Tag: ice crystals
Mysterious Flashes Coming From Earth That Puzzled Carl Sagan Finally Have An Explanation
Back in 1993, Carl Sagan encountered a puzzle. The Galileo spacecraft spotted flashes coming from Earth, and nobody could figure out what they were. They called them 'specular reflections' and they appeared over ocean areas but not over land.
The images were taken by the Galileo space probe during one of its gravitational-assist flybys of Earth. Galileo was on its way to Jupiter, and its cameras were turned back to look at Earth from a distance of about 2 million km. This was all part of an experiment aimed at finding life on other worlds. What would a living world look like from a distance? Why not use Earth as an example?
Fast-forward to 2015, when the National Oceanographic and Atmospheric Administration (NOAA) launched the Deep Space Climate Observatory (DSCOVER) spacecraft. DSCOVER's job is to orbit Earth a million miles away and to warn us of dangerous space weather. NASA has a powerful instrument on DSCOVER called the Earth Polychromatic Imaging Camera (EPIC.)
Every hour, EPIC takes images of the sunlit side of Earth, and these images can be viewed on the EPIC website. (Check it out, it's super cool.) People began to notice the same flashes Sagan saw, hundreds of them in one year. Scientists in charge of EPIC started noticing them, too.
One of the scientists is Alexander Marshak, DSCOVR deputy project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. At first, he noticed them only over ocean areas, the same as Sagan did 25 years ago. Only after Marshak began investigating them did he realize that Sagan had seen them too.
Back in 1993, Sagan and his colleagues wrote a paper discussing the results from Galileo's examination of Earth. This is what they said about the reflections they noticed: "Large expanses of blue ocean and apparent coastlines are present, and close examination of the images shows a region of [mirror-like] reflection in ocean but not on land."
Marshak surmised that there could be a simple explanation for the flashes. Sunlight hits a smooth part of an ocean or lake, and reflects directly back to the sensor, like taking a flash-picture in a mirror. Was it really that much of a mystery?
When Marshak and his colleagues took another look at the Galileo images showing the flashes, they found something that Sagan missed back in 1993: The flashes appeared over land masses as well. And when they looked at the EPIC images, they found flashes over land masses. So a simple explanation like light reflecting off the oceans was no longer in play.
"We found quite a few very bright flashes over land as well." – Alexander Marshak, DSCOVR Deputy Project Scientist
"We found quite a few very bright flashes over land as well," he said. "When I first saw it I thought maybe there was some water there, or a lake the sun reflects off of. But the glint is pretty big, so it wasn't that."
But something was causing the flashes, something reflective. Marshak and his colleagues, Tamas Varnai of the University of Maryland, Baltimore County, and Alexander Kostinski of Michigan Technological University, thought of other ways that water could cause the flashes.
The primary candidate was ice particles high in Earth's atmosphere. High-altitude cirrus clouds contain tiny ice platelets that are horizontally aligned almost perfectly. The trio of scientists did some experiments to find the cause of the flashes, and published their results in a new paper published in Geophysical Research Letters.
"Lightning doesn't care about the sun and EPIC's location." – Alexander Marshak, DSCOVR Deputy Project Scientist
As their study details, they first catalogued all of the reflective glints that EPIC found over land; 866 of them in a 14 month period from June 2015 to August 2016. If these flashes were caused by reflection, then they would only appear on locations on the globe where the angle between the Sun and Earth matched the angle between the DSCOVER spacecraft and Earth. As the catalogued the 866 glints, they found that the angle did match.
This ruled out something like lightning as the cause of the flashes. But as they continued their work plotting the angles, they came to another conclusion: the flashes were sunlight reflecting off of horizontal ice crystals in the atmosphere. Other instruments on DSCOVR confirmed that the reflections were coming from high in the atmosphere, rather than from somewhere on the surface.
"The source of the flashes is definitely not on the ground. It's definitely ice, and most likely solar reflection off of horizontally oriented particles." -Alexander Marshak, DSCOVR Deputy Project Scientist
Mystery solved. But as is often the case with science, answering one question leads to a couple other questions. Could detecting these glints be used in the study of exoplanets somehow? But that's one for the space science community to answer.
As for Marshak, he's an Earth scientist. He's investigating how common these horizontal ice particles are, and what effect they have on sunlight. If that impact is measurable, then it could be included in climate modelling to try to understand how Earth retains and sheds heat.
A search for life on Earth from the Galileo spacecraft
Terrestrial glint seen from deep space: oriented ice crystals detected from the Lagrangian point
If Pigs Could Fly – A Quick Guide to Solar Halos and Other Curiosities
Call it a porcine occultation. It took nearly a year but I finally got help from the ornamental pig in my wife's flower garden. This weekend it became the preferred method for blocking the sun to better see and photograph a beautiful pair of solar halos. We often associate solar and lunar halos with winter because they require ice crystals for their formation, but they happen during all seasons.
