# 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, et al.

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Peter Dalin, Hidehiko Suzuki, Nikolay Pertsev, Vladimir Perminov, Linda Megner, et al. Studies of noctilucent clouds from the stratosphere during the 2024 TRANSAT balloon flight. 2025. hal-05415841

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

Peter Dalin¹, Hidehiko Suzuki², Nikolay Pertsev³, Vladimir Perminov³, Linda Megner⁴, Johan Kero¹, Peter Voelger¹, Jonas Hedin⁴, Gerd Baumgarten⁵, Anne Réchou⁶, Denis Efremov⁷

¹ Swedish Institute of Space Physics, Kiruna, Sweden

² School of Science and Technology, Meiji University, Kawasaki, Japan

³ A.M. Obukhov Institute of Atmospheric Physics RAS, Moscow Russia

⁴ Department of Meteorology (MISU), Stockholm University, Stockholm, Sweden

⁵ Leibniz Institute of Atmospheric Physics, Rostock University, Kühlungsborn, Germany

⁶ Lacy Laboratory of Atmosphere and Cyclones, Université de La Réunion, Réunion, France

⁷ Aerospace Laboratory “Stratonautica”, Moscow, Russia

Correspondence to: Peter Dalin (pdalin@irf.se)

**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.

## 1 Introduction

Noctilucent clouds (NLC) are the highest clouds in the Earth's atmosphere, forming in the summer mesopause region between 80 and 90 km altitude at middle and polar latitudes in both hemispheres. They consist of water-ice particles with radii of 30–100 nm that scatter sunlight, making the clouds visible against the twilight sky. NLCs are typically observed from May to September in the Northern Hemisphere and from November to February in the Southern Hemisphere (Gadsden and Schröder, 1989; Liu et al., 2016). When observed from space, these clouds are traditionally called Polar Mesospheric Clouds (PMC) (Thomas, 1984).
<figure><img src="image_3.png" /></figure>

NLC and PMC are systematically studied using ground-based optical cameras, spectrographs, lidars as well as dedicated spaceborne instruments (e.g., Karlsson and Gumbel, 2005; Dalin et al., 2008; Bailey et al., 2009; Fiedler et al., 2011; DeLand and Thomas, 2015; Tsuda et al., 2022; Wallis et al., 2025). In addition, irregular campaign-based NLC observations are carried out using sounding rockets and aircraft (Zadorozhny et al., 1993; Gumbel and Witt, 2001; Reimuller et al., 2011; Suzuki et al., 2022; Pertsev et al., 2024). Each observational technique has specific advantages and limitations. In particular, ground-based imagers offer high horizontal (~20 m) and temporal (~1 s) resolution (Dalin et al., 2010; Baumgarten and Fritts, 2014), and lidars provide high vertical resolution (50–150 m; Baumgarten et al., 2009), but both are restricted by tropospheric weather and limited geographic coverage. Conversely, satellite observations deliver global PMC coverage but suffer from low spatial (~1 km) and temporal (minutes to hours) resolution. Moreover, sun-synchronous satellites introduce large spatial gaps (hundreds of km) between adjacent orbits at mid- and subpolar latitudes (45–60°N) due to a spacecraft orbiting the Earth. Consequently, no single existing technique can fully resolve the microphysical and dynamical processes governing NLC/PMC.

Balloon-borne observations from stratospheric altitudes (25–40 km) have a potential for comprehensive studies of NLC on a regular basis. Such observations provide several advantages: independence from tropospheric weather, uninterrupted 24 h coverage, and proximity to the NLC layer (80–85 km), yielding the highest achievable spatial resolution. From this vantage point, both large-scale (up to 2500 km, limited by Earth's curvature) and small-scale (down to meters) NLC features can be observed. Furthermore, balloon-borne observations at 30–40 km altitude occur above the ozone-rich layer (20–25 km), thereby avoiding light absorption in the Chappuis band (400–650 nm).

To date, four balloon-borne experiments have been dedicated to NLC observations from the stratosphere (Miller et al., 2015; Dalin et al., 2019, 2020, 2022; Fritts et al., 2019). These missions have provided unique datasets for studying both large- and small-scale wave dynamics and turbulence within NLC layers. In this study, we present results from a new long-duration balloon experiment aimed at investigating NLC in the polar mesopause. We describe the observation technique and instrumentation as well as early scientific findings. The stratospheric NLC observations were complemented by ground-based lidars and, for the first time, by spaceborne measurements from the Swedish MATS satellite. Additionally, temperature and water vapor data from the Aura/MLS satellite were used to characterize the mesopause environment during the TRANSAT flight, and JAWARA model data were employed to simulate the thermodynamic and wind conditions in support of the NLC observations.

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## 2 Technique and method

### 2.1 Technical characteristics of the imager

The Stratospheric Observations of Noctilucent Clouds (SONC) experiment is a balloon-borne scientific mission designed to investigate NLC dynamics across a broad range of spatial scales—from large-scale structures (30–2500 km) to fine-scale features (5 m–30 km)—as well as the microphysical properties of NLC ice particles.

