India's Sun-watching spacecraft, Aditya-L1, has produced its most detailed evidence yet of a phenomenon inside solar flares that gives researchers a fresh angle on one of solar physics' longest-running puzzles: why the sun's outer atmosphere runs so much hotter than the surface beneath it.
An X-ray instrument aboard the Indian Space Research Organisation's Aditya-L1 spacecraft, the Solar Low Energy X-ray Spectrometer, or SoLEXS, has recorded what scientists call "iron fluorescence" across 47 major X-class flares observed in 2024, near the peak of the current 11-year solar cycle. The effect occurs when intense X-rays generated in the corona during a flare strike neutral iron atoms in the cooler, denser layer of the sun below, the photosphere. Those atoms briefly absorb the energy, then re-emit it as a distinctive X-ray glow at 6.4 kiloelectronvolts, the atomic signature of iron.
The results, detailed in a paper describing the SoLEXS observations, are described as the first comprehensive catalogue of the effect from a single instrument. Researchers found the fluorescence signal's strength depended on where a flare occurred on the visible solar disk: flares near the centre produced strong signals, while those near the edge were markedly suppressed, matching theoretical predictions. That gives scientists a new diagnostic for estimating how high above the surface a flare's X-ray source sits.
An old riddle in solar physics
The broader mystery the observations bear on is the coronal heating problem. The corona, the sun's faint outer atmosphere visible to the naked eye only during a total eclipse, burns at more than 2 million degrees Celsius, while the visible surface below it, the photosphere, sits at a comparatively cool 5,500 degrees. Physicists have never fully explained how the outer layer gets hotter than the layer generating its energy, or how the corona avoids running out of energy despite regularly hurling material into space during flares and eruptions.
Aditya-L1 is well placed to probe such questions from its post roughly 1.5 million kilometres from Earth, at the L1 Lagrange point, where it has an uninterrupted view of the sun unlike observatories in Earth orbit. ISRO said the spacecraft, launched in September 2023 with seven indigenously built instruments, recorded "a strong fluorescence signal" for flares near the disk's centre, while the signal was "heavily suppressed" for flares near the edge.
If the sun is losing such huge amounts of energy with each eruption and it's not replenished, the star at the centre of our solar system would lose all its energy.
Prof. R. Ramesh, Indian Institute of Astrophysics
Solar physicists say diagnostics like this could eventually feed into space-weather forecasting, since flares and coronal mass ejections can disrupt satellites, power grids and radio communications on Earth. ISRO says Aditya-L1 will keep monitoring the sun as the current solar cycle eases from its peak toward a quieter minimum later in the decade.
