Discovery: Why the Sun’s Atmosphere is Hotter than its Surface

The surface of the Sun is around 5500 degrees Celsius.  It’s hot, and it’s a completely different state of matter than the solid, liquid, and gas states that we are used to. However, this is a pretty balmy temperature compared to the solar atmosphere, which is heated to over a million degrees.  But how does it get so hot compared to the surface? We know the core of the Sun, where fusion happens, is where temperatures can reach 14 Million degrees, but how is that energy radiated outward? And more importantly, how does it bypass the solar surface and make for a blindingly hot atmosphere?

The Sun’s surface and magnetic field. Credit: © Tahar Amari / Centre de Physique Théorique and S. Habbal / M. Druckmüller

We can definitely measure this strange temperature profile of the Sun, but even though scientists agree it’s accurate, there has been little more than theory to explain how it works….until now.  Astronomers have been running numerical simulations on powerful supercomputers at the Centre de Physique Théorique (CNRS/École Polytechnique) and GENCI at IDRIS-CNRS.  They performed a simulation for several hours, based on a model of the Sun that is made of several layers, one inside the Sun and the others representing its atmosphere.

To their surprise, they found a thin layer beneath the surface of the Sun that behaved like a shallow pan of boiling water, creating bubbles of plasma that generate and maintain the Sun’s magnetic field as it moves beyond the surface. As the magnetic field moves past the surface, tiny clumps form that look like pepper on the surface.  Dubbed ‘mesospots,’ they are surrounded by a forest like structure of magnetic roots that are driven up higher into the atmosphere. Eruptions at the surface and below travel along these roots like sound waves on a string, sending energy past the surface to the corona, which is heated up as the waves dissipate.

Calculations also show that as the ejected matter falls towards the surface it forms thin plasma jets and tornadoes, as well as other phenomena that haven’t yet been explained and were once thought to be separate.  It seems they are all related to the heating mechanism.  Pretty cool science.

A great example of how science has driven us to find answers, and our new found understanding of the Sun can be used to make hypotheses about other stars.

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