Night owls and insomniacs who stay up to watch the twilight and night sky in the summer have probably noticed wispy clouds of silvery color that start to appear in the twilight sky. Sometimes they appear as a weak, foggy spot, but other times they are bright, with beautiful wavy surfaces like the surface of the sea. These waves can move in unison in a certain direction, or they can move in different and even in opposite directions. Occasionally, strange waveforms resembling a fish skeleton or small-scale waves suddenly grow above the silvery surfaces of these clouds.
These are noctilucent clouds (NLCs), and they form in the upper atmosphere between 80 and 90 kilometers above Earth’s surface. NLCs are composed of small ice crystals about 50 nanometers in radius. Because they scatter sunlight effectively, these clouds are easily visible against the twilight sky.
On the night of 5–6 July 2018, our research group, a collaboration of scientists from the Swedish Institute of Space Physics, A. M. Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences, Aerospace Laboratory Stratonautica, and the Moscow Association for Noctilucent Cloud Research, observed NLCs from a vantage point between instruments on the ground and satellite instruments in space. We sent instruments aloft on a meteorological sounding balloon in an experiment we call Stratospheric Observations of Noctilucent Clouds (SONC). We dedicated this stratospheric experiment to studies of NLC dynamics over horizontal distances of more than 100 kilometers.
Noctilucent Clouds from Above and Below
NLCs are regularly observed and studied from the ground using optical imagers and lidars, as well as from space using satellite measurements [Bailey et al., 2009; Dalin et al., 2008; DeLand and Thomas, 2015]. Each of the techniques has its advantages and disadvantages.
Ground-based lidar and optical measurements provide high horizontal and vertical resolutions of approximately 20 and 50–150 meters, respectively [Baumgarten and Fritts, 2014], as well as high temporal resolution, on the order of 10 seconds. However, such measurements are limited by tropospheric weather conditions—the sky must be clear or partly clear—and ground-based instruments can cover only a small region of the globe.
Satellite measurements provide a global view of the NLC positions, and they are independent of weather conditions. However, they have a low horizontal resolution (~2 kilometers) and low temporal resolution (~1.5 hours between orbital trajectories). Thus, there is no perfect technique to observe and study spatial-temporal evolutions of NLCs so far.
A stratospheric balloon would seem to be a preferred platform to observe NLCs from a vantage point between ground-based instruments and satellites, but until now, only a single published experiment has been conducted from a stratospheric balloon. The E and B Experiment (EBEX)—named after two modes of the cosmic microwave power spectrum—provided NLC observations over Antarctica between 29 December 2012 and 9 January 2013 [Miller et al., 2015]. While EBEX was gathering data on polarization in the cosmic microwave background, two of its star cameras, having a narrow field of view (~4° by 3°), suddenly captured fine structures of NLCs and their turbulence dynamics on small horizontal scales, from several kilometers down to 10 meters.
The observations from EBEX and the absence of other stratospheric observations of NLCs on large scales motivated us to conduct a new balloon-borne experiment in the summer of 2018. This experiment opens up new horizons (in the literal and figurative senses) for studies of large-scale dynamic processes in the middle atmosphere.
Launching the Experiment
We launched the SONC experiment on the night of 5–6 July 2018. We used a meteorological sounding balloon to lift a full-frame, high-resolution, highly sensitive digital camera (Sony Alpha A7S, 35 millimeter, 12 megapixels) into the stratosphere over Moscow, Russia (~56°N, 41°E). The camera was equipped with a wide-angle lens that captured an image over an area spanning approximately 110° by 82° (14 millimeters, f-number f/2.8).
Since a payload gondola constantly rotates and shakes during a flight, the NLC camera was placed onto a specially designed three-axis motorized gimbal stabilized platform built by Aerospace Laboratory Stratonautica. The NLC camera took images every 6 seconds during the whole flight and collected several thousands of images. A flight with a long duration would require a thermostabilization system, but the SONC experiment did not require it. A camera can readily survive the low stratospheric temperatures (about –50°C) for the 2 hours allotted to this experiment.
We launched the SONC balloon at 21:34 Universal Time (UT), and it landed at 23:19 UT on 5 July 2018, with a total flight duration of about 1.7 hours. We chose a launch time on the basis of the long-term statistics of NLC observations in the Moscow region from 1962 to the present time, which demonstrated that the highest probability (~80%) of observing NLCs on a clear night is at the beginning of July.
The balloon, which was designed to reach an altitude of 30 kilometers, reached its maximum altitude at 20.4 kilometers, where it burst for unknown reasons. The NLC camera descended with a parachute and was successfully recovered. A GPS receiver installed in the gondola provided information on the balloon’s trajectory (Figure 1). Ground support was provided by three automated NLC cameras in the Moscow region. These cameras photographed the NLCs at the same time as the photos captured from the SONC balloon. This overlap allows us to estimate the altitude of NLCs and their dynamics in 3-D space using a triangulation technique.
Balloon-Borne Observation of Noctilucent Clouds
The balloon-borne camera captured a large-scale NLC field with a number of interesting features, which we are currently analyzing. Here we report the following general characteristics of the NLC display. NLCs were observed between 20:30 and 23:15 UT on 5 July 2018, and they were located between 82.6 and 85.1 kilometers altitude. The NLC field extended along the horizon from the northwest to northeast at a low elevation, as seen from the balloon, and the NLCs traveled from the south to the north at an observed mean speed of approximately 43 meters/second.
The horizontal extent of the NLC field from its western to eastern observable border was about 1,400 kilometers, and it extended about 800 kilometers from the northern to southern border. Such large distances are impossible to observe from the ground because of Earth’s curvature and the limited area of the twilight sky illuminated by the Sun. An observer on the ground watched only the central part of the NLC field (about 1,000 by 500 kilometers) because its far eastern and western wings were located below the ground observer’s local horizon.
Thus, a balloon-borne NLC observation provides an obvious great advantage over a ground-based observation, yielding much larger spatial coverage. This coverage is comparable to observations made from space, but it comes at a much lower cost. At the same time, we are able to resolve NLC details as small as 100 meters, a resolution that is unachievable by current space observations of NLCs.
New Horizons and Next Steps
The combination of large spatial coverage (1,400 kilometers or more) and high-resolution images (~100 meters) is a unique characteristic inherent to stratospheric balloon-borne observations of noctilucent clouds. Such a combination of resolutions is currently impossible to achieve from either the ground or space. In general, a balloon-borne observation provides us with several new opportunities:
- For long-duration flights spanning several days, we can observe NLCs for 24 hours a day. This duration is possible because the sky is always almost black above 20 kilometers altitude (there is very little Rayleigh atmospheric scattering).
- This vantage point lets us detect noctilucent clouds with very low brightness levels (see below).
- We can measure neutral wind velocity at the mesopause and the large-scale trajectory of NLC fields (over 1,400 kilometers).
- We can obtain quantitative information on a wide range of atmospheric waves (gravity and planetary waves, solar tides) propagating through the summer mesopause.
- A camera equipped with a narrow field of view lens (~10°) can provide quantitative information on small-scale turbulent dynamics (down to 1 meter).
For the next phase of our project, we will build a new NLC imager, which we plan to launch for several days. This imager will have four cameras equipped with wide- and narrow-angle lenses to resolve large-scale atmospheric gravity waves and small-scale dynamics like wave instabilities and turbulent processes in NLCs. We may be making a circumpolar flight around the North Pole for 15–20 days using a big scientific balloon whose volume is kept approximately constant, enabling it to keep a constant altitude for a long time.