Three Indian astronomers have discovered a drumbeat of magnetic explosions across the face of the sun in an advance towards resolving a decades-old mystery of why the corona, the sun’s atmosphere, is hotter than its surface.

The scientists at the National Centre for Radio Astronomy, Pune, have recorded tiny radio flashes they believe are the “smoking guns” for the magnetic explosions to explain the coronal-heating mystery.

US solar physicist Eugene Parker had suggested 22 years ago that such small magnetic explosions, which he called “nanoflares”, might be one way to explain why the temperature of the corona is two million Celsius while the sun’s surface is around 6,000 degrees Celsius. But no one had observed them until now.

At the NCRA, Divya Oberoi and his students Surajit Mondal and Atul Mohan used data from a radio astronomy observatory called the Murchison Widefield Array in western Australia and sensitive tools developed at the NCRA to analyse solar radio images.

“These magnetic explosions as we call them are actually tiny, billions of times weaker than typical solar flares,” Oberoi said. “The radio flashes we’ve seen are the smoking guns for these explosions.”

The researchers believe the explosions provide the mechanism through which the energy trapped in the magnetic fields are “dumped” into the corona causing it to remain much hotter than the sun’s surface. “The flashes are present everywhere on the sun and at all times,” said Mondal.

Astronomers have known for seven decades that the temperature of the corona — an envelope of gas surrounding the sun and visible only during a total solar eclipse — is much higher than those on the solar surface. What heats the corona has been one of the most challenging puzzles about the sun.

“Our estimates suggest that the magnetic explosions collectively produce enough energy to solve the coronal heating problem,” said Mohan. The astronomers have published their findings this month in the Astrophysical Journal Letters.