| 10 Apr 2026
Mesoscale Structure of Flickering Aurora from Wide-Field High-Speed Imaging
Abstract. We report wide-field observations of flickering aurora obtained with a fast sCMOS camera and a diagonal fisheye lens at Poker Flat Research Range, Alaska, on 8 February 2016. The system recorded 512×512 pixel images at 80 Hz, enabling us to investigate the mesoscale organization of flickering along a discrete auroral arc over spatial scales of several hundred kilometers. Flickering occurred intermittently with dominant frequencies between 3 and 20 Hz, most commonly within a narrower band of 4–12 Hz. Spatial maps of the peak frequency reveal that regions with similar periodicities sometimes formed coherent clusters on scales of∼10 km, and that multiple clusters with different frequencies (e.g., ∼8 and∼13 Hz) could coexist simultaneously along the same arc, separated by ∼150 km. Some of these clusters moved together with the background arc, suggesting that the modulation is closely tied to the local plasma environment and inverted-V potential structures associated with discrete aurora. An automated patch detection analysis showed that, although individual events may locally suggest an inverse relationship between flickering frequency and patch size, this trend does not persist statistically. Instead, flickering at a given dominant frequency occurs over a wide range of patch sizes, with a typical north–south scale of 4.4±2.4 km at 110 km altitude. These results are consistent with generation scenarios in which electron precipitation is modulated by interference among multiple EMIC waves in the auroral acceleration region, extending previous narrow-field studies to the mesoscale and demonstrating the diagnostic value of wide-field, high-cadence imaging for wave–particle interactions in the auroral ionosphere.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Annales Geophysicae.
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Review of «Mesoscale Structure of Flickering Aurora from Wide-Field High-Speed Imaging» by S. Nanjo et al.
The paper presents a novel investigation of the mesoscale organization of flickering aurora using a wide-field, high-cadence camera at Poker Flat. Unlike most prior studies, which rely on narrow-field or photometric measurements, the authors use a 180° fisheye lens and 80 Hz sampling to study auroral contexts across hundreds of kilometers, while retaining sufficient temporal resolution to resolve 3–20 Hz flickering. The paper adds important observational evidence for EMIC wave scenarios of flickering aurora. The analysis is technically sound, clearly written, and presents new findings. But some extra clarification and improvement is recommended.
Major comments
The line-of-sight and fisheye lens distortions need more careful consideration. While the authors provide reasonable mentioning at a qualitative level, more quantitative estimates of projection effects (particularly far from zenith) would strengthen the conclusions. Points to consider are mapping distortions vs. elevation angle, uncertainties in patch size and cluster boundaries, and systematic biases due to variable emission altitudes.
As the authors point out, the patch detection via keograms may potentially bias the size statistics. So it remains unclear if the authors have analyzed how their patch detection criteria may influence their results (e.g. with different count thresholds or vertical extent criteria)? How sensitive is the patch-size distribution to the threshold for ∆Count? Did the authors try different values than the threshold of 30, or why was this value chosen? Have the authors looked at how often the keogram slice misses the true center of a patch? A more careful analysis would further strengthen the significance of the conclusions.
The patch size and spatial separation rely on a fixed emission altitude that the authors have chosen (110 km). However, literature shows that there can be significant spread in the peak auroral emission heights (Whiter et al., 2023). Some reports also claim that there can be 557.7 nm emissions below 90 km altitude in the summer mesosphere (Lee et al., 2017), or higher than 120 km (Mende et al., 1993). What is the scientific justification that the authors have used for choosing a fixed emission height of 110 km? How will the spatial estimates be affected if the chosen emission altitude is slightly inaccurate? A more careful analysis of uncertainties, and some quantitative numbers on potential errors, will further strengthen the results.
The analysis focuses solely on the auroral observations, with no reference to a broader geophysical context. The paper does not mention the solar wind activity level, geomagnetic activity indices, substorm phase, etc. It may not be necessary with additional figures here, but the broader implications of the results can easily be improved if the authors add a few lines of text to the manuscript about the overall activity level during the observation period. Was this a particularly quiet or active time, in terms of solar wind, magnetic indices, or substorms?
The paper does not mention any in-situ or ground-based radar/magnetometer data, that might be of relevance. The conclusions might be further strengthened and supported by additional ground magnetometer pulsation data, Poker Flat ISR electron density/temperature measurements, or spaceborne EMIC wave observations (THEMIS, Arase, etc). If unavailable, the authors should at least state this explicitly and discuss the limitations.
Minor comments
Line 202: The video supplement is very helpful. However, please consider referencing a few specific times in the text that show key features.
The authors may also want to add a few more references to small-scale structures in flickering aurora (Whiter et al., 2008), patch size evolution during pulsating aurora (Partamies et al., 2019), or wave-like structures in the aurora before substorm onset (Wu et al., 2025).
References:
Lee, Y.-S., Y.-S.Kwak, K.-C.Kim, B.Solheim, R.Lee, and J.Lee (2017), Observation of atomic oxygen O(1S) green-line emission in the summer polar upper mesosphere associated with high-energy (≥30 keV) electron precipitation during high-speed solar wind streams, J. Geophys. Res. Space Physics, 122, 1042–1054, https://doi.org/10.1002/2016JA023413
Mende, S. B., G. R.Swenson, S. P.Geller, R. A.Viereck, E.Murad, and C. P.Pike (1993), Limb view spectrum of the Earth's airglow, J. Geophys. Res., 98(A11), 19117–19125, https://doi.org/10.1029/93JA02282
Partamies, N., Bolmgren, K., Heino, E., Ivchenko, N., Borovsky, J. E., & Dahlgren, H. (2019). Patch size evolution during pulsating aurora. Journal of Geophysical Research: Space Physics, 124, 4725–4738. https://doi.org/10.1029/2018JA026423
Whiter, D. K., B. S.Lanchester, B.Gustavsson, N.Ivchenko, J. M.Sullivan, and H.Dahlgren (2008), Small-scale structures in flickering aurora, Geophys. Res. Lett., 35, L23103, https://doi.org/10.1029/2008GL036134
Whiter, D. K., Partamies, N., Gustavsson, B., and Kauristie, K. (2023), The altitude of green OI 557.7 nm and blue N2+ 427.8 nm aurora, Ann. Geophys., 41, 1–12, https://doi.org/10.5194/angeo-41-1-2023
Wu, S.Y., Whiter, D.K., Lamy, L. et al. (2025), Radio emissions reveal Alfvénic activity and electron acceleration prior to substorm onset. Nat Commun 16, 10553, https://doi.org/10.1038/s41467-025-65580-8