Is the Sun's corona hotter than other parts?

Eyección de la corona solar observada por el satélite 'Soho' en 2002.
Observed solar corona ejection by satellite ‘Soho’ in 2002.

The Sun’s corona is hotter than the Sun’s surface, but not hotter than its core. The inner part of a star is made up of a nucleus, a radioactive zone, a tachocline that is a very thin layer, and finally a convection zone that reaches the surface. Hydrogen atoms fuse in the nucleus at temperatures of about 15 million degrees. This reaction by means of which hydrogen is converted into helium is what provides energy to the Sun. The energy leaves the nucleus in the form of photons, passes through all the layers and, after many years, the photons manage to reach the surface. Remember that the surface of a star is not solid, it is a ball of gas. And from the surface the atmosphere of the Sun begins, which is very different from that of the Earth.

The shocking thing is that while the surface of the Sun is about 5. 800 ° C, what is called the solar corona is millions of degrees. And this is very counterintuitive, for everyone, for the general public and also for people who are dedicated to science. In a heat source, such as the Sun, as we move away from its surface, the temperature should drop. And that in the Sun does not happen. It is a problem that has been giving headaches to physicists and solar physicists for some seventy years. And the problem is not solved.

There are quite a few theories, all feasible. In the year 2000 some solar physicists made an exhaustive compilation of the models that explain why the corona is so hot and they found some 22 models and all are plausible. I believe that the group that solves it will win the Nobel Prize in Physics.


Interesting to know also how this was discovered. 70 years ago, during an eclipse of the Sun, which is the only time we can actually see the Sollar corona, the Swedish physicist Bengt Edlén saw some colors, some spectral lines, and in the laboratory he verified that these emissions can only occur if the temperature is millions of degrees. It was something very shocking. In fact, there were many who believed it was impossible. But the investigation proved that Edlén was right and today we have no doubt that the temperature of the solar corona is millions of degrees.

Among the theories that explain it there are two schools. Eugene Parker, who is a very important solar physicist, developed a hypothesis in the seventies and eighties with which this heating could be explained by a phenomenon called nanoflares, whose translation into Spanish would be nanoflames. The Sun’s atmosphere is completely invaded by magnetic fields that come from the solar interior. Therefore we have a kind of carpet that would be the surface of the Sun filled with lines of magnetic fields that rise towards the solar atmosphere and that are constantly moving. At times, these magnetic field lines get extremely close and can even cross. Every time these lines intersect, they release energy. They are called nano because they would be about a trillion times less energetic than the normal flares of the Sun, those flares that occur from time to time and that even affect terrestrial communications. Those in Parker’s theory would be much smaller, but they would occur much more frequently and everywhere. What the theory says is that these small explosions would be the ones that cause the heating of the corona.

The other great theory is that of Alfvén waves by a Swedish solar physicist named Hannes Alfvén. This would be based on waves that also come out of the solar surface. These waves would propagate along the field lines and cause the plasma in their path to resonate and move sideways in a way that dissipates energy. And that energy would be the one that would heat the solar corona.

In recent years, works have been published that observationally confirm the existence of Alfven and nanoflar waves. But it remains to be seen whether these phenomena are responsible for the heating of the solar corona. My personal opinion is that it is probably a combination of different phenomena.

Ada Ortiz Carbonell is a doctor in Physics, researcher in the field of solar astrophysics, is part of the Scientific Advisory Committee of the Solar Telescope European and is a data scientist at Expert Analytics (Norway).

Question sent via email by José Flores

Coordination and writing: Victoria Toro

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