To achieve these objectives, three high-resolution, high-sensitivity Sony α7 Mark III cameras were used. Each camera has a full-frame 35 mm, 24-megapixel sensor (6000 × 4000 pixels). Two cameras were equipped with wide-angle lenses (field of view, FoV, =105.4° × 81.8°), while the third carried a narrow-angle lens
<figure><img src="image_4.png" /></figure>

(FoV=15.0° × 10.1°). This configuration, two wide-angle and one narrow-angle camera, provides a horizontal coverage of about 180° for studying mesospheric NLC structures up to 2500 km across, while simultaneously enabling spatial resolution as fine as about 5 m when viewing the NLC layer at 83 km from a 40 km altitude and 35° elevation angle. Note that such simultaneous access to both large and small scales is currently unattainable from either ground-based or satellite platforms. The three cameras formed the SONC imager, illustrated in Fig. 1. Each camera stored data on two 1 TB SD cards. Given the expected 4–5 day duration of the transatlantic flight and the storage limits, an image acquisition cadence of 50 s was chosen. This temporal resolution enables tracking the evolution of small-scale wave activity within the NLC layer as well as monitoring large-scale cloud dynamics. The optical axes of the two wide-angle cameras were inclined at 36° to the horizontal plane in order to minimize light contamination from bright tropospheric scattering and from above coming from a big white balloon envelope (~70 m in diameter). The narrow-angle camera was tilted at 35° to the horizontal plane to resolve fine-scale structures of NLC.

<figure><img src="image_5.png" /><figcaption>Figure 1: The SONC imager consists of three Sony a7 Mark III cameras and the electronic control unit (gray box with red button). Two cameras are equipped with wide-angle lenses, and one with a narrow-angle lens positioned between them. The instrument was used to observe noctilucent clouds during the TRANSAT transatlantic balloon flight (22–26 June 2024).</figcaption></figure>

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Automatic exposure bracketing was applied, capturing 5 images sequentially with exposure times varying between 1/1600 s and 1.6 s. This approach accommodated a wide dynamic range of NLC brightness under variable atmospheric background illumination conditions. One wide-angle camera (left in Fig. 1) operated flawlessly throughout the 3.8-day flight, producing a total of about 40,000 images. The other wide-angle and the narrow-angle cameras functioned for the first 13 h and 12 h, respectively, before ceasing operation for unknown reasons. The imager survived the gondola landing on the ice of Baffin Island without damage. Post-flight tests of all the cameras and electronics confirmed full functionality of the SONC imager and no performance degradation.
<figure><img src="image_6.png" /></figure>

106 The TRANSAT mission is a long-duration stratospheric balloon flight organized and operated by the French
107 Space Agency (CNES) (https://cnes.fr/en/news/balloons-crossing-atlantic-wind) for multidisciplinary
108 atmospheric and cosmic research. The gondola, named Carmen, had a total mass of about 900 kg and carried
109 nine scientific instruments. Two of these were from Sweden: the SONC imager and an infrasound instrument
110 dedicated to studies of infrasound atmospheric waves and their sources. The infrasound experiment and its
111 scientific results will be described in a separate paper.

112 The Carmen gondola utilized a three-axis motorized gimbal stabilized platform, providing an elevation-angle
113 stability of 0.15°, an absolute azimuth pointing accuracy of 1° and a stability of ±10 arcmin around it. Because
114 stratospheric balloons rotate continuously, such stabilization was critical to avoid image smearing and to
115 maintain consistent NLC monitoring at fine and large scales.

116 As the transatlantic flight took place under solstice conditions with continuous solar illumination, scientific
117 instruments were mounted behind a solar shield to prevent overheating and stray-light interference. To ensure
118 continuous shading, the gondola was slowly rotated to keep the solar shield facing the Sun throughout the flight.

119 The TRANSAT balloon was launched from Esrange, northern Sweden (67.89° N, 21.08° E) at 18:57 UTC
120 (20:57 LT) on 22 June 2024 and landed on Baffin Island, northern Canada, at 12:14 UTC (08:14 LT) on 26 June
121 2024. The total flight time was about 90 h (3.8 days), with an average horizontal speed of about 11 m s⁻¹. The
122 horizontal trajectory and altitude profiles of the TRANSAT mission are shown in Fig. 2.
123
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Figure 2: (A) Horizontal trajectory (the red line) of the TRANSAT balloon mission between 22 and 26 June 2024. The blue crosses mark the balloon's position every 12 h. (B) Altitude profile of the TRANSAT flight.</figcaption></figure>

## 2.3 Image processing

Prior to launch, all three Sony cameras underwent geometrical calibration on the ground using night-sky images containing reference stars. A second-order polynomial camera model was fitted by comparing theoretical and measured horizontal coordinates for over 200 identified reference stars. Six free coefficients describing focal length, image orientation, and optical distortion were determined. These coefficients were used to compute relative horizontal coordinates (elevation and azimuth angles relative to the center of the image) for every pixel of each camera.
<figure><img src="image_9.png" /></figure>

Given the known position and orientation of the SONC imager within the gondola reference frame, provided by CNES, the absolute horizontal coordinates of all pixels were computed. Subsequently, a georeferencing procedure projected each pixel onto the Earth's surface, assuming a mean NLC altitude of 83 km. Details of the calibration, georeferencing, and error analysis are available in Dalin et al. (2015).

NLC identification was performed using both manual and automatic approaches. In the manual procedure, about 6200 images, suitable for scientific analysis, from the left camera (Fig. 1) were carefully examined to mark NLC presence or absence. The number of images is 6200 because almost every image in a series of 5 images, taken in bracketing mode, was analyzed. A video sequence was then assembled to verify temporal continuity. This video, containing all images with synchronized flight parameters, timestamps and balloon coordinates, is available at the Harvard Dataverse repository (https://doi.org/10.7910/DVN/1PHRZU).

