the order of a wavelength has been investigated in the past by several authors. Although different physical mechanisms have been proposed, significant discrepancies between the theory and observations remain. Recent high-resolution observations captured simultaneously by two different instruments reveal distinct damping lengths for slow magneto-acoustic waves although their passbands are similar. These results suggest a possible contribution of instrumental characteristics on the measurement of damping lengths. Here, we analyse the behavior of slow waves using a different pair of instruments in order to check the prevalence of such results. In particular, the cotemporal observations of active region NOAA AR12712 by the High-Resolution Coronal Imager (Hi-C 2.1) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) are utilised. The estimated oscillation periods of slow magneto-acoustic waves identified from these data are 2.7{\,}$\pm${\,}0.2{\,}min from SDO/AIA, and 2.8{\,}$\pm${\,}1.2{\,}min from Hi-C 2.1. The corresponding propagation speeds are found to be 46.0{\,}$\pm${\,}1.7{\,}km{\,}s$^{-1}$ and 48.1{\,}$\pm${\,}0.6{\,}km{\,}s$^{-1}$, respectively. Damping lengths were calculated by two different methods, the Phase Tracking Method (PTM) and the Amplitude Tracking Method (ATM). The obtained values from PTM are 4.0{\,}$\pm${\,}2.1{\,}Mm and 4.1{\,}$\pm${\,}0.3{\,}Mm while those from ATM are 3.4{\,}$\pm${\,}1.0{\,}Mm and 3.7{\,}$\pm${\,}0.1{\,}Mm, respectively, for the AIA and Hi-C data. Our results do not indicate any notable difference in damping lengths between these instruments.
Figures
Figures from the paper (2 more)
Forward citations
Cited by 1 Pith paper
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Co-existence of longitudinal and transverse oscillations in polar plumes observed with Solar Orbiter/EUI
In the same polar plumes, EUI saw both outward-moving intensity disturbances (115 to 125 km/s) and transverse oscillations (50 to 250 s periods), coexisting up to 20 Mm.
Reference graph
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Reviewed August 7, 2026 · model on record in the stance chip above.
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