In a pot on the stove, water starts to churn from the bottom long before the whole thing boils. Bubbles of steam rise from the base even though the burner itself hasn't heated up yet. That's roughly how the Sun's atmosphere behaves before a flare: streams of plasma surge upward from the deep layers, even though the main heating source, according to previous ideas, hasn't switched on.
A solar flare is a sudden release of magnetic energy stored in the star's outer atmosphere. The standard model, developed half a century ago, painted this picture: magnetic field lines reconfigure, accelerate electrons, and those, like a hot shower, rain down on the lower atmosphere, the chromosphere. The heated material expands and rushes upward. This process is called chromospheric evaporation, and it was long believed to be triggered precisely by the electron impact.
The SUSI instrument scanned an active region in the ultraviolet and achieved a resolution of about 59 kilometers on the solar surface, the size of a small city. The researchers measured shifts in iron spectral lines that form at an altitude of 500–1000 kilometers. It turned out that 5–10 minutes before the flare onset, these lines show clear signs of motion: some flows rise, others descend. Speeds reach 10 kilometers per second.
That's like seeing steam over the pot before the stove is turned on. According to the standard model, upflows should appear only after the arrival of energetic electrons, which are detected via hard X-ray emission. But the observations show that the flows start earlier. So energy must be delivered downward by some other means, perhaps through thermal conduction, magnetic waves, or local field rearrangements right in the lower layers.
This result forces a reconsideration of the standard solar flare model. If evaporation begins before the electrons arrive, then the lower atmosphere has its own heating mechanism. The work opens a new window into diagnosing solar storms and could improve space weather forecasts, on which satellites and communications on Earth depend. The water in the pot boils not only from the stove below; sometimes the heat comes from within the water itself.
The stratospheric observatory Sunrise III ascended to an altitude of 37 km, where the sky is 100 times clearer for ultraviolet light than at sea level. This allowed achieving a resolution of 59 km on the Sun, comparable to the size of a small city.
In Arthur C. Clarke's novel 'A Solar Storm', solar flares threaten Earth, but the mechanisms of their occurrence are described schematically. This work shows how much more complex the real physics is: even in the lower layers of a star's atmosphere, processes occur that we are only beginning to understand.