Automatic image processing involved several steps: (1) Background subtraction: A second-order polynomial fit was used to estimate the sky background along each vertical column of pixels, which was then subtracted to yield a residual brightness of each pixel. (2) Two-dimensional Fourier filtering: applied to a residual brightness to extract gravity-wave-like patterns on image (see Fig. 3b). (3) Wavelet analysis: a Morse analytic wavelet transform was applied to selected images to quantify horizontal wavelengths of identified wave patterns. Each image was first projected onto Earth geographical coordinates (in km) prior to the analysis. An example (Fig. 4) shows dominant horizontal wavelengths of about 30 and 40 km, which are frequently observed in NLC (Pautet et al., 2011; Demissie et al., 2014).

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<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>Figure 3: (A) Example of NLC image acquired at 10:17:21 UTC on 25 June 2024 during the TRANSAT balloon flight. (B) Two-dimensional Fourier-filtered version of (A), highlighting residual brightness variations associated with NLC bands seen in the middle and the lower part of the image.</figcaption></figure>

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<figure><img src="image_13.png" /><figcaption>Figure 4: Example of a wavelet transform for the NLC image obtained at 10:17:21 UTC on 25 June 2024 during the TRANSAT flight. Gravity waves having prevailing horizontal scales of about 30 and 40 km are clearly seen as the red color code.</figcaption></figure>

4) A keogram representation of a sequence of all analyzed images helps to identify the presence or absence of NLC as a function of time. Keogram is a slice of an image along a specific axis, and many such slices are put together as a function of time to produce brightness variations of an observed atmospheric phenomenon over time. Keogram is commonly used to demonstrate a presence or absence of aurora in all-sky images. In the present study, based on the manual careful inspection of 6200 images suitable for scientific analysis, we have selected horizontal pixel lines in the range of 3000-3400 pixels (counting along the Y-axis from the top of the image) at which NLC preferred to appear. The image coordinates are present in Fig. 3b. These pixels were summed to produce an integrated brightness for every column between 3000 and 3400 pixels. The mean value and its standard deviation (std) were then calculated for the given slice along the X-axis of the image. Next, a selection rule was applied: brightness values greater than 1 std of the mean were selected to classify the given slice as NLC present or absent. Then keogram was created by putting together slices of about 6200 images (Fig. 5), demonstrating nearly continuous presence of NLC, that is discussed in detail in Section 4. This automatic keogram procedure fits well with the manual procedure (described above) in identifying the presence or absence of NLC as a function of time.
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Figure 5: Keogram compiled from about 6200 images during the TRANSAT balloon flight on 22-26 June 2024. The red arrows mark four time intervals (white-grey areas) when NLC were absent. Corresponding local solar times (LST) are shown for four time intervals on the x-axis.</figcaption></figure>

# 3 Ground-based and space complementary measurements

## 3.1 Lidar measurements

During the initial phase of the TRANSAT balloon flight, complementary lidar measurements were performed at Esrange (67.8°N, 21.1°E, Sweden) and Andøya (69.3°N, 16.0°E, Norway), with the aim to provide simultaneous and common volume (or in proximity) measurements of NLC layers above northern Scandinavia. At Esrange, we use a Rayleigh/Mie/Raman (RMR) backscatter lidar developed by the Bonn University to monitor aerosols in the troposphere, stratosphere and mesosphere as well as to determine temperature profiles in the aerosol-free part of the atmosphere (Blum and Fricke, 2005). The vertical and time resolution of the obtained measurements by the Esrange lidar is 150 m and 4.4 min, respectively. At Andøya, we use an RMR-lidar which has been operated as part of the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR). Using measurements of the ALOMAR lidar one can study temperatures and winds in the middle atmosphere, aerosol layers in the stratosphere, polar stratospheric clouds in the lower stratosphere in winter and noctilucent clouds in the mesopause region in summer (Baumgarten, 2010). We use lidars backscattered signals from the 532-nm wavelength channel to measure NLC over Esrange and Andøya during the TRANSAT flight in the stratosphere over northern Scandinavia. The vertical and time resolution of the obtained measurements by the ALOMAR lidar is 40 m and 30 s, respectively.
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Figure 6: Lidar measurements at Andøya, Norway (upper panel) and Esrange, Sweden (lower panel) during the initial phase of the TRANSAT balloon flight on 22-23 June 2024.</figcaption></figure>

The results of the lidar measurements during the initial phase of the TRANSAT flight are shown in Fig. 6. The NLC layer is clearly seen in both lidar measurements between 81 and 86 km altitude. Between 20 and 22 UTC, the NLC layer lifted up from 84 to 86 km, then it sank to 81-82 km between 22 UTC on 22 June and 01 UTC on 23 June as demonstrated by both lidars. Note that the ALOMAR lidar registered a double-layer NLC structure between 00 and 01 UTC. Another double-layer NLC structure will be discussed in section 4. These height variations of the NLC layer are due to propagating atmospheric gravity waves and turbulent vorticities. Note that it is a very rare case of conducting simultaneous lidar measurements at Andøya and Esrange (separated by ~250 km) on both sides of the Scandinavian range, registering about the same NLC layer. After 01 UTC, Esrange lidar measurements become too noisy to distinguish the NLC signal from noise. The ALOMAR lidar operated until 12 UTC on 23 June, continuously registering the enhanced NLC layer until 09 UTC, with height
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

variations between 81 and 84 km. The extended NLC field was registered from the stratosphere above northern Scandinavia (not shown in figure) shortly after the TRANSAT balloon lunch and during the lidar measurements, thus continuously observing nearly the same NLC layer from the ground and stratosphere from evening hours on 22 June until morning hours on 23 June. Thus, lidar measurements complement balloon-borne observations, providing information on vertical dynamics of the NLC layer observed from the stratosphere.

