Sun's Atmosphere's Scorching Heat Explained: Why the Corona is Hotter Than Its Surface

You might be surprised to learn that the sun's atmosphere is actually hotter than its surface. In fact, its atmosphere can be as high as 1 million degrees Celsius while its surface is only about 6,000 degrees. Now, scientists may have found out why this is.

The sun's temperature, which is about 15 million degrees Celsius in its core, steadily decreases with distance from the core, falling to about 6,000 degrees Celsius at its surface. In the atmosphere, though, it heats up again, rising to about 10,000 degrees Celsius in the chromosphere and over a million degrees in the corona. In order to find out why this is, the researchers turned to models.

The scientists used powerful numerical models run on computers in order to create a simulation of the different layers. The researchers found that the thin layer under the sun's surface actually behaves like a shallow pan containing boiling plasma, heated from below and forming "bubbles" associated with granules. This boiling plasma soup generates a dynamo process that amplifies and maintains the magnetic field. As the field emerges from the surface, it takes on a salt-and-pepper appearance, forming concentrations dubbed "mesospots" that are larger, fewer in number and more persistent, all of which is consistent with observations.

The researchers also discovered that in the chromosphere, the heated atmosphere is due to multiple micro-eruptions in the roots that carry intense electric current, in pace with the bubbles from the boiling plasma. In addition, larger but less numerous eruptive events take place near the mesospots.

The eruptive process generates "magnetic" waves along tree trunks rather like sound traveling along a plucked string. These waves then transport energy to the upper corona, which is heated by their progressive dissipation. As the ejected matter falls back toward the surface, it forms tornadoes.

The findings reveal a bit more about how the sun's atmosphere manages to be hotter than its surface. This, in turn, may tell scientists a bit more about other stars and also about the processes that occur on the sun.

The findings are published in the journal Nature.

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