Suzuki et al.
Progress in Earth and Planetary Science(2022) 9:11
https://doi.org/10.1186/s40645-022-00469-4

Progress in Earth andPlanetary Science

Open Access

METHODOLOGYCapability of airline jets as an observation
platform for noctilucent clouds at middle
latitudes
Hidehiko Suzuki1* , Ayako Matsumoto2, Peter Dalin3, Yuriko Nakamura1, Satoshi Ishii1, Kazuyo Sakanoi4,
Kaori Sakaguchi5, Taku Takada6, Takuo T. Tsuda7 and Yuta Hozumi5

Abstract
The exact occurrence frequency of noctilucent clouds (NLCs) in middle latitudes is significant information because
it is thought to be sensitive to long-term atmospheric change. We conducted NLC observation from airline jets in
the Northern Hemisphere during the summer 2019 to evaluate the effectiveness of NLC observation from airborne
platforms. By cooperating with the Japanese airline All Nippon Airways (ANA), imaging observations of NLCs wereconducted on 13 flights from Jun 8 to Jul 12. As a result of careful analysis, 8 of these 13 flights were found to successfully detect NLCs from middle latitudes (lower than 55° N) during their cruising phase. Based on the results of thesetest observations, it is shown that an airline jet is a powerful tool to continuously monitor the occurrence frequency ofNLCs at midlatitudes which is generally difficult with a polar orbiting satellite due to sparse sampling in both temporaland spatial domain. The advantages and merits of NLC observation from jets over satellite observation from a point ofview of imaging geometry are also presented.
Keywords: Noctilucent cloud, Mesosphere, Middle latitudes, Global warming, Airline, Aircraft

1 Introduction
Noctilucent clouds (NLCs) are the Earth’s highest clouds,
and they appear at the mesopause region during summer
in both subpolar regions (Gadsden and Schröder 1989).
NLCs are a proxy for monitoring climate change because
their occurrence is highly sensitive to the mixing ratio ofwater vapor and atmospheric temperature in the mesopause region (Thomas 1996; Thomas and Olivero 2001).
Several models predict that global warming in the lower
atmosphere caused by global increases of greenhousegasses promotes cooling of the upper atmosphere (e.g.,
Lübken et al. 2013). In addition, photochemical reactions
involving greenhouse gasses, such as methane, increase
the mixing ratio of water vapor in the mesosphere, which*Correspondence: suzuhide@meiji.ac.jp
1
Meiji University, Kawasaki, Kanagawa, Japan
Full list of author information is available at the end of the article

is a source of water ice clouds (i.e., NLCs) (Hervig et al.
2016). Thus, enhancement of global warming due togreenhouse gasses can extend the NLC region toward
latitudes lower than the current typical subpolar latitudes(Thomas 1996). Therefore, it is important to know theoccurrence of NLCs in middle latitudes precisely. A Japanese research group started NLC observation from theground in northern Japan in 2012. They have reported
the first detection of NLCs from multiple sites in Hokkaido, Japan (43.17° N–45.36° N), on June 21, 2015 (Suzuki
et al. 2016). However, they have not reported further
NLC events from Japan from 2016 to 2018 although radarobservations sometimes detect mesospheric echoes. One
possible cause that prevents the detection of visible NLCs
from the ground is poor weather conditions during theearly summer season in Japan (Suzuki et al. 2016). Satellite
observations are a powerful tool to investigate the global
distribution and a long term variability of NLC (or Polar
Mesospheric Clouds, PMCs, as observed from space). In© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Suzuki et al. Progress in Earth and Planetary Science(2022) 9:11