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## 3.2 Space measurements

During the TRANSAT flight, measurements of PMC from space were also conducted with the MATS satellite. MATS (Mesospheric Airglow/Aerosol Tomography and Spectroscopy) is a Swedish satellite mission, launched in November 2022, designed to investigate polar mesospheric clouds, temperature regime and atmospheric gravity waves in the mesosphere (Gumbel et al., 2020). The MATS satellite utilizes airglow emissions coming from excited molecular oxygen in the near infrared range of the spectrum (760-780 nm, the O₂ (0-0) A-band) as well as scattered light from PMC in the ultraviolet (270 and 340 nm). The primary scientific instrument on-board the satellite is the limb imager, a telescope with 6 wavelength channels that continuously captures high-resolution images of the atmospheric limb with a field of view of about 40 km in the vertical and 250 km across track at the tangent point. Using a tomographic analysis of acquired MATS images, one can reconstruct waves in three dimensions and provide 3-dimensional fields of airglow, PMC properties and temperature (Megner et al., 2025).

Figure 7 demonstrates an example of MATS images in the UV channel 1 at 270 nm taken at 10:48:08 UTC on 25 June 2024. This measurement was done at the tangent point in the mesopause close to the west coast of Greenland as illustrated in Fig. 7c. One can clearly see the presence of the PMC layer in Fig. 7b as the bright yellow band on the atmospheric limb between 80 and 85 km tangent altitude. The time and location of the PMC by MATS agree very well (time difference of 6 seconds) with the NLC observation obtained from the stratosphere, with the MATS tangent point being in the field of view of the SONC imager as seen in Fig 7c. Such simultaneous common-volume observation of NLC from the stratosphere and space was made for the second time; the first one was made by the PMC Turbo stratospheric flight and the AIM satellite in July 2018 (Fritts et al., 2019). But in the present study this was done for the first time when such a large-scale NLC/PMC field (extending at least 1500 km from north to south) was seen from the stratosphere and from space based on the limb-viewing observation using the MATS satellite. It should also be noted that MATS registered the PMC layer before and after the time shown in Fig. 7, i.e., MATS observed an extended PMC layer in a large volume of the mesopause at least over Greenland, Baffin Bay and Baffin Island. Part of this large-scale NLC field was registered from the stratosphere by the SONC imager. At the same time, images taken from the stratosphere have much higher spatial resolution, showing small- and medium-scale wave dynamics (including smaller gravity waves and turbulent structures) that MATS cannot resolve using the limb-viewing geometry. Thus, studies of spatial-temporal variability of NLC/PMC can be carried out simultaneously from the stratosphere and space, complementing each other.

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<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>Figure 7: Example of NLC measurements obtained from the stratosphere and space on 25 June 2024 during the TRANSAT balloon flight. (A) Image, taken by the SONC imager from the stratosphere at 10:48:02 UTC, shows the extended NLC field covering almost the whole image area. (B) Image, taken by the MATS satellite at 10:48:08 UTC, illustrates part of the atmospheric limb, with the bright yellow PMC layer seen between 80 and 85 km. Data are shown for the UV channel 1 at 270 nm. (C) Projection of the NLC image shown on Panel A onto the Earth's surface. The black dot is the position of the TRANSAT balloon, the red asterisk marks the MATS tangent point which is in the field of view of the SONC imager.</figcaption></figure>

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# 4 Results and Discussion

## 4.1 Nearly continuous observations of NLC

The first unexpected scientific result is the nearly continuous presence of mesospheric clouds during 3.8 days of the TRANSAT balloon flight. In total, there were four intervals of NLC disappearances as shown by the red arrows in Fig. 5. These events occurred on 23 June at 09:30–10:50 UTC on 23 June, at 18:20–21:20 UTC on 24 June, at 17:45–02:40 UTC on 25-26 June, and at 09:00–11:00 UTC on 26 June. Outside these intervals, NLC were continuously observed, often with varying brightness and wave modulation.

