the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Dec 2025
High temporal resolution photos of SAR arc rays lead to a new interpretation of the physical causes: Wave-particle interactions and energetic electron precipitation
Abstract. High spatial and temporal resolution images of red auroras over Germany during the 10–11 May 2024 magnetic storm have added new information concerning stable auroral red (SAR) arc formation. The high-altitude red aurora displayed brightness streaks/rays, which continued to lower altitude green auroral brightness rays, indicating that energetic electron precipitation along the Earth’s magnetic field lines are causing both auroras. Both the red and green auroras are diffuse in nature, indicating that instability of high-energy ring current particles inside the plasmasphere followed by wave-particle interactions is the most likely cause of the precipitating electrons. The 5577Å diffuse green aurora below the SAR arcs is a new scientific finding. This is named Stable Auroral Green (SAG) arcs. The SAR and SAG arc images were taken during the first and second steps of the 2024 superstorm. They occurred during intense substorms.
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Status: open (until 21 Jan 2026)
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CC1: 'Comment on egusphere-2025-5536', Ryuho Kataoka, 10 Dec 2025
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AC1: 'Reply on CC1', Yasuhito Narita, 13 Dec 2025
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Comment: This manuscript utilizes the full-color photographs as obtained from digital cameras, including a smart phone camera of Google Pixel, to discuss that the greenish feature in the sub-auroral region (around 50 MLAT) is a new type of aurora. However, it is difficult for readers to be confident about the conclusion by reading though the manuscript.
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Reply: We respectfully disagree.
Comment: First of all, since the clear ray structure appeared in Figures 3, 4, and 5 is one of typical evidence used for distinguishing auroras from SAR, the starting point of the scientific discussion is confusing. Also, for example, well-known quiet arcs before auroral breakup look diffuse and greenish, but there is no discussion about the essential difference between the quiet arcs and the newly discovered auroral type.
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Reply: Please remember that this is a magnetic storm interval with SAR arcs clearly visible. Those are SAR arcs, are you not convinced of that? The high time resolution images allowed clear red (and green) streaks to be shown for the first time. Older images did not have the
temporal resolution to see this. The green aurora below the red aurora is a new finding as you mention, leading to the discussion of a new mechanism for causing the SAR arcs.There is no need to discuss quiet-time arcs here. Clearly there are no small (< 1 km) dim green quiet arcs in Figures 3, 4 and 5.
Comment: More technically, it is unclear how we can understand that the auroral structures in the photographs shown in Figures 3, 4, and 5 are different from pre-existing categories of auroras. Looking like diffuse and greenish is not likely scientifically enough to convince readers. For example, even if those data are from non-scientific cameras, time-lapse movies may help to partly discuss that the targeted structure in the photograph may or may not be like SAR.
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Reply: This was a very lucky occurrence where the red SAR arc and green SAG arc became visible over Germany. As discussed in Figure 1, this is not the typical case and most people have thought that red arcs occur by themselves, leading to incorrect auroral excitation mechanisms. It certainly would be nice to have time lapsed movies of these auroras, but unfortunately this did not happen. Hopefully in the future we will be lucky enough to be able to make such movies. But even if one had such movies, what new science could be learned from that?
Comment: YouTube link is also provided to partly solve that issue by checking the relative motion. However, the time-lapse movie is an artistic collection of many beautiful photographs without exact time stamps, and it is still hard to understand how the authors scientifically distinguish the auroral types. We can see from the YouTube movie that many different types of auroras exist and the mixture of them simultaneously appears in the sub-auroral region during the storm-time when the auroral oval widely extended toward mid-latitude areas.
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Reply: Beautiful movies are nice, but we think this is not pertinent to the results of our paper.
Comment: I also noticed that there might be typical SAR-like feature widely appeared to the South at around the 1:20 YouTube-movie time, although it is still very hard to convince readers about SAR by just referencing this particular YouTube movie. It seems that further critical clarifications and examination of photographs are likely necessary before concluding the discovery/hypothesis, and working on many other citizen science photograph contributions may significantly help to improve the analysis.
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Reply: Thank you. We mention that all the photographers are PhD scientists working in high level research institutes and universities in Braunschweig and Goettingen, Germany. Their camera equipment (and scientific knowledge) will be far better than the average amateur photographer. Each photographer was able to provide the exposure time and pointing direction which is given in the paper. We think that these images are very strong proof of both streaks in the red aurora and the discovery of the SAG arcs beneath the SAR arcs. We feel that no further justification is needed.
For the paper, we used only a few of the many pictures taken. We have collected all of the photos with accurate time stamps and will include this in the paper at the reviewing stage.
Citation: https://doi.org/10.5194/egusphere-2025-5536-AC1
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AC1: 'Reply on CC1', Yasuhito Narita, 13 Dec 2025
reply
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CC2: 'Comment on egusphere-2025-5536', Laura Kranich, 19 Dec 2025
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I do not recommend to publish this paper because it strongly contradicts multiple sources of evidence collected about the May 10-11 2024 geomagnetic storm in Germany.
