Our f lashy planet
Linking reflections in deep space to the sparkling linings of clouds.
DIDYOU KNOW there’s a spacecraft between Earth and the Sun that stares unblinkingly at our planet and every hour or so snaps an image of its sunlit side? Its name is DSCOVR (for Deep Space Climate Observatory) and it sits 1.5 million kilometres away, at a stable point where the Sun’s gravitational pull just balances that of Earth.
Launched in early 2015, DSCOVR’s main stocks-in-trade are climate and space weather observations for NASA and the National Oceanic and Atmospheric Administration. But its operators soon noticed something else. A peppering of 866 bright flashes on Earth’s surface occurred during DSCOVR’s first 14 months of operation.
Your first thought, as mine was, is probably “Sun-glint from the ocean”. But these flashes were seen over land as well as sea. Most of us have spotted bright reflections of the Sun in lakes or ponds from aircraft windows, so that might not seem surprising. But the flashes are too bright to be coming from small bodies of water. So researchers looked elsewhere, eventually concluding water in another form causes the reflections.
Meteorologists know that, under certain conditions, atmospheric ice crystals can form in the shape of miniature hexagonal plates floating horizontally in the air. Myriads of tiny reflections in cirrus clouds extending over many kilometres could be the source of the mysterious flashes. But how could the idea be tested?
First, researchers checked that the flashes only occurred when the angle between the Sun, the flash and the spacecraft was correct for reflected light. But the clincher was to use the rainbow spectrum of the flashes themselves to determine how much atmospheric oxygen the light had passed through on its journey. It turned out to be much less than would have been expected for flashes occurring at ground level. Sure enough, they were able to demonstrate that the reflections were coming from clouds 5–8km above sea level. Problem solved.