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. A helical dynamo deep near the center of the Sun's mass produces a strong electric current flowing deep within the star, following Ampère's law. A second chaotic dynamo near the surface of the Sun is responsible for weaker fluctations in the Sun's magnetic field which often result in the most noticeable solar activity.
A dynamo converts motional energy into electric-magnetic energy. An electrically conducting fluid with shear or more complicated motion, such as turbulence, can, at least temporarily, amplify a magnetic field through Lenz's law: fluid motion relative to a magnetic field will induce electrical currents in the fluid that distort the initial magnetic field. If the fluid motion is sufficiently complicated, it can sustain its own magnetic field, with adjective fluid amplification essentially balancing diffusive or ohmic decay of the field. Such systems are called self-sustaining dynamos. The Sun is a self-sustaining dynamo with convective motion and differential rotation within the Sun constantly being converted to electric-magnetic energy.
Dynamo cycle and solar cycle
The most prominent of time variation of the solar magnetic field is related to the quasi-periodic 11-year solar cycle waxing and waning in the number and size of sunspots. Sunspots are visible as dark patches on the Sun's photosphere and correspond to concentrations of magnetic field. At a typical solar minimum, few sunspots are visible, and occasionally none can be seen at all. Those that do appear are at high solar latitudes. As the solar cycle progresses towards its maximum, sunspots tend form closer to the solar equator, a phenomenon known as Spörer's law.
An 11-year sunspot cycle is half of a 22-year Babcock–Leighton solar dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. At solar-cycle maximum, the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength, but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the convection zone forces emergence of toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west and having footprints with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon known as the Hale cycle.
During the solar cycle’s declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number. At solar-cycle minimum, the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare, and the poloidal field is at its maximum strength. With the rise of the next 11-year sunspot cycle, differential rotation converts magnetic energy back from the poloidal to the toroidal field, but with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change, then, in the overall polarity of the Sun's large-scale magnetic field.
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