The Solar Dynamo

Magnetic fields are a little like rubber bands. They consist of continuous loops of lines of force that have both tension and pressure. Like rubber bands, magnetic fields can be strengthened by stretching them, twisting them, and folding them back on themselves. This stretching, twisting, and folding is done by the fluid flows within the Sun.

The Omega Effect

Magnetic fields within the Sun are stretched out and wound around the Sun by differential rotation - the change in rotation rate as a function of latitude and radius within the Sun. This is called the omega-effect after the Greek letter used to represent rotation. The Sun's differential rotation with latitude can take a north-south oriented magnetic field line and wrap it once around the Sun in about 8 months.

The Alpha Effect

Twisting of the magnetic field lines is caused by the effects of the Sun's rotation. This is called the alpha-effect after the Greek letter that looks like a twisted loop. Early models of the Sun's dynamo assumed that the twisting is produced by the effects of the Sun's rotation on very large convective flows that carry heat to the Sun's surface. One problem with this assumption is that the expected twisting is far too much and it produces magnetic cycles that are only a couple years in length. More recent dynamo models assume that the twisting is due to the effect of the Sun's rotation on the rising "tubes" of magnetic field from deep within the Sun. The twist produced by the alpha effect makes sunspot groups that obey Joy's law and also makes the magnetic field reverse from one sunspot cycle to the next (Hale's law).

The Interface Dynamo

Early models of the Sun's magnetic dynamo worked on the idea that the dynamo activity occurs throughout the entire convection zone. It was soon realized, however, that magnetic fields within the convection zone would rapidly rise to the surface and wouldn't have enough time to experience either the alpha or the omega effect. Since a magnetic field exerts a pressure on its surroundings, regions with a magnetic field should push aside the surrounding gas and make a bubble that would continue to rise all the way to the surface. This buoyancy is not produced in the stable layer below the convection zone. Within the radiative zone the magnetic bubble would rise only a short distance before it would find itself just as dense as its surroundings. This led to the idea that the Sun's magnetic field is being produced in the interface layer between the radiative zone and the convection zone. This interface layer is also a place where we find rapid changes in rotation rate as we look inward or outward across it.

The Meridional Flow

The Sun's meridional flow - the flow of material along meridian lines from the equator toward the poles at the surface and from the poles to the equator deep inside - must also play an important role in the Sun's magnetic dynamo. At the surface this flow is a slow 20 m/s (40 mph) but the return flow toward the equator deep inside the Sun where the density is much higher must be much slower still - 1 to 2 m/s (2 to 4 mph). This slow return flow would carry material from the polar regions to the equator in about 20 years. This rate of flow is very similar to that of the sunspot activity bands seen in the butterfly diagram.

Web Links

Solar Dynamo Position - Stanford University dynamo discoveries

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