Internal Dynamo

Where do the Sun's magnetic fields come from and how are they made? We know that the motion of electric charges can produce magnetic fields, and that changing magnetic fields produce electric currents. And the Sun's interior is totally electrically charged, consisting of electrons and protons. In deep layers, these charged particles are so hot that they conduct electric current as well as copper does at room temperature, tte hot circulating gases generate electrical currents that create magnetic fields; these fields in turn sustain the generation of electricity, just as in a power-plant dynamo.

FIG. 5.16 Solar cycle magnetic variations These magnetograms portray the polarity and distribution of the magnetism in the solar photosphere. They were made with the Vacuum Tower Telescope of the National Solar Observatory at Kitt Peak from 8 January 1992, at a maximum in the sunspot cycle (lower left:) to 25 July 1999, well into the next maximum (lower right). Each magnetogram shows opposite polarities as darker and brighter than average tint. When the Sun is most active, the number of sunspots is at a maximum, with large bipolar sunspots that are oriented in the east-west (left-right) direction within two parallel bands. At times of low activity (top middle), there are no large sunspots and tiny magnetic fields of different magnetic polarity can be observed all over the photosphere. The haze around the images is the inner solar corona. (Courtesy of Karel J. Schrijver, NSO, NOAO and NSF.)

tte Earth's magnetic field is supposed to be generated by such a dynamo, operating on a much smaller scale within its molten core.

tte magnetic fields of the Sun are entrained and "frozen" into the conducting gas whose particles carry the magnetism with them. As they move along with the gas, the embedded magnetic fields are deformed, folded, stretched, twisted and amplified, tte mechanical energy of the motion of the charged gas particles is thereby converted into the energy of magnetic fields, ttis dynamo mechanism does not explain how the magnetic fields originated, but rather how they are amplified and maintained, tte process of field amplification is nevertheless cumulative, so a dynamo can generate an intense magnetic field from an initially weak one.

tte solar dynamo is now thought to operate in a thin layer, called the tacholine, located at the interface between the deep interior, which rotates with one speed, and the overlying layer that spins faster in the equatorial middle. Sound waves also speed up more than expected in this shear layer, indicating that turbulence and mixing associated with a dynamo are most likely present. Below the tacholine the Sun rotates like a solid object, with too little variation in spin to drive a solar dynamo. And above this boundary layer, the rotation rates at different latitudes diverge over broad areas that are not focused enough to play much of a role in the dynamo.

Global rotation, differential rotation, and convective motion are supposed to interact strongly at the tacholine, producing dynamo action. In this deep-seated region, the dipolar magnetic field, which runs inside the Sun between its north and south pole, is apparently wound up and stretched into an azimuthal, or toroidal, coil that rises radially through the overlying convective zone to produce sunspots and active regions with their 11-year cyclicbehavior.

In technical terms, a global dipole magnetic field is supposed to be sheared and stretched into a toroidal field by differential rotation inside the Sun, and dipolar sun-spot pairs are supposed to be produced by a lifting and twisting process related to the rising toroidal magnetic fields, tte global dipole and toroidal magnetic field components are alternately destroyed and recreated in a cycle that lasts a total of 22 years. Deep meridional flow, or north-south circulation, is supposed to help regenerate the dipole field, also accounting for its reversal, and explain why sunspots do not form at high latitudes.

Nevertheless, despite all the mathematical complexity, or perhaps because of it, there is no solar dynamo model that explains the different aspects of the Sun's magnetic activity cycle in detail. And no one has yet observed the magnetic fields deep inside the Sun.