Right from the beginning of the results chapter, there are unfounded claims made for which, in my opinion, there is no objective basis whatsoever, let alone any proof.
The central statement, referring to the photo taken on May 10, 2024 (Fig. 3), on which the argument is based, is:"Several interesting features are noted in this closeup of the aurora shown in Figure 3. The red portion of the aurora is not a featureless reddish display similar to the aurora shown in Figure 1, but there are rays in the red auroras from the upper right downward slightly to the left. This has never been reported before in scientific articles, to our knowledge. Another new feature is the green oxygen 5577 Å display below and to the left of the red aurora. The rays in the red aurora are continued into the green aurora. The rays are enhancements in brightness. In general, the green aurora is featureless other than the rays. This also has never been reported before.”
How do the authors come to the conclusion that anything extraordinary is visible there, even something never observed before? There's a complete lack of context for this claim. In fact, all that can be seen on the image is just "normal" aurora borealis, which, due to the strong southward magnetic field of the arriving CME, had simply shifted very far south. There were SAR arcs present at the time when the image was taken but they were seen in southerly directions and this can be proven beyond doubt thanks to several all-sky time-lapses from members of the AKM citizen science forum (Arbeitskreis Meteore e.V.) that night.
This unfortunately renders the rest of the paper entirely meaningless.
Image No. 3 shows nothing more than the northeastern edge of the main phase of the second substorm (after sunset) shortly after 10:00 PM UT (the constellation Cygnus with Deneb are in the center of the image and Vega can be seen at the top right side).
Further down in the paper, the authors claim that the three photos provided show a SAR arc, which is actually only the case with the last one (Fig. 5).
According to the paper, image no. 3 was taken at 22:16 UT in Braunschweig, and the SAR aurora - quite expectedly, given the extreme values of rt solar wind measurements - was already visible high in the southern sky at that time. Even in Bavaria, several hundred kilometers to the south, the SAR arc (or at times times multiple SAR arcs) was already visible in the southern sky way before 22:00 UT. Rays were later visible within it, but these were mostly accopmanied by green Ragda aurora also visible in southerly directions. Thanks to the AKM forum, this is very well documented, for example, on the first page of this thread:
https://forum.meteoros.de/viewtopic.php ... 5&#p244952
or here: https://forum.meteoros.de/viewtopic.php ... 40#p245009
or here: https://forum.meteoros.de/viewtopic.php ... 80#p245047
or here: https://www.ofd-wetter.de/PL-Galerie/20 ... -00-08.mp4
or here: https://www.youtube.com/watch?v=td-kTEX5p9A
or here: https://www.carl-herzog.de/wetter/astro ... ichter.mp4
or here: https://www.youtube.com/watch?v=wkney3hUCi0
The paper's central claims about the existence and successful capture of a supposed “SAG” aurora can therefore be clearly refuted with some simple research. I therefore strongly suggest a withdrawal of this paper until actual evidence for its claims can be provided.Citation: https://doi.org/10.5194/egusphere-2025-5536-CC2 -
RC1: 'Comment on egusphere-2025-5536', Anonymous Referee #1, 19 Dec 2025
reply
The authors used German auroral photos during the May 2024 storm to find a new type of stable auroral red arcs and classified the greenish feature as "stable auroral green arcs. This is an interesting attempt. However, the authors did not show much evidence except for their morphological discussions. The authors' auroral photos and the morphological descriptions rather reminds me of typical midlatitude aurorae with ray structure, in contrast with the typical SAR arc morphology (Kozyra et al., 1997). Moreover, the term of "stable auroral green arcs" was used in Mendillo et al. (2016) that the authors did not cite. Unless the authors show substantial independent evidence, their manuscript does not seem convincing enough to satisfy the threshold of publication in Annales Geophysicae. Of course, I am happy to take a closer look if the authors provide further supporting evidence.
Major Comments
The SAG (stable auroral green) arcs have been once used in Mendillo et al. (2016) that the authors did not cite. What is the difference of their SAG arcs but Mendillo et al.'s SAG arcs? What is the authors' novelty on SAG arcs against what Mendillo et al. (2016) discussed?
Apart from the authors' own articles, their references are extremely old. Their latest external references date only down to 2008, except for the SuperMAG dataset (Gjerloev, 2009, 2012; Newell and Gjerloev, 2011) and the OMNI dataset (King and Papitashvili, 2020). This gives me a worry whether the authors have seriously surveyed the recent research developments upon writing this manuscript.
Especially for the May 2024 storm, we have more than tens of publications. Apart from their self citations, none has been properly addressed.
The authors claim that Figure 3 represents SAR arcs. However, this shows non reddish coronations and evident vertical structures, in contrast with SAR arcs that appear monochromatic reddish glows without evident structure (Kozyra et al., 1997). The authors explicitly admit such a discrepancy as, "The red portion of the aurora is not a featureless reddish display similar to the aurora shown in Figure 1, but there are rays in the red auroras from the upper right downward slightly to the left". Figure 3 probably shows no new feature for SAR arcs but normal auroral displays. The authors need to show proper independent measurements to interpret Figure 3 as SAR arcs.
