Studies of noctilucent clouds from the stratosphere during the 2024 TRANSAT balloon flight

Dalin, Peter; Suzuki, Hidehiko; Pertsev, Nikolay; Perminov, Vladimir; Megner, Linda; Kero, Johan; Voelger, Peter; Hedin, Jonas; Baumgarten, Gerd; Réchou, Anne; Efremov, Denis

doi:10.5194/egusphere-2025-5757

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https://doi.org/10.5194/egusphere-2025-5757

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11 Dec 2025

| 11 Dec 2025

Studies of noctilucent clouds from the stratosphere during the 2024 TRANSAT balloon flight

Peter Dalin, Hidehiko Suzuki, Nikolay Pertsev, Vladimir Perminov, Linda Megner, Johan Kero, Peter Voelger, Jonas Hedin, Gerd Baumgarten, Anne Réchou, and Denis Efremov

Abstract. A transatlantic scientific balloon flight (TRANSAT) was conducted between 22 and 26 June 2024. The TRANSAT balloon, operated by the French Space Agency (CNES), floated in the stratosphere at approximately 40 km altitude between Esrange (Sweden) and Baffin Island (Canada) for about 3.8 days. The scientific payload comprised nine instruments, including two from the Swedish Institute of Space Physics: an optical imager for noctilucent cloud (NLC) studies and an infrasound instrument for atmospheric infrasound wave investigations. The NLC imager consisted of three identical visible-range optical cameras, one of which operated successfully throughout the entire flight, capturing thousands of NLC images. The TRANSAT balloon campaign was supported by ground-based lidar measurements and spaceborne observations from the Swedish MATS satellite. Here, we describe the technical characteristics of the balloon experiment and present early results. Nearly continuous observations of NLC were obtained during the entire flight. A localized warm region in the mesopause was identified as the cause of temporary NLC disappearance, while complex NLC structures exhibiting different motions were found to probably result from horizontal wind rotation with altitude within the mesopause region.

Received: 19 Nov 2025 – Discussion started: 11 Dec 2025

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Peter Dalin, Hidehiko Suzuki, Nikolay Pertsev, Vladimir Perminov, Linda Megner, Johan Kero, Peter Voelger, Jonas Hedin, Gerd Baumgarten, Anne Réchou, and Denis Efremov

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-5757', Anonymous Referee #1, 14 Jan 2026
Review of " Studies of noctilucent clouds from the stratosphere during the 2024 TRANSAT balloon flight " by Peter Dalin et al.
General Comment:
The prime objective of this manuscript is to investigate the dynamics and variability of noctilucent clouds (NLC) through unprecedented stratospheric balloon-based imaging, supported by ground-based lidar and satellite observations. Key results from the comprehensive observations and multi-instrument analysis indicate that NLC were detected nearly continuously over 3.8 days, with their disappearance linked to a localized warm region in the mesopause caused by the intrusion of mid-latitude air. Furthermore, observed double-layer NLC structures moving in opposite directions were consistent with wind shear and inertial-gravity wave activity. Overall, most of the analysis in this manuscript is well executed and the conclusions are well-reasoned. However, the present manuscript has some shortcomings, and that is why the manuscript can be considered for publication after minor revisions from my point of view. I would like to give my comments below that need to be addressed.
Specific Comments:
In Section 2.1, Sony α7 M3 is an SLR camera, and its operation is generally manual. It is suggested that the author add a description here to explain how to realize automatic shooting and how to transform the cameras.

I noticed that there are four different gaps in the observation of NLCs in Figure 5, and then I mapped the gap time to the flight location in Fig.2a. It seems that the regions where these gaps appear are not consistent with the warm regions in Fig.9. Can the author provide some explanations? In addition, I suggest marking the location and coverage of each gap in Fig. 2a or b.

In Section 4.3, I downloaded and watched the video provided by the author carefully. I can see that the structures of NLCs have different directions of movement, but I am not sure whether it is a double-layer cloud, and whether it is just the change caused by gravity waves in different directions of propagation? Can the author provide some more detailed and convincing evidence and identification process for distinguishing double-layer clouds?

