Studying thunderstorms from space

The photograph of the September 8, 2015, thunderstorm over eastern India that Andreas Mogensen took from the ISS.

WHEN a thunderstorm strikes, one sees a spectacle of bright white flashes accompanied by loud cracks. From laboratories on the earth, it is difficult to see past the dense clouds to the sky above the storm. What kind of lightning flashes occur in the higher levels of the earth’s atmosphere is an area of research that is poorly studied. For an elevated view, scientists have traditionally launched expensive equipment into space or flown specialised aircraft to altitudes of 18,000 metres, almost twice the height that commercial planes fly at. Torsten Neubert, a physicist at the Technical University of Denmark (DTU), had a different idea: ask astronauts aboard the International Space Station (ISS) to photograph a storm from high up.

A team of Danish scientists teamed up with the Danish astronaut Andreas Mogensen and photographed storms raging over California, eastern India, Thailand and Costa Rica from his position on the ISS. The images were part of a pilot test of the proposal, named Thor after the Nordic god of thunder.

But imaging thunderstorms from space is not simple. Mogensen needed to know precisely when to be in position so that he could capture the storm as the ISS flew over it. According to Olivier Chanrion—also from the DTU, who presented the findings at the December 2016 meeting of the American Geophysical Union in San Francisco —while Mogensen was in orbit his team transmitted thunderstorm forecasts to the ISS. Once the scientists knew where and when to expect a storm, they used software to determine when it would be visible to the astronaut. They predicted four storms that Mogensen would be able to observe during his 10-day mission in September 2015, and the astronaut successfully captured them all. The images revealed that thunderstorms contained far more lightning flashes than expected. According to Chanrion, the eastern Indian storm captured on the evening of September 8 was the most spectacular. In Mogensen’s three-minute-long video (for a short clip, see www.youtube.com/watch?v=dY5F_gEexAY), storm clouds are constantly lit up by flashes. Pulsating blue “jets” of lightning travel upwards from the cloud’s top, shooting to altitudes of 40 km, well past where the clouds end. A “sprite”—a flare tens of kilometres in size that originates high above the thunderstorm—explodes in the night sky. And all the while the cloud’s upper surfaces are awash with smaller flashes from blue discharges that flitter every 30 seconds.

“The observations are the first of their kind,” said Neubert. He hopes Thor’s success will open the door to further imaging of thunderstorms by astronauts during the European Space Agency’s Atmospheric-Space Interactions Monitor project. The two-year experiment, which begins in 2017, will survey the X-ray and gamma ray emissions of thunderstorms from the ISS. Such data will enable scientists to understand better the origin of high-energy X-ray and gamma ray flashes, and the impact of the flashes on the upper levels of the atmosphere. “We don’t know why the blue discharges are so numerous,” Chanrion said. He and his colleagues hope to find out.

R. Ramachandran