The visible surface of the Sun, or 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, also called the corona, is hundreds of times hotter, reaching a million degrees celsius or higher. This spike in temperature, despite the increased distance from the Sun’s main energy source, has been observed in most stars, and represents a fundamental puzzle that astrophysicists have mulled over for decades. In 1942, the Swedish scientist Hannes Alfvén proposed an explanation. He theorized that magnetized waves of plasma...