The Sun has a layer called the chromosphere, sitting just above the visible surface. Bright arcs often flare up there, visible through telescopes, but to understand what ignites them, you need to look at invisible magnetic fields.

Astronomers aimed the Swedish Solar Telescope at a region where the magnetic field is twisted like a rope of many strands. They measured how iron and calcium atoms emit light and used that data to reconstruct plasma temperature and velocity. They found that above strong sunspots, the chromosphere is about 300 degrees hotter than in quiet areas. And along a bright loop, plasma first rises and then falls, like water in a fountain, but scorching hot.

The picture matched the calculations: energy is released where the twisted rope touches neighboring loops. It is exactly there that plasma accelerates and glows brightly. This is not final proof, but it is now clear that such observations can link chromospheric flares to the three-dimensional structure of the magnetic field.

Solar flares affect space weather near Earth: they can knock out satellites and disrupt radio communications. The better we understand how they are born, the more accurately we can predict them.