Research done through the clouds

 Research done through the clouds

Research done through the clouds

When it comes to talking about the universe, I’m  not usually at a loss for words. But every so   often, I see something so awe-inspiring and  unusual that words don’t do it justice. And   it seems that when I posted an image of this  last year, it looks like I’m not alone here.   Such is the case with noctilucent clouds, a  strange and beautiful phenomenon that occurs   high in the Earth’s mesosphere and requires  a unique set of conditions to form. In fact,   these cloud formations are so captivating  that they’ve inspired communities of watchers,   who share atmospheric data online in  the hope of seeing one for themselves.   So, what are noctilucent clouds? What gives them  their ghostly luminescence? And what surprising   insights can they offer, not just about Earth’s  atmosphere, but perhaps the future of our planet?   I’m Alex McColgan, and you are watching Astrum.  Join me today as we view spectacular images of   these eerily beautiful cloud formations,  learn how they form, and understand why   they are giving scientists a surprising  window into Earth’s changing atmosphere.   

Imagine a clear summer night. The Sun has dipped  below the horizon, and Capella is shining brightly   to the North. Suddenly, a bright streak erupts low  in the northern sky. It might look like shining   silver thread, or maybe an icy blue whirl.  Slowly, the streaks get brighter and clearer,   until finally, the whole sky glistens with a  patchwork of eerily shining clouds. Night shining   clouds such as this formation photographed  over the Baltic Sea are truly spectacular,   and no, they don’t actually emit light. They  get their glowing appearance by reflecting   solar radiation during astronomical twilight,  when the sun is between 6 and 16 degrees below   Earth’s horizon and remnant light is scattered  in the upper atmosphere. Because noctilucent   clouds form at very high altitudes (around 76 to  85 kilometres above the terrestrial surface), they   can reflect the Sun long after an observer has  fallen into Earth’s shadow. 

So, while noctilucent   clouds also form during daylight hours, they  are only visible to the unaided eye at night.   To make a noctilucent cloud, you need water,  dust particles and incredibly cold temperatures.   Unlike normal clouds, which form in the Earth’s  troposphere, where 75% of the atmosphere’s mass   and 99% of its water vapour occurs, noctilucent  clouds form high in the upper mesosphere,   which is the atmosphere’s third  layer, above the stratosphere.   Just a reminder, the lowest layer of atmosphere  on the Earth is the troposphere, the boundary of   which is the tropopause, then the stratosphere,  stratopause, mesosphere, mesopause, thermosphere,   thermopause, and then finally the exosphere. For  more about all these layers, check out this video.   The mesosphere is a tricky region to study,  since its too high for aircraft to fly and   too low for orbital spacecraft due to atmospheric  drag. 

Research done through the clouds

Noctilucent clouds form slightly below the   mesopause, which is the upper boundary between the  mesosphere and thermosphere. The mesopause is also   the coldest region of Earth, with temperatures  that can plummet below 100 degrees Celsius. (By   comparison, the coldest temperature ever recorded  in Antarctica was minus 89.6 degrees Celsius!)   These frigid conditions produce tiny ice  crystals less than 100 nanometers in diameter,   which then gather on dust particles. Researchers  have learned that the mesosphere must reach   minus 120 degrees Celsius for these ice  crystals to form. That’s pretty cold!   But if noctilucent clouds require such cold  conditions, why do they occur during summer?   Well, one unusual property of the mesopause  is that it is colder in summer than in winter.   Known as the mesopause anomaly, this happens  because hot air in the lower troposphere expands,   resulting in upswelling gasses that decompress  in the mesosphere, causing adiabatic cooling.   

Because noctilucent clouds require very low  temperatures, they only appear for 60 to 80   days out of the year, peaking around 20 days  after the solstice. If you want to see a night   shining cloud formation, you’ll need a lookout  spot between ±50 and ±70 degrees latitude.   Although satellites have spotted many mesospheric  clouds north of the 70th parallel, here on Earth,   the polar regions have too much ambient light  during summer for optimal viewing. So, if you   happen to live in northern Norway or Alaska,  you’ll have to travel south for your NLC watching!   That’s a pretty good survey of what we  know, so let’s talk about what we don’t.   Saying the mesosphere is a very dry place would  be a bit of an understatement. In fact, it is   one million times dryer than air from the Sahara  Desert. Not exactly a place you’d expect to find   clouds! We currently think upswelling air from the  troposphere is the likely source of this moisture. 

More puzzling, however, is the question of where  the dust is coming from. One theory is that it   comes from space in the form of debris. The Earth  gets bombarded daily with thousands of meteorites   and other space debris. Could they leave enough  dust to create such massive cloud formations?   Or could there be other causes? Interestingly,  NLCs were first sighted in 1895, two years after   Krakatoa’s massive volcanic eruption in Indonesia  which spewed debris into the upper atmosphere.   It has to be something big like this because  otherwise the atmospheric layers don’t mix   very much, and so dust wouldn’t usually make it to  the mesosphere. We also know that manmade sources,   such as exhaust from space shuttles, can  sometimes trigger noctilucent clouds,   such as a 2014 incident when the SpaceX Falcon 9  caused noctilucent clouds over Orlando, Florida.   In 2009, the United States Naval Research  Laboratory successfully created an artificial   NLC using exhaust particles from a suborbital  sounding rocket. 

Research done through the clouds

And in 2018, the University of   Alaska created a noctilucent cloud by releasing  water from a suborbital rocket. But without   such proximate causes, the source isn’t easy to  pinpoint. Luckily, NASA has launched a satellite,   known as the Aeronomy of Ice in the Mesosphere, or  AIM, to answer these and other pressing questions.   It was first launched in 2007 but  as of 2022 is still operational.   AIM is powered by two solar panels and is  equipped with three payload instruments.   The Cosmic Dust Experiment, which emits pulses  to measure speeding dust particles. The Cloud   Imaging and Particle Size, has four cameras  that image mesospheric clouds from variable   angles to create a detailed 2D panoramic view.  And the Solar Occultation for Ice Experiment,   which measures particles, temperature and  atmospheric gasses in order to identify chemicals   and conditions for NLC formation. 

There have also  been several experiments to synchronize AIM’s   observations with those of low-flying aircraft,  the first of which was conducted in July 2009.   By synchronizing data between satellites and  aircraft, scientists can construct far more   detailed models of mesospheric cloud formation  and their features than they could from AIM alone.   For NASA and other atmospheric researchers,  answering these questions is no idle matter.   The mesosphere is a remarkably sensitive indicator  of changes that are happening elsewhere in the   atmosphere. The same features that make it so  unusual, its rarified gasses and sensitivity   to changes far down in the troposphere, make it  a useful canary in the coal mine, so to speak.   Decades of NLC study have made it clear that  they are becoming more frequent, and climate   researchers are now beginning to link changes  in NLC distribution to global climate change.