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Every eleven years (more or less precisely) sunspots are seen to rise in numbers. The magnetic field of the sun changes polarity over eleven years and this is the reason the number of spots can vary. But I'm not sure I understand why a flip occurs. On Earth, this can be seen at times too, but only at a longer time scale. Is this difference due to the sun being gaseous rather than liquid? Why is the periodicity so steady? Why does the flip occur in the first place? It surely has to do with the sun's rotation, but I don't see clearly how.

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Why is the periodicity so steady?

It's not so steady. Some solar cycles have lasted for nine years, others for fourteen years. Of the 26 solar cycles between 1700 and the present, 21 had a span between 10 and 12 years.

Why does the flip occur in the first place?

Nobody knows. There are various conjectures, but they're just conjectures.

The solar cycle, which is observed in terms of sunspots, is definitely associated with the polarity of the Sun's polar magnetic field; the flip from one polarity to the other occurs near the peak of a solar cycle. The polar magnetic field drives the solar cycle, but why the polar magnetic field alternates from one polarity to another, and then back again, nobody really knows.

The Earth's magnetic field also flips and flops. The 1950s discovery that the Earth has experienced alternations in its magnetic fields over time was one of the driving discoveries that led to the theory of plate tectonics. Dynamos can change polarity. Why that happens? Nobody knows. Nobody knows if the flips and flops in the Earth's magnetic field and the flips and flops in the Sun's magnetic field have the same underlying cause.

One last thing: It's perhaps better to look at the cycle as lasting 22 years. That is how long it takes for the Sun's magnetic field to flip its polarity and then flip back again. There are differences between the even and odd 11 year solar cycles.

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    $\begingroup$ I think you could put in more here than "nobody knows". The cycle is associated with the large-scale, dynamo-generated magnetic field on the Sun. Periodicities in dynamos are nothing strange, but you are right that there is no definitive explanation as to why it should specifically be 22 years. $\endgroup$ – ProfRob Jun 20 at 15:43
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    $\begingroup$ I know that this answer is a bit unsatisfactory to some. Science, being science rather than religion, does not have an answer to every question. Isaac Asimov put it best: "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' but 'That's funny...'" $\endgroup$ – David Hammen Jun 20 at 16:00
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    $\begingroup$ I tried to write my own answer, and I'm having a hard time making anything organized out of it, but even though we don't really know how the solar dynamo works in detail we can say that there's a pendulum-like energy exchange going on — somehow the solar dipole puts the brakes on the very process that generates it, so we get something like harmonic motion, and the periodicity comes from the ratio between the size of that force and the (huge) momentum of all of the swirling solar plasma, so that it takes a decade for it to do a turnaround. $\endgroup$ – hobbs Jun 21 at 5:27
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    $\begingroup$ In other words, even if we don't fully understand it, we are at least aware of ways that it's like things we understand :) $\endgroup$ – hobbs Jun 21 at 5:32
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    $\begingroup$ There is also the matter of the "Maunder Minimum" between 1645 and 1715, 70 years where sunspots were as rare as hen's teeth. It was accompanied by a Little Ice Age (I remember it well). $\endgroup$ – Paul_Pedant Jun 21 at 15:24

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