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The Sun’s atmosphere is way, WAY hotter than its surface — here’s why

todayJune 5, 2021 4

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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 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.

The theory had been tentatively accepted – but we still needed proof, in the form of empirical observation, that these waves existed. 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.

Burning questions

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.

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. 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.

Over many decades of study, the photosphere’s temperature has been consistently estimated at around 6,000°C. Edlén and Grotrian’s finding that the Sun’s corona is so much hotter than the photosphere – despite being further from the Sun’s core, its ultimate source of energy – has led to much head-scratching in the scientific community.

The extreme heat of the Sun’s corona is one of the most vexing problems in astrophysics.

Scientists looked to the Sun’s properties to explain this disparity. The Sun is composed almost entirely of plasma, which is highly ionized gas that carries an electrical charge. The movement of this plasma in the convection zone – the upper part of the solar interior – produces huge electrical currents and strong magnetic fields.

These fields are then dragged up from the Sun’s interior by convection, and burble onto its visible surface in the form of dark sunspots, which are clusters of magnetic fields that can form a variety of magnetic structures in the solar atmosphere.

This is where Alfvén’s theory comes in. He reasoned that within the Sun’s magnetized plasma any bulk motions of electrically charged particles would disturb the magnetic field, creating waves that can carry huge amounts of energy along vast distances – from the Sun’s surface to its upper atmosphere. The heat travels along what are called solar magnetic flux tubes before bursting into the corona, producing its high temperature.