I suggest that figure 6, figure 9 and figure 10 all indicate a, b, c, d….
Citation: https://doi.org/10.5194/egusphere-2025-5757-RC1
- AC1: 'Reply on RC1', Peter Dalin, 22 Jan 2026
  
  We are grateful to the Reviewer 1 for reviewing our manuscript and for the useful comments that allowed us to improve the manuscript. Below are our responses to the Reviewer's comments marked in bold.
  Specific Comments:
  1. In Section 2.1, Sony α7 M3 is an SLR camera, and its operation is generally manual. It is suggested that the author add a description here to explain how to realize automatic shooting and how to transform the cameras.
  This camera has a standard Micro USB terminal for power supplying, battery charging and USB communications, into which an external trigger can be connected to activate the shutter button. For this purpose, we have developed an external trigger (the gray box shown in Fig. 1) that simultaneously transmits the trigger signal to all three NLC cameras at a certain frequency (50 s in this experiment). Thus, automatic shooting is carried out by three cameras throughout the balloon flight. In addition, this external device distributes power to all three cameras from an external power supply (a chemical battery on the gondola). We have added this information to the revised manuscript.
  2. I noticed that there are four different gaps in the observation of NLCs in Figure 5, and then I mapped the gap time to the flight location in Fig.2a. It seems that the regions where these gaps appear are not consistent with the warm regions in Fig.9. Can the author provide some explanations? In addition, I suggest marking the location and coverage of each gap in Fig. 2a or b.
  We have added the locations of each gap (four gaps comprise 8 points) in Fig. 2a. The second gap does include the warm region shown in Fig. 9, taking into account the field of view of the camera projected on the Earth’s surface as shown in Fig. 8.
  3. In Section 4.3, I downloaded and watched the video provided by the author carefully. I can see that the structures of NLCs have different directions of movement, but I am not sure whether it is a double-layer cloud, and whether it is just the change caused by gravity waves in different directions of propagation? Can the author provide some more detailed and convincing evidence and identification process for distinguishing double-layer clouds?
  As we noted in Section 4.3, we cannot measure the heights of the NLC layers in this experiment. Therefore, we cannot 100% assert the presence of a double NLC layer in this case. The Reviewer 1 is partly right in this. At the same time, we showed that the double NLC layer was previously detected by the ALOMAR lidar in Norway. In addition, we have provided a list of theoretical and experimental studies showing the mechanism of formation of a double NLC layer and statistics on double and multiple layers in NLC. Thus, a double NLC layer is not such a rare phenomenon but it is formed relatively often.
  Also, we analyzed this event based on JAWARA model data, which showed the presence of an inertia-gravity wave and the rotation/change of neutral wind with height, which is an inherent property of a gravity wave. There is good agreement between the measured NLC velocities and the model wind velocities. Therefore, it can be assumed with a high probability that there was the double NLC layer in this space-time domain, and the different NLC motions may be a consequence of the passage of this wave through the NLC layer.
  Other potential explanations in this regard are speculative and cannot clarify the presence or absence of the double NLC layer. We want to avoid unnecessary speculative reasoning in the present manuscript.
  4. I suggest that figure 6, figure 9 and figure 10 all indicate a, b, c, d….
  We agree with this comment and have added abbreviations A, B, C, D of the panels in Figs. 6, 9 and 10.
  
  Citation: https://doi.org/10.5194/egusphere-2025-5757-AC1
RC2:
'Comment on egusphere-2025-5757', Anonymous Referee #2, 09 Feb 2026

