The solar dynamo is the physical process that generates the Sun's magnetic field. The Sun is permeated by an overall dipole magnetic field, as are many other celestial bodies such as the Earth. The dipole field is produced by a circular electric current flowing deep within the star, following Ampère's law. The current is produced by shear (stretching of material) between different parts of the Sun that rotate at different rates, and the fact that the Sun itself is a very good electrical conductor (and therefore governed by the laws of magnetohydrodynamics).
Any electrically conducting fluid can form a dynamo simply by shear within the fluid itself, because of a consequence of Lenz's law of induction: moving the fluid through a magnetic field will induce electrical currents in the fluid that distort the external magnetic field. The direction of the distortion is such that a previously existing magnetic field tends to be dragged along with the fluid, as with swirls of cream in a cup of coffee. If the flow has a strong shear component then the individual magnetic field lines are stretched by the flow, amplifying the existing magnetic field. Such systems are called MHD dynamos.
Depending on the structure of the flow, the dynamo may be self-exciting and stable, self-exciting and chaotic, or decaying. The Sun's dynamo is self-exciting: the direction of the field reverses itself about every 11 years, causing the sunspot cycle as ropes of magnetic flux rise to the surface of the Sun and manifest themselves as sunspots on the surface. The solar dynamo is thought to be located at the tachocline, a region inside the Sun that exhibits a very large shear profile.
The detailed mechanism of the solar dynamo is not known and is the subject of current research.
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