These NLC disappearances are not related to local solar time (see four local solar time intervals on the x-axis in Fig. 5). That is, these NLC interruptions were of a rather spontaneous nature and were not related to solar thermal tides, which are regular atmospheric variations. Indeed, Fritts et al. (2019) demonstrated more or less regular NLC disappearances in the evening (18-24 LST) and morning hours (6-12 LST), observed during the PMC Turbo balloon flight from Sweden to Canada in July 2018. The authors associated this with solar semidiurnal and diurnal thermal tides in the polar summer mesopause, the amplitudes of which reach 10-20 K, which is comparable to ones by gravity waves in the mesopause region (Rapp et al., 2002). It should be noted that this result was obtained on the basis of lidar measurements on the PMC Turbo balloon, that is, obtained with a very small field of view of a few meters in the mesopause. Therefore, for such a small volume of the mesopause one can expect the dominant influence of large-scale waves such as solar tides. In the case of the TRANSAT balloon flight NLC observations were carried out on large scales of about 1000-1500 km at a time. At such large scales, not only solar tides manifest themselves but also propagating gravity waves of various scales from 1 to 1000 km. Indeed, a typical picture seen from the stratosphere was that we saw a disappearance of NLC in one area of the mesopause (for example, in the eastern part), but simultaneous appearance of other NLC in another part of the mesopause (for example, in the western part). In other words, large-scale NLC fields were generally represented by numerous wavy patterns of sporadic nature. One can assume that gravity waves, continuously coming from the lower atmosphere, form a new NLC and/or modulate an existing NLC layer in different parts of the mesopause (as observed in the present experiment), thereby dominating solar thermal tides on large scales. At the same time, we should note that this result was obtained on the basis of a single 3.8-day flight and a limited range of latitudes (60-75°N) and longitudes (45°E-95°W). That is why the present case study cannot claim to be a general statement about the relative importance of solar tides compared to gravity waves.

## 4.2 A case study of the NLC disappearance

The second result is connected to the disappearance of NLC at 18:20-21:20 UTC (16:23-19:06 LST) on 24 June 2024 (Fig. 5). This NLC disappearance was observed over the Atlantic Ocean near the east coast of Greenland and over Iceland as demonstrated in Fig. 8.
<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /><figcaption>Figure 8: Projection of the field of view of the SONC camera on the Earth's surface at 18:20 UTC (16:23 LST) and 21:20 UTC (19:06 LST) on 24 June 2024, when no NLC were observed from the stratosphere. The black dots mark the position of the TRANSAT balloon.</figcaption></figure>

We have tried to find out the reason for this NLC disappearance. For this data of the Aura/MLS temperature and water vapor measurements have been used to obtain a comprehensive picture of the mesopause environment in the region of interest. Aura/MLS temperature and water vapor measurements of ver.5.0 and level 2 data quality were obtained from the NASA public web-site:

https://acdisc.gesdisc.eosdis.nasa.gov/data/Aura_MLS Level2/. According to the Aura/MLS data quality and description document (version 5.0x level 2 and 3), for Aura/MLS temperature measurements, the vertical resolution in the mesopause region is 11 km at 0.01 hPa (~80 km) and 12 km at 0.001 hPa (~90 km). The horizontal resolution (along track) is ~250 km at 0.01 hPa and ~280 km at 0.001 hPa. The pressure range recommended for scientific use is from 261 hPa (~10 km) to 0.00046 hPa (~93 km). Typical precisions for individual Aura/MLS temperature profiles at this altitude range are ±3.4–3.6 K. For Aura/MLS water vapor measurements, the vertical resolution is 8.8 km at 0.01 hPa and 10.3 km at 0.002 hPa. The horizontal resolution is ~725 km at 0.01 hPa and ~350 km at 0.002 hPa. The pressure range recommended for scientific use is from 316 hPa to 0.001 hPa. Typical precisions for individual Aura/MLS water vapor profiles are 55% at 0.01 hPa and 450% at 0.001 hPa. The description on the MLS temperature product and its validation can be found in
<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

313 Froidevaux et al. (2006) and Schwartz et al. (2008). The validation of water vapor data is described in detail by
314 Read et al. (2007) and Lambert et al. (2007). The frost point temperature in the mesopause was calculated using
315 Aura/MLS water vapor data based on thermodynamics of the vapor pressure of ice (Murphy and Koop, 2005).

316 The results are shown in Fig. 9 demonstrating the following peculiar feature. A prominent localized warm
317 area in the mesopause region at the pressure levels of 0.0022 hPa (about 86 km geometrical height) and 0.0046
318 hPa (about 83 km) was present between Greenland and Scandinavia, with temperatures 20-30 K higher than
319 those in the adjacent mesopause regions at 0.0022 hPa. The location of this warm spot coincides well with the
320 position of the mesopause visible from the TRANSAT balloon when no NLC were seen (Fig. 8). The right
321 column in Fig. 9 shows the differences between the actual temperature measurements and the frost point
322 temperature as calculated using Aura/MLS water vapor measurements, with positive temperature differences
323 being as much as 10-15 K in the center of this warm spot. We double-checked all the Aura/MLS data
324 (temperature and water vapor) which show good quality data suitable for scientific analysis and we could not
325 find any error in these data.

326
<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /><figcaption>Figure 9: (Left column) Aura/MLS temperature measurements at the pressure level of 0.0022 hPa (~86 km) and at 0.0046 hPa (~83 km) on 24 June 2024. (Right column) Differences between the temperature and the frost point temperature as estimated based on Aura/MLS water vapor measurements at 0.0022 hPa and 0.0046 hPa (see the text).</figcaption></figure>

In Fig. 9 (especially in plots for the 0.0046 hPa level), one can note that the warm local region at high latitudes was an extension of a warm vast area located at mid-latitudes. That is, one can assume that this local warm region was caused by a jet of warm air mass from middle latitudes to high latitudes. To test this hypothesis, we apply the Japanese Atmospheric General circulation model for Upper Atmosphere Research Data Assimilation System (JAGUAR-DAS), which is a data assimilation system for the whole neutral atmosphere (Koshin et al., 2020; 2022). Specifically, JAGUAR-DAS Whole neutral Atmosphere Reanalysis (JAWARA) has been considered in the present study. JAWARA is a long-period reanalysis, starting from September 2004, which covers the altitude range from the surface to the lower thermosphere (~110 km). The vertical resolution of
<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

341 JAWARA data in the mesopause region between 80 and 90 km is about 0.7 km. JAWARA outputs, distributed
342 on a horizontal grid spacing of 2.81°, include the following hourly data: temperature, zonal, meridional and
343 vertical wind velocities, geopotential height, temperature tendency due to diabatic heating, zonal and meridional
344 forcing from the GW parameterization (Koshin et al., 2025). Note that JAWARA assimilates MLS temperature,
345 meaning that the temperature fields in MLS and JAWARA are not fully independent and the mutual agreement
346 might be expected because of the data assimilation.