2007, NASA launched the Aeronomy of Ice in the Mesosphere (AIM) satellite to monitor the PMCs (Russell et al.
2009; Lumpe et al. 2013). The AIM satellite, which is still in
operation, provides data regarding the temporal and spatial variations of PMCs over both polar regions. However,data availability in middle latitudes is quite limited due toan observation geometry and a sparse sampling caused bya polar orbit. Recently, the observation of PMCs from Geostationary Earth Orbit (GEO) meteorological satellite isreported (Tsuda et al. 2018). This method is quite effective
to continuously monitor the PMCs in both hemispheres.
Hozumi et al. (2021) performed derivation of horizontal
winds at altitudes of PMCs by tracking a motion of the
PMCs observed by Japanese GEO satellite, Himawari-8.
However, GEO satellite can monitor the PMCs in fixed latitude and longitudinal ranges. Therefore, we have started
a feasibility study to monitor NLCs in middle latitudes
from airplanes and balloons in addition to current groundbased imagers since June 2019. International jets fly at analtitude of about 10 km during the cruising phase. From
this altitude, NLC observation is possible without obstruction by lower clouds because most tropospheric clouds are
below this altitude. Our project has collaborated with theJapanese airline All Nippon Airways (ANA). This paper
reports the results of NLC observations in the middle
latitude region using airline jets. An initial result of NLC
observations with small cameras installed in the cockpit
of passenger jets connecting North America, Europe, andJapan is reported in detail. The instrumental overview,
details of the observation plan, and the analysis method
are described in Sect. 2. Results of NLC observations byairline jets conducted in the Northern Hemisphere during the summer 2019 are shown in Sect. 3. Occurrence of
NLCs in middle latitudes and the advantages of observation with jets are discussed in Sect. 4.

2 Instrumentation and analysis method
We entrusted small action cameras (GoPro HERO7
Black) to ANA and conducted test observation of NLCs
from cockpits in the northern summer season of 2019.
We also tested a GoPro camera to check its sensitivity bytaking star images in a dark sky. It was found that using
the “nightlapse mode,” the GoPro can capture star field
images. If an exposure time and sensitivity are set to 30 s
and ISO 800, respectively, Milky way can be detected (Itmeans GoPro can capture faint stars with visual magnitude of up to 5–6). Because NLCs are brighter than visible stars, the GoPro HERO 7 Black was confirmed to besensitive enough to detect NLCs. The shooting interval
was set to 1 min during each flight except for a flight on
Jul 12 (see below for details).

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Several ANA flights from Japan to Europe and the USA
cruise subpolar regions. We checked which flight routes
are suitable for NLC observation during the summer solstice before an observation planning. The criteria were
(1) the flight route includes a latitude higher than 40° N
and (2) the solar elevation was between − 15° and − 5°during the cruising in a subpolar region. Table 1 lists the
ANA flights connecting Japan–Europe and Japan–North
America that satisfy these criteria. These flights arenominated by considering their typical route provided
by Flightradar24, which is a web service providing realtime and past commercial aircraft flight tracking information (https://​www.​fligh​trada​r24.​com/). A position of
the Sun in celestial coordinates is assumed to be that on
the summer solstice for the calculation of a local solar
elevation angle during a flight. These flights are regularroutes which are nominally operated every day. We haveconducted test observations with some of these flights on
several days. The dates on which the NLC observations
were conducted are also listed. Underscored dates meanthat NLCs were successfully detected on that date. Dates
with an asterisk mean that NLCs were detected from alatitude lower than 55°N. It is noted that the observation on Jul 12 during flight NH176 was conducted withanother compact digital camera (Canon PowerShot G7
X Mark II) belonging to a crewmember of the flight. In
total, NLC imaging observations were conducted on
13 flights in the Northern Hemisphere during the summer of 2019. NLCs were detected from middle latitudes(lower than 55°N) on eight flights of these flights.
We then checked all taken sequential images for apotential presence of NLCs during each flight (seeTable 1). The NLC visibility condition is defined as asolar elevation between -15 and -5° and an altitude of jets
is above lower clouds. Actual routes of each flight wereobtained from Flightradar24 (https://​www.​fligh​trada​
r24.​com/). The solar elevation angle during each observation was calculated to judge whether each image wastaken under the NLC condition by combining a timecode of each image and the flight route log (time, latitude, longitude, and altitude). If an image was found to betaken under the possible NLC condition, then the image
was carefully checked to determine whether it captures
NLC features. The detection of NLCs in each image wascompletely made manually because NLCs have apparent features (bluish color and shining against a dark sky)in color images and are easily distinguished from lower
clouds. Figure 1a shows a typical NLC image taken during flight NH105 at 12:45 UTC on Jul 8, 2019, over thenorthern Pacific Ocean. The solar elevation angle at thisobservation time was -12.3°, satisfying the NLC visibility
condition. As shown by this example, many small-scaleSuzuki et al. Progress in Earth and Planetary Science(2022) 9:11

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Table 1 List of flights operated by ANA considered to be suitable for NLC observation
Flight ID

Departure

Destination

Total flight time Flight time satisfying(h)the criteria (h)Observation date Day/Month (of 2019)Japan–Europe flights
NH203

