Situated about 1.6 million kilometers (1 million miles) from Earth toward the Sun, NOAA’s Deep Space Climate Observatory (DSCOVR) maintains a constant view of the sunlit face of our planet. Over the past two years, the satellite has observed something that researchers were not expecting when they launched the satellite. The main imager, NASA’s Earth Polychromatic Imaging Camera (EPIC), has detected hundreds of flashes of light reflecting off the planet.
EPIC acquires images hourly, and they are posted to the mission’s web site for public viewing. In the months after the satellite launched, keen observers from outside NASA wrote to the science team, questioning the source of these lights. Alexander Marshak, DSCOVR deputy project scientist at NASA’s Goddard Space Flight Center, first noticed light flashes occasionally appearing over oceans as he looked through daily EPIC images.
Investigating further, Marshak and colleagues found that similar flashes had caught the attention of astronomer Carl Sagan in 1993 when he was looking at images taken by the Galileo spacecraft. During one if its swings past Earth, Galileo turned its instruments on and collected data as a test before heading to Jupiter. “Large expanses of blue ocean and apparent coastlines are present,” Sagan and colleagues wrote, “and close examination of the images shows a region of [mirror-like] reflection in ocean but not on land.”
Flashes of light off of the oceans have a relatively simple explanation, Marshak noted. When sunlight shines on a relatively smooth part of an ocean or lake and reflects directly back to the sensor, it is like taking a flash photograph in a mirror.
But when the scientists took another took a look at the Galileo images, they saw something Sagan and his colleagues missed: bright flashes over land. And those flashes appeared in the EPIC images as well. The EPIC team thought at first that sunlight could be glinting off of some other surface bodies of water. But the location and size of the flashes just didn’t support that explanation.
Marshak and colleagues Tamas Varnai of the University of Maryland, Baltimore County, and Alexander Kostinski of Michigan Technological University turned their thoughts to another source of water in the Earth system: the sky. They conducted a series of experiments, detailed in a new paper published in Geophysical Research Letters, to confirm that the cause of the distant flashes is ice high in the atmosphere.
The team cataloged 866 prospective sunlight glints over land between June 2015 and August 2016. The flashes show up in three distinct colors because the camera takes the red, green, and blue images several minutes apart. (Note the faint colors around the flash in our image above.) The scientists reasoned that if these flashes were caused by reflected sunlight, they would be limited to certain spots on the globe where the angle between the Sun and Earth is the same as the angle between the spacecraft and Earth.
When they plotted the locations of glints against the angles, the two matched. This helped confirm that it was not lightning or some other phenomenon causing the flashes. “Lightning doesn’t care about the Sun and EPIC’s location,” Marshak said. The researchers also determined that the light was reflecting off of ice particles floating in the air nearly horizontally.
The EPIC data also helped confirm that the flashes are coming from a high altitude, not simply water on the ground. Two channels on the instrument are designed to measure the height of clouds. According to the observations, high cirrus clouds—5 to 8 kilometers (3 to 5 miles) up in the atmosphere—appeared wherever the glints were located.
“The source of the flashes is definitely not on the ground,” Marshak said. “It is definitely ice, and most likely solar reflection off of horizontally oriented particles.”
Marshak is now investigating how common these horizontal ice particles are, and whether they are common enough to have a measurable impact on how much sunlight passes through the atmosphere. If so, it is a feature that would need to be incorporated into computer models of how much heat is reaching and leaving Earth.
NASA image courtesy of the DSCOVR EPIC team. Story by Kate Ramsayer, NASA GSFC, with Mike Carlowicz.