347 JAWARA temperature and neutral wind data in the mesopause region are shown on 24 June 2024 in Fig. 10,
348 demonstrating the following features. The warm localized spot was present between Greenland and Scandinavia,
349 and it was most pronounced at 0.0042 hPa, ~83 km, (lower plots), which agrees well with the position of the
350 warm spot represented by Aura/MLS. At the same time, wind velocity vectors show that the local wind jet was
351 formed, which moved from middle to high latitudes in the region with this warm spot. Thus, this warm spot in
352 the mesopause was caused by the intrusion of a warm air mass from the middle to high latitudes between
353 Greenland and Scandinavia. This warm spot induced sublimation of ice particles in the mesopause, which caused
354 NLC to disappear during the TRANSAN balloon flight over the North Atlantic and the east coast of Greenland
355 in the evening of 24 June 2024. The mechanism of this air intrusion from middle to high latitudes is beyond the
356 scope of the present paper.
357
<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /><figcaption>Figure 10: JAWARA model temperature (color code) and neutral wind velocity vectors (arrows) on 24 June 2024. The data are shown at the pressure level of 0.0023 hPa (upper panels) and at 0.0042 hPa (lower panels), and at 17:00 UTC (left column) and at 18:00 UTC (right column).</figcaption></figure>

## 4.3 A double-layer structure in NLC

The third early result addresses a double-layer event in NLC observed over Greenland on 25 June 2024. The reader is recommended to look at the video of NLC image sequence at 05:08-06:49 UTC (02:35-04:08 LST) on 25 June (https://doi.org/10.7910/DVN/AKJK4P). One can see at least two NLC layers moving in opposite directions: one cloud layer visually moved approximately from the top to the bottom of the image frame, whereas another cloud layer moved in the opposite direction from the bottom to the top. Double-layer and multiple-layer structures in NLC/PMC have been previously observed and modeled in a number of studies (Baumgarten et al., 2012; Dubinskii and Popel, 2012; Kaifler et al., 2013; Gao et al., 2017; Li et al., 2017).

We have carefully estimated speeds and directions of seven individual NLC points in each of these two different layers. The results of this analysis are as follows. For the NLC layer visually moving from the bottom to the top, the average NLC speed was about 28 m/s, with the average azimuth of about 18° (counting clockwise

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<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>

from the north). For the NLC layer moving from the top to the bottom, the average NLC speed was about 60 m/s, with the average azimuth of about 193°. These statistical results are summarized in Table 1 as well as shown in Fig. 11, demonstrating NLC velocity vectors (green and yellow arrows) estimated for the two different layers. Note that we have taken into account the velocity vector of the TRANSAT flight (zonal and meridional speed was -10.5 m/s and -7.1 m/s, respectively) when calculating these NLC wind velocities vectors. To calculate NLC speeds, the average height of the entire cloud layer was chosen as 83 km. Uncertainty in the height of the NLC layers between 82 and 86 km gives an error of 3-4% of the average NLC speed.

It can be assumed that different movements of the two NLC layers were caused by different wind systems located at different heights in the mesopause region. We cannot measure the heights of the different NLC layers but we can consider a model simulation to check this assumption. We utilize JAWARA zonal and meridional wind data in the mesopause region taken at 06:00 UTC on 25 June 2024, i.e., when and where different motions of the NLC were observed.

JAWARA wind data are shown in Fig. 11 by the vector field as well as by the absolute wind speed (color code). Panel A represents JAWARA wind data at the pressure level of 0.0042 hPa (~82.9 km) above Greenland and seven NLC velocity vectors (green arrows) observed in the NLC layer which was moving in the NNE direction (visually from the bottom to the top). The average wind speed was about 21 m/s in the direction of 13° (counting clockwise from the north) in the vicinity of the observed NLC. Panel B illustrates JAWARA wind data at the pressure level of 0.0017 hPa (~86.8 km). One can see about the opposite direction (to SSW) of the model wind field to that shown on panel A. The average JAWARA wind speed was about 56 m/s, with the average azimuth of 188°, in the vicinity of the observed NLC. Seven NLC velocity vector are shown by the yellow arrows which have about the same speed and the direction of motion. All these statistical data are summarized in Table 1, which demonstrate good agreement between the measured NLC and JAWARA wind velocities both in the absolute value and in the propagation direction, taking into account their uncertainties. Note that the JAWARA model has a low horizontal spatial resolution (2.81° or ~310 km in latitude and ~113 km in longitude in the analyzed space domain over Greenland) and cannot reproduce wave dynamics due to small-scale gravity waves. However, in this particular case, the JAWARA model does correspond well to the observed motions of the two different NLC layers.

