The Sun exhibits a periodic cycle for solar activity as manifested by solar activity (e.g. sunspots, solar flares). This activity is related to magnetic fields which emerge from the interior of the Sun. The magnetic fields are created by the solar dynamo. The Earth has a dynamo as well but the Earth's magnetic field is (mostly) constant in time. What is some current theories (assuming there isn't one already accepted) that explains for why the solar dynamo generates magnetic fields periodically?

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    $\begingroup$ "Best" is a bit argumentative, could you define your criteria? Maybe "currently accepted" or something similar that would point to the need of stating by whom that is believed to be so? $\endgroup$ – TildalWave Oct 8 '13 at 18:39
  • $\begingroup$ Isn't the solar cycle 11 years? $\endgroup$ – user51 Oct 9 '13 at 8:33
  • $\begingroup$ @Evert, it is technically 22 years as the magnetic field only returns to its original configuration after two 11-year activity cycles. $\endgroup$ – ehsteve Oct 11 '13 at 14:04
  • $\begingroup$ @TildalWave, you are right. I updated the question. I also changed the time period to 11 year as it more "recognizable" though the fact that it takes 22 years for the magnetic field to go back to its original configuration may be important in the answer. $\endgroup$ – ehsteve Oct 11 '13 at 14:08
  • $\begingroup$ I think the earth's field is loosely periodic, it's just that the period is much longer: tens of thousands of years $\endgroup$ – Steve Linton Jun 15 '19 at 10:10

It is linked to the solar dynamo and the dynamical behavior of the so-called $\alpha$-effect. One way to understand easily the solar dynamo and the the $\alpha$-effect with the hand is the following: you can generate a magnetic field by differential rotation (the $\Omega$-effect, $\Omega$ being the classical notation for the angular velocity). But this sole effect, twisting the magnetic field, would fold it so much that it will reconnect and dissipate. If you don't want to lose the magnetic field, you need a process to "regenerate" it, and a good way to do it is to create a poloidal component of the magnetic field. This poloidal component can be generated by convection in the solar interior; the magnetic field is, in the same time, twisted and pulled and that explains the solar dynamo.

Now, you can show that dynamo waves will propagate from the equator to the poles, and when the poles are reached, a reversal is observed. This propagation of dynamo waves is well observed by the sunspots patterns during the solar cycle.


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