One of the most enduring mysteries in astrophysics is why the Sun's outer atmosphere, the corona, is millions of degrees hotter than its surface. Despite frequent eruptions that release enormous energy, the corona remains scorching hot. India's first solar observation mission, Aditya-L1, has provided vital clues to solve this puzzle, as reported by infohub.kz.
Prof. R. Ramesh of the Indian Institute of Astrophysics (IIA), who led the study, explains that temperature variations across the Sun's layers defy conventional physics. Deep inside the core, temperatures reach 15 million degrees Celsius. Moving outward to the photosphere, the visible surface, it's about 5,500°C. But the outermost layer, the corona, blazes at around 2 million degrees Celsius, sometimes spiking to 40 million.
The corona is the birthplace of extreme space weather events like solar flares and coronal mass ejections (CMEs), which release colossal energy. CMEs can trigger beautiful auroras but also disrupt power grids and satellites. During quiet periods, the Sun emits two to three CMEs daily, but during solar maximum, which occurs every 11 years, that number can exceed ten per day.
Prof. Ramesh warns that if the Sun didn't replenish the energy lost in each CME, our star would eventually cool, plunging Earth into an irreversible deep freeze. Since that hasn't happened, there must be a mechanism maintaining the corona's extreme heat.
Scientists attribute this to two factors: first, boiling motions on the Sun's surface generate waves that carry energy outward to the corona, much like ocean waves deposit foam on a shore. Second, tangled magnetic field lines in the Sun's atmosphere constantly snap and reconnect. Prof. Ramesh describes CMEs as occurring when these twisted, braided loops rupture, ejecting magnetized plasma and gas into space, often near sunspots where magnetic fields are intense.
"But then these lines reconnect and the Sun replenishes the lost energy within hours," he adds. In their paper, Prof. Ramesh and his team quantified how much energy each system contributes to the corona. The study reveals that the second system—magnetic reconnection—supplies the majority of the energy. "The waves from the Sun's boiling surface provide only 7% of the required energy," says Prof. Ramesh. "The remaining 93% comes from the Sun reconfiguring itself and replenishing the lost energy."