It is better to examine auroral electron precipitations to figure out whether they are looking at SAR arcs or aurora caused by broadband electron precipitations. The authors are requested to consult DMSP satellite data to figure out whether they can justify their claim with the observational data. At least, from this picture, what they photograph does not look like a SAR arc.
I am not convinced with their comparison of Figures 1 and 3. Figures 1 and 3 are photographed at Los Angles on 12 Apr 1981 and at and Braunschweig at 22:16 UT on 10 May 2024. They are different events. Numerous publications have documented colorful auroral displays that extended to the European sector during the May 2024 storm (e.g., Spogli et al., 2024; Grandin et al., 2024). This is not something terribly new either. The authors need to contextualise their photos and movies to such citizen-science approaches.
Minor Comments
Page 1 Line 10 The authors need some references to justify their statement. Who made such an interpretation? Who examined the spectrum? The spectroscope showed up decades after the French Revolution.
Page 1 Line 12 The authors need to clarify from where "SAR arcs were seen ∼20% of the time".
Figure 1 The authors need to add proper credits for this figure. We can see James Young's name in the photo, while it is important to address that in the figure caption.
Page 2 Line 20 The authors need to show proper references to justify their statement on their statement: " the announcer told viewers to go outside and experience the unusual red aurora". If this is one of the authors' personal recollection, the authors should clarify so.
Page 4 Line 56 Tsurutani et al. (2003) documented the aurora extending down to L = 1.3, rather than L ≈ 3. It should be better to use their own value.
Page 4 Line 62 How did the authors define the auroral duration as ∼19:00 and 24:00 UT? I could not find any references to justify this statement. Did they make some survey? Did they rely on some citizen science surveys (e.g., Grandin et al., 2024) for this duration? Or did they indicate this duration from one of the German authors' personal experience?
Page 5 Line 76 I am slightly confused here. Wy are they comparing Figures 1 and 3? They are from different storms with different magnitude in the most negative Dst suppressions. We learn extremely little from comparison of the red aurora in Los Angeles in Apr 1981 and red and green aurorae in Germany in May 2024.
Page 6 Line 93 In contrast with the authors' claim, rays are visible at least in the supposed "SAR arc".
References
Grandin et al. (2024) The Gannon Storm: Citizen Science Observations during the Geomagnetic Superstorm of 10 May 2024, Geoscience Communication, 7, 297–316
Kozyra et al. (1997) High-Altitude Energy Source(s) for Stable Auroral Red Arcs, Reviews of Geophysics, 35, 155-190
Mendillo et al. (2016) A Stable Auroral Red (SAR) Arc with Multiple Emission Features, Journal of Geophysical Research: Space Physics, 121, 10564-10577
Spogli et al. (2024) The Effects of the May 2024 Mother’s Day Superstorm over the Mediterranean Sector: From Data to Public Communication. Annals of Geophysics, 67, PA218
Citation: https://doi.org/10.5194/egusphere-2025-5536-RC1
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- 1
This manuscript utilizes the full-color photographs as obtained from digital cameras, including a smart phone camera of Google Pixel, to discuss that the greenish feature in the sub-auroral region (around 50 MLAT) is a new type of aurora. However, it is difficult for readers to be confident about the conclusion by reading though the manuscript.
First of all, since the clear ray structure appeared in Figures 3, 4, and 5 is one of typical evidence used for distinguishing auroras from SAR, the starting point of the scientific discussion is confusing. Also, for example, well-known quiet arcs before auroral breakup look diffuse and greenish, but there is no discussion about the essential difference between the quiet arcs and the newly discovered auroral type.
More technically, it is unclear how we can understand that the auroral structures in the photographs shown in Figures 3, 4, and 5 are different from pre-existing categories of auroras. Looking like diffuse and greenish is not likely scientifically enough to convince readers. For example, even if those data are from non-scientific cameras, time-lapse movies may help to partly discuss that the targeted structure in the photograph may or may not be like SAR.
YouTube link is also provided to partly solve that issue by checking the relative motion. However, the time-lapse movie is an artistic collection of many beautiful photographs without exact time stamps, and it is still hard to understand how the authors scientifically distinguish the auroral types. We can see from the YouTube movie that many different types of auroras exist and the mixture of them simultaneously appears in the sub-auroral region during the storm-time when the auroral oval widely extended toward mid-latitude areas.
I also noticed that there might be typical SAR-like feature widely appeared to the South at around the 1:20 YouTube-movie time, although it is still very hard to convince readers about SAR by just referencing this particular YouTube movie. It seems that further critical clarifications and examination of photographs are likely necessary before concluding the discovery/hypothesis, and working on many other citizen science photograph contributions may significantly help to improve the analysis.