Nature keeps it simple. Light refracting through or reflecting from six-sided plate and column (pencil-shaped) ice crystals in high clouds is responsible for almost all halos and their variations.
Lower clouds, like the puffy cumulus dotting the sky on a summer day, are composed of water droplets. A typical cumulus spans about a kilometer and contains 1.1 million pounds of water. Cirrostratus clouds are much higher (18,000 feet and up) and colder and formed instead of ice crystals. They're often the first clouds to betray an incoming frontal system.
Cirrostratus are thin and fibrous and give the blue sky a milky look. Most halos and related phenomena originate in countless millions of hexagonal plate and pencil-shaped ice crystals wafting about like diamond dust in these often featureless clouds.
This is the top end of a hexagonal column-shaped ice crystal. Light refracting (bending) through the 60-degree angled faces of millions of these crystals is concentrated into a ring of light 22 degrees from the sun. As light leaves the crystal, the shorter blue and purple wavelengths are refracted slightly more than red, tinting the outer edge of the halo blue and inner edge red. Credit: Donalbein with additions by the author
In winter, the sun is generally low in the sky, making it hard to miss a halo. Come summer, when the sun is much higher up, halo spotters have to be more deliberate and make a point to look up more often. The 22-degree halo is the most common; it's the inner of the two halos in the photo above. With a radius of 22 degrees, an outstretched hand at arm's length will comfortably fit between sun and circle.
Light refracted or bent through millions of randomly oriented pencil-shaped crystals exits at angles from 22 degrees up to 50 degrees, however most of the light is concentrated around 22 degrees, resulting in the familiar 22-degree radius halo. No light gets bent and concentrated at angles fewer than 22 degrees, which is why the sky looks darker inside the halo than outside. Finally, a small fraction of the light exits the crystals between 22 and 50 degrees creating a soft outer edge to the circle as well as a large, more diffuse disk of light as far as 50 degrees from the sun.
The sun on Dec. 6, 2013 with a 22-degree halo and two luminous canine companions or sundogs. Similar halos and 'moondogs' can be seen around a bright moon. Credit: Bob King
Sundogs, also called mock suns or parhelia, are brilliant and often colorful patches of light that accompany the sun on either side of a halo. Not as frequent as halos, they're still common enough to spot half a dozen times or more a year. Depending on how extensive the cloud cover is, you might see only one sundog instead of the more typical pair. Sundogs form when light refracts through hexagonal plate-shaped ice crystals with their flat sides parallel to the ground. They appear when the sun is near the horizon and on the same horizontal plane as the ice crystals. As in halos, red light is refracted less than blue, coloring the dog's 'head' red and its hind quarters blue. Mock sun is an apt term as occasionally a sundog will shine with the intensity of a second sun. They're responsible for some of the daytime 'UFO' sightings. Check this one one out on YouTube.
An especially colorful sundog with a 'tail'. Red light is bent less than blue as it emerges from the ice crystal, tinting the sundog's inner edge. Blue is bent more and colors the outer half. If you look closely, all colors of the rainbow are seen. Credit: Bob King
Wobbly crystals make for taller sundogs. Like real dogs, ice crystal sundogs can grow tails. These are part of the much larger parhelic circle, a rarely-seen narrow band of light encircling the entire sky at the sun's altitude formed when millions of both plate and column crystals reflect light from their vertical faces. Short tails extend from each mock sun in the photo above.
About 2 hours after the flying pig image, the sun climbed beyond 50 degrees altitude. The circumscribed halo vanished! Credit: Bob King
There's almost no end to atmospheric ice antics. Many are rare like the giant 46-degree halo or the 9 and 18-degree halos formed from pyramidal ice crystals. Oftentimes halos are accompanied by arcs or modified arcs as in the flying pig image. When the sun is low, you'll occasionally see an arc shaped like a bird in flight tangent to the top of the halo and rarely, to its bottom. When the sun reaches an altitude of 29 degrees, these tangent arcs – both upper and lower – change shape and merge into a circumscribed halo wrapped around and overlapping the top and bottom of the main halo. At 50 degrees altitude and beyond, the circumscribed halo disappears … for a time. If the clouds persist, you can watch it return when the sun dips below 29 degrees and the two arcs separate again.
Maybe you're not a halo watcher, but anyone who keeps an eye on the weather and studies the daytime sky in preparation for a night of skywatching can enjoy these icy appetizers.