Tokyo

Frankfurt

11.7

1.7

12/Jun* (Fig. 3d)

NH218

Munich

Tokyo

10.9

1.9

23/Jun*(Fig. 3f )

NH206

Wien

Tokyo

10.4

2.9

Not tested

Japan–America flights
NH6

San Francisco

Los Angeles

9.8

1.3

Not tested

NH106

Tokyo

Los Angeles

9.8

0.5

Not tested

NH105

Los Angeles

Tokyo

11.1

5.0

08/Jun*(Fig. 3a), 09/Jun*(Fig. 3b)

NH8

Tokyo

San Francisco

9.3

1.3

13/Jun

NH172

Tokyo

San Jose

9.3

0.9

Not tested

NH178

Tokyo

Seattle8.8

0.8

12/Jun*(Fig. 3c)

NH109

New York

Tokyo

13.2

0.5

Not tested

NH174

Tokyo

Houston

12.0

1.4

Not tested

NH12

Tokyo

Chicago11.8

3.1

20/Jun*(Fig. 3e), 30/Jun*(Fig. 3g)

NH112

Tokyo

Chicago11.8

1.8

Not tested

NH111

Chicago

Tokyo

12.3

0.1

22/Jun

NH116

Tokyo

Vancouver

8.7

0.9

Not tested

NH180

Tokyo

Mexico11.4

1.0

29/Jun, 30/Jun

NH176

Tokyo

Los Angeles

9.8

1.3

07 Jun, 12/Jul*(Fig. 3h)

Duration time satisfying the criteria for the NLC observation (see text) is also shown. The dates on which NLC observations were conducted are also listed.
Underscored dates denote successful NLC detection on that date. Dates with an asterisk mean that NLCs were detected at a latitude lower than 55°N

structures, modulation by gravity waves, are also seenin addition to the NLC features mentioned above. Thisis also a typical feature of an NLC image taken from theground (Pautet et al. 2011).
The images with NLC features were then analyzed using
the scheme described in Suzuki et al. (2015, 2016). This
scheme involves deducing camera parameters (optical distortion coefficients) and fitting of a local horizontal coordinate system (elevation and azimuthal angles) to each pixel
using reference star positions captured in the analyzed
images. The simple distortion model considers distortion
being proportionate to angular distance from the centerof the image. The distortion model of the camera adopted
in this study is the same as that described in Suzuki et al.
(2018). Figure 1b is the same image shown in Fig. 1a butwith the horizontal coordinates determined and embedded using this method. The horizontal and vertical curves
in Fig. 1b show elevation and azimuth angles, respectively,
in 30° intervals. The azimuth angle is set to zero at geographical south and increases toward the west-north-east
direction (i.e., clockwise increment). The notation “N” on
the curve in the center of the image represents geographical north. The area with NLC features appears at a low
elevation area and is indicated by the red dotted line. The
white dotted lines show the scattering angle (i.e., angle
between a line connecting the observer and NLCs, and aline connecting NLCs and the Sun). The scattering angle

is revisited in Sect. 4. Figure 1c shows the image projected
onto a geographical map by assuming the typical NLC altitude of 85 km (Hansen et al. 1989). The latitudinal and longitudinal extents of the NLCs shown by this projection arefrom 51°N to 55°N and from 137°W to 146°W, respectively.
The image area fairly closed to the horizontal line (elevation ~ 0) is saturated due to a strong background signal andthus unfortunately contains no information on the presence of NLCs. This situation is typical for all observations.
Because the exposure time for each observation is determined automatically by the camera, this saturation problem frequently occurs when imaging a scene with a wide
range of radiance (from a dark sky to a bright twilight sky).
This problem limits the maximum range of observation. As
shown in Fig. 1c, the effective range of the observation isabout + 5° in latitude and at about + 500 km from the location of a jet if this effect is considered. In other words, NLCs
observed at latitudes lower than 55°N means that the NLCs
exist at latitudes lower than 60°N. Therefore, we regard NLC
detection from latitudes lower than 55°N as a case of detection of mesospheric clouds at middle latitudes (< 60° N) by
jets throughout this paper.