401

<table><caption>**Table 1.** Statistical parameters of the movement of two different layers observed in NLC during the TRANSAT balloon flight on 25 June 2024. For comparison, wind data from the JAWARA model are shown. Uncertainties represent one standard deviation.</caption><thead><tr><th>Two NLC layers oppositely moved in image frame</th><th>NLC speed (m/s)</th><th>NLC azimuth (degree from north)</th><th>JAWARA wind speed (m/s)</th><th>JAWARA wind azimuth (degree from north)</th></tr></thead><tbody><tr><td>From bottom to top</td><td>27.8±10.0</td><td>18.2±8.2</td><td>21.0±3.7 at 0.0042 hPa</td><td>13.0±12.0 at 0.0042 hPa</td></tr><tr><td>From top to bottom</td><td>60.5±11.3</td><td>192.5±2.2</td><td>56.1±2.5 at 0.0017 hPa</td><td>188.0±4.3 at 0.0017 hPa</td></tr></tbody></table>

405

Panel C of Fig. 11 shows the JAWARA wind velocity vector as a function of altitude for a specific point in the vicinity of the observed NLC. The cyclic rotation of the wind velocity vector is clearly seen between 80 and 94 km altitude, i.e., there was a circular rotation of the wind vector along the azimuth of 360°. Panel D illustrates
<figure><img src="image_35.png" /></figure>

409 the corresponding hodograph of the tip of the horizontal wind vector with the subtracted mean velocity values (-
410 33 m/s and -18 m/s for the zonal and meridional wind component, respectively) between 80 and 94 km, that is,
411 perturbations of the horizontal wind velocity. It is well known that a gravity wave induces horizontal wind
412 variations rotating, in general, elliptically with height, and hodograph analysis is commonly used to derive
413 gravity wave parameters (Gossard and Hooke, 1975; Eckermann, 1996 and references therein). In the present
414 case, the hodograph has an almost circular form, implying near circular polarization (P≈0). Perturbations of the
415 horizontal wind are of 45-50 m/s, and the perturbation vector has a clockwise rotation with increasing height
416 which means that the group velocity of this particular gravity wave was propagating upward (phase velocity was
417 downward). Such an inertia-gravity wave could indeed generate winds in different directions in the given
418 volume of the mesopause, forming a multiple-layer structure from a single NLC layer. The inertia-gravity wave
419 interpretation is a plausible scenario which is consistent with JAWARA winds and NLC motions. At the same
time, it should be noted that the background wind in the mesopause region abruptly changes its speed and
420 direction with altitude in summer (Portnyagin and Solovjova, 2000; Conte et al., 2025). Therefore, other
421 mechanisms such as the vertical displacement of the NLC layer by gravity waves, may provide the observed
422 different NLC motions.
423

424 It is important to note the following. The clockwise wind rotation with altitude by JAWARA was the
425 opposite of that found by Conte et al. (2025), who have analyzed long-term meteor radar measurements in the
426 mesopause region at high latitudes over northern Norway. The authors have obtained the counter-clockwise
427 rotation of the mean wind speed with altitude in the summer time. The same effect of the wind rotation with
428 altitude is predicted by the global empirical wind model for the upper mesosphere/lower thermosphere
429 (Portnyagin and Solovjova, 2000). In this present case, the clockwise rotation of the wind speed indicates the
430 dominance of the inertia-gravity wave over the mean wind rotation speed in the space-time domain under
431 consideration. Also, we should note that even without inertia-gravity waves, winds in the mesopause region can
432 rotate with altitude (Ekman spiral) under the action of the Ekman-type mechanism involving Coriolis force and
433 turbulent viscosity (Chkhetiani and Shalimov, 2010; 2022).
434
<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /><figcaption>JAWARA Wind on 2024-06-25 at 06 UTC at 0.0042 hPa ~82.9 km</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>JAWARA Wind on 2024-06-25 at 06 UTC at 0.0017 hPa ~86.8 km</figcaption></figure>

<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>

435

436 **Figure 11**: JAWARA wind data (thin black arrows) at the pressure level of 0.0042 hPa, ~82.9 km (A) and at the pressure level of 0.0017 hPa, ~86.8 km (B) at 06 UTC on 25 June 2024. The color code is the absolute horizontal

437
<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /></figure>

438 wind speed. The green and yellow arrows show velocities of NLC moving in opposite directions (see the text).

439 (C) JAWARA wind velocity vector selected for a specific location (68.4°N, 39.4°W) as a function of altitude
440 between 80 and 94 km. (D) Hodograph of disturbances of the horizontal wind velocity vector between 80 and 94
441 km. The thin arrows show the direction (clockwise rotation) of wind velocity disturbances with altitude.

442

443 **5 Conclusions**

444 Observations of noctilucent clouds were performed from the stratosphere during the TRANSAT transatlantic
445 long-duration balloon flight between Sweden and Canada on 22-26 June 2024. NLC were observed with the
446 SONC imager composed of three high-resolution SONY α7 Mark III cameras, stabilized on the TRANSAT
447 gondola. NLC cannot be detected by an imager from the ground during the midnight Sun season due to strong
448 background scattering in the lower atmosphere. However, an observation from the stratosphere can detect NLC
449 during 24 hours. These measurements offer an unprecedented view of mesospheric cloud dynamics on small-
450 and large-scales, bridging the observational gap between ground-based and satellite platforms. The main results
451 of this study can be summarized as follows:

452 1. One wide-angle camera was completely operational during the whole flight for 3.8 days, taking a total of
453 40,000 images, of which about 6200 images are suitable for scientific analysis.

