Explosions on the sun’s surface explain its extremely hot outermost layers

This summer’s total solar eclipse revealed rare views of the sun's corona, its outermost layers of plasma millions of degrees in temperature. But the solar corona has long baffled scientists: Why is it so searingly hot compared with the sun’s visible surface, which is about 1000 times cooler? Now, researchers have suggested that relatively small explosions known as “nanoflares” may be responsible for the corona’s extreme temperatures. Working in the New Mexico desert, the scientists launched a sounding rocket called FOXSI containing seven telescopes on a 15-minute trip into space to observe the sun (shown above in x-ray light). The telescopes, with more sensitive detectors than previous x-ray telescopes, recorded high-energy light indicative of temperatures exceeding 10 million°C from one region of the sun. But solar flares—the brief, intense flashes of light caused by the sun’s magnetic fields changing shape suddenly—couldn’t be causing the heating because none were observed. Instead, many nanoflares, a million times weaker than traditional solar flares but still packing enough of a punch to meet the United States’ energy needs for a year, were acting in concert to heat the corona, the team reports today in Nature Astronomy. Upcoming FOXSI launches and other space-based telescopes may reveal more about nanoflares, the researchers suggest.