3 Results
All results of the NLC observations in 2019 are summarized in Fig. 2. Actual flight routes for all 13 flights aretaken from Flightradar24. The flight ID (e.g., NH203)

Suzuki et al. Progress in Earth and Planetary Science(2022) 9:11

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Fig. 1 a Typical NLC image taken during flight NH105 at 12:45 UTC on Jul 8, 2020, over the northern Pacific Ocean. b Same image as (a), but withthe horizontal coordinates embedded. The horizontal (vertical) lines show the elevation (azimuth) angle in 30° intervals. The notation “N” on thevertical line in the center of the image represents the geographical north. The area with NLC features is indicated by a red dotted line. The white
dotted lines show the scattering angle. c Image projected onto geographical map by assuming an NLC altitude of 85 km. The plus symbol indicatesthe location of the jet during the observation

Suzuki et al. Progress in Earth and Planetary Science(2022) 9:11

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Fig. 2 Flight routes and summary of the NLC observations conducted on 13 flights in the Northern Hemisphere during the summer 2019 (seeTable 1). Gray and black circles on flight routes show the locations without and with a possibility of NLC detection, respectively. Red circles show
locations from which NLCs were successfully detected

and date of departure are also indicated for each flight.
Gray circles in a flight route show the locations where
the jet location did not satisfy the condition for NLC
detection (see the “Method” section for the criteria).
The black circles in a flight route show locations witha chance of NLC detection from the jet. The red circles show locations from which NLCs were detected.
Intervals between these locations are not constantthrough each pass; they are sometimes intermittent andsometimes continuous due to irregular timing of flightreports from jets during cruising.
NLCs were detected from latitudes lower than 55°N
on 8 of these 13 flights. Most detections were achieved
over the Pacific Ocean. Because the Pacific Ocean contains few land areas with large populations, previous
reports of NLCs from these areas are quite rare. Again
note that locations with red circles plotted in Fig. 2 arenot locations where NLCs existed but the locations
from which NLCs were seen. The actual locations of
NLCs observed from jets would be more poleward, as
shown in Fig. 1c. This paper focuses on NLC detections
from latitudes lower than 55° N to discuss the actual
occurrence of NLCs in middle latitudes (< 60° N).

Figure 3 shows temporal and longitudinal distributionof sampling points of NLC observations by the jets andthe CIPS instrument onboard the AIM satellite. The vertical and horizontal axes represent days from north hemispheric summer solstice (DFS) in 2019 and longitudes,
respectively. Red and black symbols show the locations
and times of NLC observations conducted by jets. As in
Fig. 2, red crosses mean that NLCs were detected “from”
these points, but plots are limited to latitudes lower than
55°N. The blue and gray symbols represent locations andtimes of PMC observations from space by the AIM/CIPS
instrument. The horizontal width of gray lines roughly
corresponds to the zonal distance of a foot print of CIPS
camera at latitude 55°N. The version of data provided by
the AIM/CIPS team is level 3c, version 05.20, revision 5.
In this case, blue symbols mean that NLCs were detected“at” these points, but data are limited to latitudes between
50 and 60° N. Therefore, Fig. 3 is a combined result of
the NLC presence in middle latitudes (< 60° north), confirmed by both the jets and AIM satellite in the Northern
Hemisphere during the summer 2019. The observation
coverage of test flights is shown in this figure. Someflights show remarkably wide longitude coverage of anSuzuki et al. Progress in Earth and Planetary Science(2022) 9:11

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Fig. 3 Temporal and longitudinal variation of NLCs detected by jets and the CIPS instrument onboard the AIM satellite. The vertical and horizontal
axes represent days from north hemispheric summer solstice (DFS) in 2019 and longitudes, respectively. Red crosses and black plus symbols show
the locations and times of NLC observations conducted by jets. As in Fig. 3, red crosses mean that NLCs were detected “from” these points, but plots
are limited to latitudes lower than 55°N. The blue plus and gray plus symbols represent NLCs detected from space by the AIM/CIPS instrument. The
horizontal width of grey lines roughly corresponds to the zonal distance of a foot print of CIPS camera in at latitude 55°N. All sampling points by
AIM/CIPS between latitudes 50°N and 60°N are shown in this plot.

NLC observation. In particular, the flight NH105 tested
on Jul 8 and 9 (marked in Fig. 3) covers nearly 70 degrees
width in longitude above middle of Pacific Ocean. Since
all flights are daily scheduled, an NLC observation is possible with one day interval in this longitude range if thecamera is installed on every jets. An expected distribution of sampling points in time and longitude of flight
NH105 through the NLC season is shown and discussed
in the next section.
It is also shown that in several cases, NLCs weredetected only by jets, and AIM did not detect NLCs at thesame time and location. For example, focusing on longitudes near 180°, NLC occurrence in the middle latitudes
is only one detection between DFS-20 and DFS + 25,
according to the AIM data. However, it increases to four
events if both data sets are combined. This shows a difference in detection sensitivity for NLCs in middle latitudes
between the jet and AIM satellite observations. We discuss this in more detail in the next section.