This work is a summary of the NLC dataset obtained by the SONC imager onboard the TRANSAT balloon flight from Sweden to Canada in June 2024. It is an accomplishment that the flight and recovery were successful! The work includes highlights on specific features such as NLC disappearances and potential double-layer structure. The authors made an effort to compare to previous results and other datasets, e.g. satellite data and reanalysis. Especially the analysis on wind rotation with altitude coinciding with the different motions of NLC layers is very interesting. The imager data was made available and can be used for future research.
The manuscript is more of an overview motivated by the observations rather than a work based on a specific scientific hypothesis or objective that is related to some open question in this field of research. Several methods or ideas are employed, like wavelet analysis (but only to a single image?), or comparing to reanalysis temperatures for a specific case, which all give reasonable but not necessarily new scientific results. Nevertheless a record of the mission is valuable, and I recommend it for publication after revision. My questions and comments are sorted by line number:
line 100: Is it correct that all cameras were recovered undamaged and fully functional? So why did two of three not provide data? Was it a software/datastorage/telemetry issue? Investigating the problem might inform future missions.
line 107: Generally, I suggest to put urls into the bibliography. For this particular one, however, I think it is uncommon to cite news articles in scientific publications. It is safer to remove the link and give the relevant information in the text (to avoid a broken link at some time).
line 109: The infrasound experiment is not used in this study at all, so I suggest to not mention it here.
line 118: From watching the video, it seems the anti-sun pointing of the gondola was not active during all of the flight. This should be clearly indicated as it likely affects the data analysis.
Fig. 2: When watching the video, one notices the change in azimuth. I think it would be helpful to plot azimuth as a function of time in addition to height in Figure 2.
line 145: In the video, around 2024-06-23 6:02 UTC, there are strong reflections on the camera image. Are these from dust on the camera lens? Was this data removed from the keogram analysis?
line 145: There seems to be a vibration (up-down movement of the horizon) at 25 June 8:51 UTC and 26 June 4 UTC. What caused this? Does this effect the keogram analysis?
line 145: A white object enters the field of view on 26 June 6 UTC. I am curious what this is?
Figure 3: On the image, the structures are hardly visible. It is a bit better in the video. Please add the projected image and the relative brightness from the wavelet analysis to this figure.
line 146: For how many images was the automatic image processing done? Only one example is shown.
line 146: More information on the automatic analysis should be given, including plots for each step. Please show the projected image for the example in Fig. 3 and 4. In Figure 4, why is the x-axis labeled with "Pixels along Y-axis of image", when it the text it is stated that the wavelet transform was applied to the projected image? The projected image' dimensions must be latitude/longitude or in units of km, not pixels. Please mark the positions in the original image, where the waves of 30 and 40 km horizontal wavelength were detected. Is it on the top or the bottom of the original image? Is Figure 4 shown for a single vertical column of the image, or many?
line 170: "at which NLC preferred to appear" Could it be that the sensitivity of the camera or the viewing geometry is most favourable in this part of the image? Close to the horizon, the camera observes the NLC layer at a lower angle compared to the top of the image closer to zenith. Please state at which distance from the gondola positions these pixel positions are. These must be farthest away, maybe a 1000 km?
line 171: Is the keogram constructed from projected or unprojected images? Fig. 3b, where we can see pixels 3000-3400, is unprojected, but then it makes no sense that (l. 151) "each image was projected prior to the analysis" in this (1)-(4) sequence of steps.
line 177: Does "the manual procedure" refer to the video? It is stated in line 141 that the images in the video were marked for NLC presence or absence, but no results are shown. Can this be added to Fig. 5?
line 174: Is one standard deviation enough? Did you check other values, does the result change? Is the mean brightness (and sensitivity of the camera to NLC) constant for all times? Does it depend on the azimuth, or its deviation from anti-sun?
Figure 5. It is worth noting that the times in the caption do not correspond to the times of the gondola position shown in Fig. 2a because it shows NLC observed at a long distance (the NLC position depending on azimuth).
Fig. 5: Please add the number of y-pixels for which this keogram was constructed to the caption.
Fig. 5: I think during the first 12 h the effect from the unstabilized gondola is seen as vertical stripes. The keogram can only be interpreted knowing the azimuth.
