NASA said on May 15, 2017 that one of its cameras – on a spacecraft a million miles from Earth – is capturing unexpected sunglint from our planet. The homeward-facing camera on NOAA’s Deep Space Climate Observatory (DSCOVR) caught hundreds of these flashes over the span of a year. Scientists have determined that a tiny cause for the big reflections: high-altitude, horizontally-oriented ice crystals in Earth’s atmosphere.
DSCOVR is primarily used to study the solar wind and its effects on Earth. The Earth Polychromatic Imaging Camera (EPIC) instrument aboard DSCOVR views the sunlit face of the Earth from sunrise to sunset, taking almost-hourly images. In addition to DSCOVR’s main mission, the camera can also shed light on atmospheric phenomena such as ozone, aerosols, cloud heights, dust and volcanic ash. NASA said that Alexander Marshak, DSCOVR deputy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, first noticed light flashes occasionally appearing over oceans as he looked through that day’s EPIC images:
Investigating the flashes, Marshak and his colleagues found that similar reflections from our pale blue dot caught the attention of astronomer Carl Sagan in 1993. Sagan was looking at images taken by the Galileo spacecraft, which launched in 1989 to study Jupiter and its moons. During one if its gravitational-assist swings around Earth, Galileo turned its instruments on this planet and collected data. Sagan and his colleagues used that to test a key question: Whether spacecraft could detect signatures of life from afar.
But the glints detected by Sagan and his team were always over an ocean. Marshak said the explanation for them is relatively simple:
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.
Sagan’s team apparently missed the fact that, strangely, some Galileo images show bright flashes of light over land. And those flashes appeared in the EPIC images as well. As the contact listed on the website that posts all EPIC images, Marshak started getting emails from people curious about what the flashes were. He said:
We found quite a few very bright flashes over land. 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.
Instead, Marshak and his colleagues Tamas Varnai of the University of Maryland, Baltimore County, and Alexander Kostinski of Michigan Technological University, thought of water elsewhere in the Earth system: ice particles high in the atmosphere. The scientists conducted a series of experiments, detailed in a new paper published in the peer-reviewed journal Geophysical Research Letters, to confirm the cause of the distant flashes.
First, the researchers cataloged all of the prospective sunlight glints over land in images from the EPIC camera. The flashes show up in three distinct colors because the camera takes the red, green and blue images several minutes apart. In all, the scientists found 866 bursts between DSCOVR’s launch in June 2015 and August 2016.
The scientists reasoned that if these 866 flashes were caused by reflected sunlight, they would be limited to certain spots on the globe – spots where the angle between the sun and Earth is the same as the angle between the spacecraft and Earth, allowing for the spacecraft to pick up the reflected light. When they plotted the locations of the glints with where those angles would match, given Earth’s tilt and the spacecraft’s location, the two matched.
This helped confirm that it wasn’t something like lightning causing the flashes. Marshak explained:
Lightning doesn’t care about the sun and EPIC’s location.
The researchers also plotted angles to determine that the light was reflecting off of ice particles floating in the air nearly horizontally.
Another feature of the EPIC data helped confirm that the flashes were from a high altitude, not simply water on the ground. Two channels on the instrument are designed to measure the height of clouds, and when the scientists went to the data they found high cirrus clouds, 3 to 5 miles (5 to 8 kilometers) where the glints were located. Marshak said:
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.
Detecting glints like this from much farther away than in this case could be used by other spacecraft to study exoplanets, he said. As an Earth scientist, however, Marshak is now investigating how common these horizontal ice particles are, and whether they’re common enough to have a measurable impact on how much sunlight passes through the atmosphere. If so, it’s a feature that could be incorporated into computer models of how much heat is reaching and leaving Earth.
Bottom line: The DSCOVR spacecraft is primarily used to study the solar wind and its effects on Earth. But a home-facing camera aboard DSCOVR – called EPIC – also caught sight of sunglint over land on Earth. The glint comes from high-altitude, horizontally-oriented ice crystals in Earth’s atmosphere.
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