Sunspots are one kind of standard stable nuclear fusion reactors.
Then, how can we know that sunspots are stable nuclear fusion reactors?
Every sunspot has a magnetic field. The magnetic field of a circular sunspot is similar to the magnetic field at the end of a long solenoid.
Some astronomers such as Hale and Cowling want to use circular electric currents to simulate the magnetic fields of the sunspots.
The circular electric currents are very large (1,000,000,000,000 Amperes for middle size circular sunspots), if we use a long solenoid to simulate the magnetic field of a circular sunspot.
In nuclear fusion experiments such as Z-pinch experiments, hollow cylindrical plasmas (a long solenoid) with large circular electric currents will pinch and form high temperature high density hollow cylindrical plasmas, and nuclear fusion reactions will happen in these plasmas.
So, strong stable magnetic fields of the sunspots just mean that there are stable nuclear fusion reactions in the sunspots.
But there is a problem, who produce the large circular electric currents of the long solenoids of the sunspots?
Astronomers think that sunspots are magnetic flux tubes, because nobody knows how to produce these large circular electric currents of long solenoids.
In high temperature plasmas, large circular electric currents just mean that large number of electrons, protons and ions move circularly in different direction or in same direction with different speed.
According to Newton’s three laws of motion (notice: Do not use Euler equation and Navier-Stokes equation of fluid mechanics), we need a centripetal force to drive the electrons, protons and ions move circularly.
But there are problems:
There are different kinds of centripetal forces, so what kind of the centripetal force drives these electrons, protons and ions of plasmas move circularly? Can this centripetal force describe all the plasmas motions of the sunspots (include Evershed flow)? Who produce this centripetal force?