Many people who viewed the images posted by NASA noticed the bright flashes and wrote to Dr. Marshak asking what they were.
Dr. Marshak began to investigate, and he came across early observations of the lights described by Dr. Sagan and colleagues in 1993. They had seen the mirrorlike reflections in satellite images taken by the Galileo spacecraft as it swung around Earth en route to Jupiter.
“He saw many, many sun glints, and he mentioned it only over the oceans,” Dr. Marshak said. The explanation offered for the flashes was that they were reflections from the smooth surface of the water. But when Dr. Marshak reviewed the images from Galileo, he saw the flashes also appeared over land, as they did in the images from Dscovr.
With his colleagues, he analyzed about 860 glints of light observed by Dscovr from the time it reached orbit through August 2016, and recorded the latitudes at which they appeared on the planet. The team thought that if the flashes were caused by reflections of sunlight, rather than something like lightning, the glints would appear only at certain latitudes on the globe, which were dependent on the angles of the sun, Earth and the satellite.
When the team plotted the glints’ locations, it found that the flashes all fell within the expected latitudes, confirming that they were reflections of sunlight.
Next the team wanted to see if the reflections came from surfaces on the ground, like lakes, or from clouds in the sky. It turned to another tool aboard Dscovr that tracks the location of cirrus clouds, which are wispy and found more than three miles high. The researchers found that the locations of the clouds matched with the locations of the glints.
The team concluded that tiny ice crystals floating within clouds at high altitudes in a horizontal arrangement were positioned just right to create the sun glints.