The visible surface of the Sun (the photosphere) is around 6,000°C. But a few thousand kilometers above it – a small distance when we consider the size of the Sun – the solar atmosphere (the corona) is hundreds of times hotter, reaching a million degrees Celsius or higher.

The reason why has been puzzling scientists for decades, having finally been confirmed experimentally for the first time: Alfvén waves, first proposed in 1942, for which the Swedish scientist Hannes Alfvén won the Noble prize in 1970.