The core of the Sun is an extremely high-temperature furnace with temperatures reaching up to 15 million Kelvin. The enormous energy generated by nuclear reactions is transmitted from the core outward, with the temperature gradually decreasing until it reaches only 5,800 Kelvin at the surface of the Sun. However, scientists have discovered a strange phenomenon on the outer side of the Sun's surface: as the distance from the surface of the Sun increases, the temperature actually rises. The corona is a vast plasma structure in the outer layer of the Sun, with a density just 1/10^12 of that of the photosphere. However, measurements show that its temperature can reach as high as 1 million to 3 million Kelvin! Between the chromosphere and the corona, there is a "transition zone" with a thickness ranging from ten kilometers to hundreds of kilometers, where the temperature suddenly rises. It is difficult to explain this phenomenon from a thermodynamic perspective, as if an electric light bulb could heat the surrounding air to a temperature even higher than that of the bulb itself. Therefore, it is necessary to seek non-thermodynamic heating mechanisms.
For decades, scientists have proposed many theories of coronal heating, of which the two most widely accepted are acoustic heating and magnetic reconnection. The acoustic heating theory was proposed in 1949. The Sun is a high-temperature plasma filled with magnetic fields, so magnetic acoustic waves and Alfvén waves, which describe plasma fluctuations in magnetic fields, are considered the most important wave modes of the Sun. On the solar photosphere, disturbances such as granulation or supergranulation can generate these two waves, which can carry energy and travel through the outer atmosphere of the Sun, eventually converting the energy into thermal energy in the form of shock waves.
Another possible heating mechanism is the magnetic reconnection theory. The Sun's magnetic field induces electric currents in the corona, which can maintain magnetic field lines forming independent magnetic loops. However, under certain circumstances, these electric currents can suddenly collapse, allowing the magnetic field to be reconnected to other magnetic poles, a process called "magnetic reconnection". During this process, a large amount of energy is released in the form of thermal energy and wave energy. There are millions of magnetized small areas on the surface of the Sun, which are constantly being stirred by granulation. The magnetic field in the corona must constantly engage in "reconnection" operations with this "magnetic carpet", and the overall generated thermal energy is not negligible.
However, so far, we only know that this heating mechanism is definitely related to magnetic fields, but no theory can quantitatively calculate the high temperature of the corona very ideally. The final solution may require the combination of two or more theories. This mystery still awaits more observational data and deeper thinking to solve it.