4 Discussion
As shown in Fig. 3, a flight route of NH105 satisfies thecriteria for an NLC observation (see the “Method” section for the criteria) with a long duration. Test observations with this flight have been conducted in two
successive days (Jul 8 and Jul 9) and both show similar

result. Since NH105 flights westward, a duration of twilight time becomes long. This allows NLC observation
with NH105 to be possible in a wide longitude range. Figure 4 shows the expected distribution of sampling points
in time and longitude of NH105 if the camera is installedin every flight through the NLC season. The flight routeFig. 4 An expected distribution of sampling points in time andlongitude of NH105 if the camera is installed on every flight throughthe NLC season. Possible sampling points by NH105 flights are shownby nearly horizontal linesSuzuki et al. Progress in Earth and Planetary Science(2022) 9:11

is assumed to be same as it on Jun 8. Variation of solar
deceleration and right ascension during the period is
considered to check the criteria for an NLC observation.
Sampling points by AIM/CIPS instrument in latitudes
between 50 and 60 degrees are also indicated by greylines as well as Fig. 3. One can clearly see that there are
significant spatial voids of 17–18 degrees between adjacent orbits of the AIM spacecraft at middle latitudes
due to the spacecraft orbiting the Earth. These voids inthe spatial midlatitude coverage produce gaps in PMCs
observations by the CIPS instrument at middle latitudes
as was mentioned in Dalin et al. (2020a). This fact is oneof the causes that makes difficult to observe NLCs in the
middle latitude region by a satellite in a near-polar orbit.
On the other hand, expected sampling points based on
the test observation by NH105 flight show much denser
distribution in longitude between 145° E and 215° E. We
also see several NLC observations as seen from jets (redmarks in Fig. 3) located between AIM orbit trajectories(stripy distributed grey marks in Fig. 3), i.e., these NLC
occurrences were missed by the CIPS cameras.
It is also shown that in several cases, NLCs weredetected only by jets, and AIM did not detect NLCs at
the same time and location. It is not straightforward tocompare the detection limit of NLC signatures between
jets and AIM/CIPS observations because of different
wavelength, geometry, and sensitivities of sensors. However, the jet observation has an advantage in terms of anobservation geometry. As shown in Fig. 1b, NLCs are typically seen poleward (or northward in the present case) at
low elevations of the twilight sky from flight observers.
Due to this geometry condition, the angle formed by theSun, NLCs, and observer tends to be close to 180°. Thismeans the scattering angle of NLCs is close to a forwardscattering condition (i.e., the scattering angle ≈ 0) whenobservations are conducted from the Earth’s surface.
This tendency is shown in Fig. 5, which provides selected
images of NLCs taken by all eight flights that successfully observed NLCs from latitudes lower than 55° N. The
precise location of each observation is indicated in eachpanel and Table 2. The scattering angle at the observation
time was calculated for all cases and shown by concentric dashed lines with an interval of 10°. The red dashedlines in each image show the image area in which NLC
features are identified. Areas with saturated signals areomitted from the NLC area. The number of pixels with
an NLC feature is sorted by the scattering angle and summarized as the histogram shown in Fig. 6. Each event isrepresented by a line with a different color. The scattering angles of pixels with NLC features are mostly distributed between 10° and 40° for all cases. In addition, clear
peaks are also seen between 10° and 20° in most cases.
These results show that NLC features observed fromPage 7 of 10

