One of the most enduring mysteries that has been puzzling scientists forever is how the Sun's outer atmosphere - the corona - is millions of degrees hotter than its surface. And how does the corona continue to remain so hot despite frequent eruptions during which it loses huge amounts of energy? Astrophysicists in India now say the latest findings from Aditya-L1, the country's first solar observation mission in space, have provided them some vital clues to unlocking those mysteries. Their findings are revealed in a recent paper published in the prestigious Astrophysical Journal Letters. The temperature variations in different regions of the Sun defy the laws of physics, says leading Indian solar astrophysicist Prof R Ramesh of the Indian Institute of Astrophysics (IIA) who led the study. If you cut through the Sun's layers, deep inside is the core where the temperature is 15 million Celsius. Travel outwards to the surface or the photosphere - the part that we see from the Earth - it's about 5,500C. Far away from the core is the Sun's outermost layer or corona where the temperature is about 2 million C - it can sometimes go up to 40 million C. Prof Ramesh says the corona is where extreme weather events - such as solar flares and coronal mass ejections (CMEs) - originate, during which large amounts of energy is released by the Sun into space. The CMEs cause beautiful auroras, and they can also affect life on Earth by causing geomagnetic storms that can knock down power grids and affect weather and communication satellites. In normal or low-activity times, the Sun launches two to three CMEs a day. During the maximum solar activity cycle that comes every 11 years, there can be 10 or more in a single day. "Now if the Sun is losing such huge amounts of energy with each CME and it's not replenished, the star at the centre of our solar system would lose all its energy and Earth would plunge into an irreversible deep freeze," Prof Ramesh says. But since that's not happening, it means there's a "mechanism" by which the corona is able to maintain its inexplicably high temperature, he says. Scientists put it down to two factors - one, the bubbling boiling motions on the Sun's surface that constantly generate waves which, as they travel outwards, carry energy to the corona - a bit like sea waves carrying foam and froth to the shore. The second is the "tangled magnetic field lines" in the Sun's atmosphere that constantly snap and then reconnect. Prof Ramesh says a CME occurs when these twisting, looping lines - which resemble braided hair - rupture, throwing out massive clouds of magnetised plasma and gas in space. They often originate near sunspots, which are darker, cooler spots on the Sun where magnetic fields are particularly strong. "But then these lines reconnect and the Sun replenishes the lost energy back within hours," he adds. In their paper, Prof Ramesh says they have been able to quantify how much energy each of the two systems supply to the corona – which explains both why it's so many times hotter to begin with and how it retains that temperature after consistently losing energy. Their study, he says, clearly shows that it's the second system that supplies most of the energy. "Though the waves generated as a result of the bubbling, boiling motions on the Sun's surface generate and transport energy, their contribution is very little - they supply only 7% of the energy requirement. "The remaining 93% comes because the Sun reconfigures itself and replenishes the lost energy."
