The power of planetary alignments is normally reserved for unscientific horoscopes, but it turns out they might have some importance after all. The sun has an 11-year cycle of activity, and it may be shepherded by alignments of Venus, Earth, and Jupiter.
The sun鈥檚 cycle includes variations in the number of sunspots, the total energy it emits, and the structure of its magnetic field. Since the cycle was first discovered in 1843, researchers have struggled to understand what determines its length and periodic nature.
Frank Stefani at the Helmholtz-Zentrum Dresden-Rossendorf in Germany and his colleagues have calculated that it could be an alignment between Venus, Earth, and Jupiter that occurs only once every 11 years.
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They used previously collected data on the last 1000 years of the solar cycle, including historical records of sunspots and aurorae, which occur more often during periods of high solar activity. They found that the solar cycle seems to have been remarkably consistent for the last millennium.
And the average length of the cycle lines up remarkably well with the time between alignments of Venus, Earth, and Jupiter, which are the three planets with the strongest gravitational effect on the sun. Just as the moon contributes to tides in Earth鈥檚 oceans, the planets could slosh the plasma within the sun.
鈥淚f the planets are aligned, you have a sort of spring tide pulling on the sun,鈥� says Stefani. 鈥淎nd then comes the point which many people consider strange and ridiculous.鈥� The gravity of those three planets displaces plasma inside of the sun by about one millimetre.
鈥淭he circulation in the sun is of the order of half a kilometre per second, cells move up and down within the sun that are the size of Texas,鈥� says Leif Svalgaard at Stanford University. 鈥淚magine that being influenced by a millimetre-sized tidal effect.鈥�
The relatively minuscule strength of the tidal effect has always thwarted astronomers trying to tie planetary alignments to the solar cycle, he says. 鈥淚t is probably a coincidence.鈥�
But Stefani and his colleagues think otherwise. They calculated a way for even a tiny displacement of plasma to dramatically affect the circulation of material inside the sun and thus its magnetic field, making the planets the 鈥渃lock鈥� that keeps the solar cycle on time.
They do this through a phenomenon in the sun鈥檚 magnetic field called the alpha effect. The alpha effect describes how the magnetic field gets twisted into loops as the sun rotates. The researchers calculated that even the small amount of energy added into the plasma by the planets鈥� alignment can cause an instability in the alpha effect.
鈥淭here is a little bit of sloshing or changing of the direction of the rotation of the plasma,鈥� says Stefani. It acts a bit like a child on a swing 鈥� they might be swinging at their own pace, but if they鈥檙e pushed every once in a while their swinging will eventually synchronise with the frequency at which they鈥檙e pushed. A push just once every 11 years could set the sun鈥檚 magnetic field swinging into the cycle that we observe now, Stefani and his colleagues calculated.
They plan to test this mechanism in the laboratory, using very hot liquid metal as a stand-in for the sun鈥檚 plasma. Svalgaard suggests that we could also test it by observing planets circling other stars to see if those stars have cycles tied to the tides from their planets.
Solar Physics