454 2. NLC were detected nearly continuously during the whole flight, showing a strong variability in space and
time. The dominant role of solar thermal tides in the summer mesopause decreases when NLC are observed
455 on large scales of about 1500 km. At such scales, gravity waves of various scales from 1 to several hundred
456 km play the largest role, evolving and modulating NLC layers in different regions of the polar mesopause.
457 Note that this result was obtained on the basis of a single 3,8-day flight and a limited range of latitudes (60-
458 75°N) and longitudes (45°E-95°W). Thus, the present case study cannot claim to be a general statement
459 about the relative importance of solar tides compared to gravity waves.
460

461 3. Ground-based support of the TRANSAT balloon campaign was represented by two lidars located at Esrange
462 and Andoya. Both lidars registered NLC at the time of the initial phase of the balloon flight over northern
463 Scandinavia. The NLC layered was continuous from 20 UTC on 22 June until 09 UTC on 23 June,
464 modulated in height between 81 and 86 km, and showing a double-layer structure.

465 4. The same mesospheric cloud layer was recorded from the stratosphere and space around noon on 25 June
466 2024. Such a simultaneous common-volume NLC detection was carried out, for the first time, using a limb-
467 geometry by the MATS satellite. The extended cloud layer was observed over at least Greenland, Baffin Bay
468 and Baffin Island, with part of this layer registered from the stratosphere over the west coast of Greenland
469 and Baffin Bay. Images from the stratosphere showed complex wave dynamics, with interference of small-
470 scale waves and turbulent regions below what MATS can partly resolve. Therefore, observations of clouds
471 from the stratosphere and space, using limb geometry, complement each other well.

472 5. Gravity waves had characteristic horizontal wavelengths of 30-40 km for the case study considered in the
473 mesopause over Greenland on 25 June 2024.

474 6. One peculiar case of the NLC disappearance in the polar mesopause in the afternoon of 24 June 2024 has
475 been analyzed. This cloud disappearance was caused by the localized warm spot in the mesopause region
476 between Greenland and Scandinavia, as shown by Aura/MLS satellite and JAWARA model data. This warm
477 spot in the mesopause region was caused by the intrusion of a warm air mass from middle to high latitudes.
<figure><img src="image_44.png" /></figure>

7. Different movements of NLC were observed in the morning of 25 June 2024 over Greenland. One part of the clouds moved to NNE, another one moved to SSW. A detailed analysis of wind data based on the JAWARA model showed that there were different wind systems between 80 and 94 km altitude over Greenland, with a clockwise wind rotation with altitude. Most likely, a large inertia-gravity wave propagated upwards, causing significant disturbances (45-50 m/s) of the horizontal wind. The wind disturbances caused NLC to move differently at different altitudes. Other mechanisms such as the vertical displacement of the NLC layer by gravity waves may explain the observed different NLC motions.

These results demonstrate the scientific value of long-duration stratospheric balloon missions for mesospheric research. Future campaigns of this type will benefit from expanded multi-spectral imaging, real-time data transmission, and coordinated ground-satellite observations to further improve understanding of mesospheric cloud dynamics. A statistical analysis of the wave dynamics observed in NLC during the 2024 TRANSAT balloon flight, and investigation of microphysical properties of NLC ice particles will be addressed in future studies.

491

## Data availability

Two video files of the SONC experiment are available on the Harvard Dataverse repository (https://doi.org/10.7910/DVN/AKJK4P; https://doi.org/10.7910/DVN/1PHRZU). Individual NLC images taken from the stratosphere by the SONC experiment that support the findings of this study are available at the HEMERA Data Centre (https://data.hemera-h2020.eu/atmospheric-balloon-experiments/#/) as well as will be made available on request.

498

## Author contributions

PD, HS, NP, VP, JK and AR have designed two balloon-borne experiments dedicated to studies of noctilucent clouds and infrasound waves. PD wrote the draft of the paper and made analyses of the SONC images, Esrange lidar, Aura/MLS and JAWARA data. LM made analysis of the MATS satellite data. PV and JH performed Esrange lidar measurements. GB performed ALOMAR lidar measurements. DE designed and produced the electric control unit for the SONC imager. All authors discussed the results and contributed to revisions of the text and the figures.

506

## Competing interests

The contact author has declared that none of the authors has any competing interests.

509

## Acknowledgments

The authors thank the CNES/INSU-CNRS Balloon Programme. The TRANSAT balloon flight was funded and performed by the French Space Agency (CNES). The authors thank the Aura/MLS team for providing temperature and water vapor data around the globe during the 2024 TRANSAT balloon flight.

514

## Financial support

The SONC imager hardware (imager and electronic control unit) was funded by the Kempe Foundation (Kempestiftelserna) under grant agreement JCK-1901.4. The infrasound instrument and flight costs of the SONC
<figure><img src="image_45.png" /></figure>

and infrasound instruments were financed under research grant 42/17 from the Swedish National Space Agency (Rymdstyrelsen). The MATS satellite and JH were financed by the Swedish National Space Agency under grant 2021-00052. LM was financed by the Swedish National Space Agency under grant 2022-00108 and the Swedish Research Council (Vetenskapsrådet) under grant 2021-04876.

522

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