jets nearly satisfy forward-scattering conditions during observations. This works as an advantage for detection of NLCs because the phase function of scattering byparticles measuring several tens of nanometers, whichmake up an NLC, shows strong scattering power aroundforward scattering (Bailey et al. 2009). In contrast, satellite observations are not conducted under such favorable conditions because they look at NLCs with a nearlynadir view from a polar orbit. The AIM/CIPS instrument consists of four nadir-viewing UV cameras thathave a total field of view of 120° along the orbit and 80°
across the orbit. These cameras are called the PX (+ X),
MX (− X), PY (+ Y), and MY (− M) cameras after theirpointing directions (Lumpe et al. 2013). A range of scattering angles sampled by AIM/CIPS varies depending on
the solar zenith angle (SZA) at the time of observation.
When the SZA is large, the observation has a small scattering angle. Typical ranges of scattering angle sampled
by the four cameras when SZA has the highest value(90°–95°) during an operation are indicated in Fig. 6. It
is noted that only the PX camera can cover the scattering
angles that are sampled by the jets. Moreover, scattering
angles between 10 and 20°, which are typical conditions
for the jet observation, are almost out of range, even with
the PX camera. Although an SZA angle of 90°–95° is thebest condition for AIM/CIPS to detect NLCs (i.e., polarmesospheric clouds), it is generally difficult to observe
NLCs under forward-scattering conditions, unlike a jet
observation. Therefore, the jets observation has advantage in observation geometry by means of a scattering
angle condition.

5 Conclusion
In June 2019, we started a project to monitor NLCs in
middle latitudes from airplanes and balloons in addition to current ground-based imaging efforts (Dalinet al. 2020b). NLC observation from air-borne platforms
has great advantage with respect to ground observation
because the NLCs are not obstructed by lower clouds.
Present work reports the initial results of observation of
middle latitude NLCs from ANA jets conducted in the
Northern Hemisphere during the summer 2019. NLC
imaging observation was conducted on a trial basis with
13 flights during this season. NLCs were detected frommiddle latitudes (lower than 55° N) on 8 flights of 13
test flights. Temporal and longitudinal variations of the
NLCs detected by jets and the CIPS instrument onboard
the AIM satellite were compared. As a result, some ANA
flights show remarkably wide longitude coverage of an
NLC observation. For example, the NH105 flights westward from USA to Japan covers the nearly 70° longitude
range above middle of Pacific Ocean since it satisfies thecondition for the NLC observation with a long duration

Suzuki et al. Progress in Earth and Planetary Science(2022) 9:11

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Fig. 5 Selected images of NLCs taken by all eight flights that successfully observed NLCs from latitudes lower than 55°N. The precise location andother parameters of each observation are listed in Table 2. The meaning of auxiliary lines in each figure is same as in Fig. 1b

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Table 2 A list of observation parameters for selected images shown in Fig. 5
Image ID (see Date and time (UTC)
Fig. 5)Flight ID

Latitude (°)

Longitude (°)

Mean local time

Flightaltitude (km)Solarelevation
angle (°)(a)

12:45, Jun 8

NH10549.177 N

155.259 W

2:24, Jun 9

9.4

(b)

12:00, Jun 9NH10548.310 N

145.300 W

2:19, Jun 99.8− 11.167(c)

13:07, Jun 12

NH178

53.104 N

175.070 E

0:47, Jun 13

12.5

(d)

17:15, Jun 12

NH203

44.989 N

137.456 E

2:25, Jun 13

9.8

(e)

11:18, Jun 20

NH12

50.143 N

164.782 E

22:17, Jun 20

10.1

(f )

20:41, Jun 23

NH218

54.003 N

19.053 E

21:57, Jun 23

11.3

(g)

12:53, Jun 30

NH1253.835 N

174.340 E

0:30, Jul 1

9.8

(h)

12:44, Jul 12

NH176

48.052 N

161.318 W

1:59, Jul 12

10.4

Fig. 6 Distribution of scattering angles of pixels with NLC features.
The distribution of the number of pixels is normalized by the total
number of pixels with NLC features for all eight events (see Table 1
and Fig. 3), and each event is plotted in a different color. Typicalranges of scattering angles sampled by the four AIM/CIPS cameras(PX, MX, PY, and MY) when the SZA has the highest value (90°–95°)
are also indicated (see Fig. 6 of Lumpe et al. 2013)flight. Moreover, since all flights are daily scheduled, an
NLC observation is possible with one day interval in widelongitude range if the camera is installed on every jets.
In several cases, NLCs were detected only by jets, andAIM did not detect NLCs at the same time and location. Though it is generally difficult to directly compare
the detection sensitivity of these instruments, differencein the observation geometry is considered to be one ofpossible factor to explain this discrepancy. Observationgeometries (i.e., scattering angle of NLC sampling) forall eight events were focused. It is found that observationof NLCs from jets possible has an advantage in detection
sensitivity because imaging is performed under nearly
forward-scattering conditions. This can be said also forground-based imaging observation. However, observation from an airborne platform is free from obstruction
by clouds in the lower atmosphere, strong troposphericturbulence, light and aerosol pollutions. Thus, NLC
observation from an elevation much higher than most− 12.298− 13.061− 14.346− 12.955− 7.952− 12.817− 15.614

