Planetary line-up excites the sun
Wednesday, 2 July 2008 Marilyn Head
ABC Australian astronomers may have found a solution to how far-away Jupiter and Saturn drive the sun's solar cycle.
In a paper published in the Publications of the Astronomical Society of Australia, astronomer Dr Ian Wilson and colleagues from the University of Southern Queensland, suggest Jupiter and Saturn affect the sun's movement and its rotation, and hence its sunspot activity.
Every 11 years the sun undergoes a period of intense solar activity, marked by flares, coronal mass ejections and sunspots. This period is known as the solar maximum and occurs twice each solar, or Hale, cycle.
"The sun can be thought of as a large bar magnet," says Wilson.
"The equatorial region of the sun spins more rapidly than the poles, and this differential rotation winds up the magnetic field lines like a rubber band."
Wilson says sunspots typically appear wherever these magnetic field lines bubble to the surface.
"Once the peak in sunspot activity is reached, a huge amount of energy is released, the magnetic poles are reversed and a new cycle begins," he says.
Unknown connection
For many years scientists have recognised an apparent connection between the strength of sunspot activity and the movement of the sun in relation to solar system's barycentre, which is driven by the combined gravitational forces of Jupiter and Saturn.
But no one has been able to explain the connection.
"There are really only two possible interactions, and neither of them is feasible," Wilson says.
"Tidal forces are too tiny. They can only produce a movement of about a millimetre on the surface of the sun.
"The alternative, that the sun's motion about the centre of mass should be able to generate internal motion within the sun, violates Einstein's equivalence principle."
Tug-o-war
The authors believe the tiny gravitational tugs of Jupiter and Saturn speed up or slow down the sun's orbital motion about the centre-of-mass, when they are aligned or separated by an angular distance of 90 degrees.
They say that when the sun's orbital motion changes, so too does its equatorial rotation rate, which provides strong circumstantial evidence that there is a spin-orbit coupling mechanism operating between Jupiter and Saturn and the sun.
The authors propose that this spin-orbit coupling takes the form of a 9:8 resonance, with the 179 year alignment cycle of the Jovian planets being equal to nine alignments of Jupiter and Saturn and eight 22-year Hale cycles.
The extent to which Jupiter and Saturn affect the sun's motion may impact on the strength of sunspot activity throughout its solar cycle.
But Wilson is cautious.
"It is one thing to show an association and quite another to show cause and effect. We have to be very careful, but we will know in a few years," he says.