line 195: It is not of relevance for this paper that the ALOMAR lidar can measure polar stratospheric clouds in winter, or stratospheric aerosols. Please only state necessary information. For example, temperature and winds would be useful for interpretation of the NLC layer shown in Fig. 6. But as these are not shown, it can be removed.
Figure 6: Can the lidar measurements be shown with the same quantity for color? It is very hard to compare "RayHi backscatter signal in Hz" to backscatter coefficients. It seems when the Esrange raw counts would be converted to backscatter coefficients, it might not be significant at this high resolution. Maybe it would be better to bin Esrange data to 1 km x 1 h?
line 206: It is not "clearly seen in both lidar measurements". There is an enhancement of signal at 3 UTC over the whole column 80-87 km at Esrange, which is very different from ALOMAR. It is hard to see from the plot if all is noise or what part is significant.
Fig. 6: Can you add a projected SONC image when the two stations are in the field of view of the imagers? Unluckily, I suppose, at 7 UTC, when the NLC brightness at ALOMAR increases, the imagers looked west, away from ALOMAR?
line 210: "turbulent vorticities": the phrasing is uncommon. Maybe "instabilities"? Turbulence is not visible at these scales, I think.
line 217: "nearly the same NLC" please add SONC images to prove this
line 219: What is the scientific conclusions from the presented measurements?
line 247: A discussion of the resolution of the SONC image data should be added to this work. From Fig. 3a and the video, it can be doubted that "smaller gravity waves and turbulent structures" can be detected. As the images were projected, please include an assessment of the largest and smallest scales that can be observed.
line 250: "complement each other". In line 231, it was stated that MATS provides high-resolution images and three-dimensional fields of NLC properties. Is Fig. 7b the high-resolution image? As the satellite likely passes several times per day, was this the only coincidence?
Fig. 7a: As a second-order polynomial was removed in the analysis, would it be useful to show this image also, in order to see the structures more clearly? The original image printed out hardly shows structure.
line 263: Why is it unexpected? There is a long history of AIM imagery of NLC. What is expected from the AIM climatology for this time of year and location? E.g. in 2022, AIM shows strong and widespread NLC https://lasp.colorado.edu/aim/browse-images
line 265: Please add mean latitude and longitude fo these time periods, as they do not relate to the gondola positions shown on the map.
line 291: The third interval on the 25th is the longest. Does JAWARA show similar temperature enhancements for this time and location?
line 371: Was the estimation of speeds performed on projected images? Can a sequence of two projected, background-removed image be shown that highlights speed and directions of the two layers?
line 449: It is not "unprecented", as there was previous imaging from stratospheric balloons
line 453: Why were only 6200 images from 40'000 images used? As the 6200 cover all times, were all the other images taken in between? Had they unsuitable exposures such that they could not be used, or what is the reason for excluding so many images?
line 472: Is point 5 based on a single image only? As the automatic analysis is not used to obtain statistics in this work, it should be motivated why it was done. Is it to determine the smallest observable scales?
line 474: The result that NLC disappearance was linked to a warming is consistent, but not surprising, as the dependence of NLC occurrence on ambient temperature is known. Section 4.2 lacks citations of previous work on this topic.
Spelling and grammar
line 161: obtaned -> obtained
line 165: Language of this paragraph should be improved, e.g. "Keogram is a slice.." -> "A keogram is a a slice..", singular, plural etc.
line 198: "during the TRANSAT flight in the stratosphere" please remove "in the stratosphere"
line 215, line 238: "from the stratosphere" -> "by the SONC imager" or "from the TRANSAT balloon"
line 216: "lunch" -> "launch"
line 228: "6" -> "six"
line 254: Delete "from the stratosphere"
line 354: TRANSAN -> TRANSAT

Citation: https://doi.org/10.5194/egusphere-2025-5757-RC2
- AC2: 'Reply on RC2', Peter Dalin, 24 Mar 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5757/egusphere-2025-5757-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-5757-AC2

Peter Dalin, Hidehiko Suzuki, Nikolay Pertsev, Vladimir Perminov, Linda Megner, Johan Kero, Peter Voelger, Jonas Hedin, Gerd Baumgarten, Anne Réchou, and Denis Efremov

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Short summary

A transatlantic scientific balloon flight (TRANSAT) was conducted in June 2024. The TRANSAT balloon floated in the stratosphere at approximately 40 km altitude between Sweden and Canada for about 4 days. An optical imager was installed on the TRANSAT balloon to study noctilucent cloud (NLC). Nearly continuous observations of NLC were obtained during the entire flight. A complex NLC structures exhibiting different motions were found to result from wind rotation with altitude between 80 and 94 km.


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