The sun is composed almost entirely of plasma, which is highly ionized gas that carries an electrical charge. Scientists looked to the sun's properties to explain this disparity. Estimating the photosphere's heat has always been relatively straightforward: we just need to measure the light that reaches us from the sun, and compare it to spectrum models that predict the temperature of the light's source. This represents temperatures up to 1,000 times hotter than the photosphere beneath it, which is the surface of the sun that we can see from Earth. The coronal heating problem has been established since the late 1930s, when the Swedish spectroscopist Bengt Edlén and the German astrophysicist Walter Grotrian first observed phenomena in the sun's corona that could only be present if its temperature was a few million degrees celsius. Our recent study has finally achieved this, validating Alfvén's 80 year-old theory and taking us a step closer to harnessing this high-energy phenomenon here on Earth. The theory had been tentatively accepted-but we still needed proof, in the form of empirical observation, that these waves existed. He theorized that magnetized waves of plasma could carry huge amounts of energy along the sun's magnetic field from its interior to the corona, bypassing the photosphere before exploding with heat in the sun's upper atmosphere. In 1942, the Swedish scientist Hannes Alfvén proposed an explanation. 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.
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