tropospheric clouds has the potential to achieve continuous monitoring of faint signatures from NLCs in middle
latitudes. Moreover, jets also allow NLCs to be observed
not only over land but also over vast ocean areas. Basedon the above facts, the advantages of an observation of
midlatitude NLCs from jets are summarized as (1) widelongitudinal coverage compared to a satellite observation in one path, (2) no obscuration by lower clouds, (3) afavorable geometric condition for strong scattering signal
from NLCs, and (4) possibility of observing NLCs over
the oceans compared to ground-based measurements.
On the other hand, a disadvantage is that an observation
plan depends on the schedule of the airline jets which are
determined by an airline company. In fact, observationplans in 2020 and 2021 have been unfortunately canceled
because of a significant reduction of the number of international flights due to the global impact of COVID-19
pandemic.
The present work shows that regular observation using
airline jets is a powerful tool to monitor the NLCs in
midlatitude region with much denser sampling intervalboth in time and space than those of existing techniques(from the ground and space). A simple small camera canmonitor NLCs in the middle latitude region covering
wide longitude range if it is installed inside a cockpit ofintercontinental regular flights.
Abbreviations
AIM: The Aeronomy of Ice in the Mesosphere; ANA: All Nippon Airways; CIPS:
The cloud imaging and particle size; DFS: Days from summer solstice; FOV:
Field of view; MSE: Mesosphere summer echo; NLC: Noctilucent cloud; NASA:
National Aeronautics and Space Administration; PMC: Polar mesosphericcloud; SZA: Solar zenith angle.
Acknowledgements
The authors acknowledge the support from the AIM/CIPS teams for providing
the cloud imaging data. The authors would also like to express gratitude
to ANA pilots and staff, Mr. Satoshi Ichikawa, Mr. Rikimaru Kakegawa, Mr.
Satoru Fuji, Mr. Yohei Nikai, Mr. Yuichi Nishiyama, Mr. Yuzo Nakai, Mr. Masami
Masumoto, Mr. Toshiki Nagase, and Masuyuki Kato for providing precious NLC
photographs taken from jets and for their cooperation on collecting data.

Suzuki et al. Progress in Earth and Planetary Science(2022) 9:11

The authors acknowledge the support from Mr. Katsuya Kokubun for helping
initial planning of the project. The authors thank Mr. Masahiro Omote for his
technical help on the satellite data analysis.
Authors’ contributions
HS carried out the analysis and image processing of NLC images. AM plannedand organized the schedule of jet observations. PD and TT (Taku Takada)
contributed to feasible study for the jet observation and scientific analysis.
YN and SI contributed to development of image processing procedures.
KS (K. Sakanoi), KS (K. Sakaguchi), TT (Takuo Tsuda), and YH contributed todata analysis. All authors contributed manuscript preparation and read andapproved the final manuscript.
Funding
The work of HS, KS (K. Sakanoi), KS (K. Sakaguchi), TT (Taku Takada), TT (Takuo
Tsuda), and YH is supported by the Japan Society for the Promotion of Science(JSPS) KAKENHI Grant Number JP19H01956. This work of YH is supported inpart by JSPS KAKENHI Grant Numbers JP19J01367. The part of this work is supported by Meiji University (Grant No. MU-RMG 2019-21).
Availability of data and materials
The NLC image data taken by jets used in this paper are available uponrequest to the corresponding author HS (suzuhide@meiji.ac.jp). The AIM-CIPS
data are provided through http://​lasp.​color​ado.​edu/​aim/​index.​php.

Declarations
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Meiji University, Kawasaki, Kanagawa, Japan. 2 ANA HOLDINGS INC.,
Minato‑ku, Tokyo, Japan. 3 Swedish Institute of Space Physics, Kiruna, Sweden.
4
Komazawa University, Setagaya, Tokyo, Japan. 5 National Institute of Information and Communications Technology, Koganei, Tokyo, Japan. 6 Tokyo
Metropolitan College of Industrial Technology, Arakawa, Tokyo, Japan. 7 The
University of Electro-Communications, Chofu, Tokyo, Japan.
Received: 25 August 2021 Accepted: 12 January 2022

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