Solar astronomers therefore often extrapolate from a conceptually simple model devised in 1961 by the American astronomer Horace W. Babcock (1912-2003). His theory begins at sunspot minimum with a global, dipolar magnetic field that runs inside the Sun from south to north, or from pole to pole. Uneven, or differential, rotation shears the electrically conducting gases of the interior, so the entrained magnetic fields get stretched out and squeezed together, tte magnetism is coiled, bunched and amplified as it is wrapped around the inside of the solar globe, eventually becoming strong enough to rise to the surface and break through it in active-region belts with their bipolar sunspot pairs (Fig. 5.17), like a stitch of yarn pulled from a woolen sweater, tte surrounding gas buoys up the concentrated magnetism, just as a piece of wood is subject to buoyant forces when it is immersed in water. As Babcock expressed it:

Shallow submerged lines of force of an initial, axisymmetric dipolar field are drawn out in longitude by the differential rotation____Twisting of the irregular flux strands by the faster shallow layers in low latitudes forms "ropes" with local concentrations that are brought to the surface by magnetic buoyancy to produce bipolar magnetic regions with associated sunspots and related activity.24

FIG. 5.17 Winding up the field A model for generating the changing location, orientation and polarity of the sunspot magnetic fields. At the beginning of the 11-year cycle of magnetic activity, when the number of sunspots is at a minimum, the magnetic field is the dipolar or poloidal field seen at the poles of the Sun (left). The internal magnetic fields then runjust below the photosphere from the south to north poles. As time proceeds, the highly conductive, rotating material inside the Sun carries the magnetic field along and winds it up. Because the equatorial regions rotate at a faster rate than the polar ones, the internal magnetic fields become stretched out and wrapped around the Sun's center, becoming deformed into a partly toroidal field (middle and right). The fields are then concentrated and twisted together like ropes. With increasing strength, the submerged magnetism becomes buoyant, rises and penetrates the visible solar disk, or photosphere, creating magnetic loops and bipolar sunspots that are formed in two belts, one each in the northern and southern hemisphere (right). [Adapted from Horace W. Babcock, Astrophysi-cal Journal 133, 572-587 (1961)]

FIG. 5.17 Winding up the field A model for generating the changing location, orientation and polarity of the sunspot magnetic fields. At the beginning of the 11-year cycle of magnetic activity, when the number of sunspots is at a minimum, the magnetic field is the dipolar or poloidal field seen at the poles of the Sun (left). The internal magnetic fields then runjust below the photosphere from the south to north poles. As time proceeds, the highly conductive, rotating material inside the Sun carries the magnetic field along and winds it up. Because the equatorial regions rotate at a faster rate than the polar ones, the internal magnetic fields become stretched out and wrapped around the Sun's center, becoming deformed into a partly toroidal field (middle and right). The fields are then concentrated and twisted together like ropes. With increasing strength, the submerged magnetism becomes buoyant, rises and penetrates the visible solar disk, or photosphere, creating magnetic loops and bipolar sunspots that are formed in two belts, one each in the northern and southern hemisphere (right). [Adapted from Horace W. Babcock, Astrophysi-cal Journal 133, 572-587 (1961)]

tte initial dipole, or poloidal, magnetic field is twisted into a submerged toroidal, or ring-shaped, field running parallel to the equator, or east to west. Apparently, the dynamo generates two toroidal magnetic fields, one in the northern hemisphere and one in the southern hemisphere, but oppositely directed, which bubble up at mid- to low-latitudes to spawn the two belts of active regions, symmetrically placed on each side of the equator, ttus, according to Babcock's scenario, we may view the solar cycle as an engine in which differential rotation drives an oscillation between poloidal and toroidal geometries.

As the 11-year cycle progresses, the internal magnetic field is wound tighter and tighter by the shearing action of differential rotation, and the two belts of new active regions slowly migrate toward the solar equator. Because the active regions emerge, on the average, with their leading ends slightly twisted toward the equator, the leading sun-spots in the two hemispheres tend to merge and cancel out, or neutralize, each other at the equator, ttis leaves a surplus of following-polarity magnetism in each hemisphere, north and south, which is eventually carried poleward at sunspot minimum.

Diffusion and poleward flows apparently sweep remnants of former active regions into streams, each dominated by a single magnetic polarity, that slowly wind their way from the low- and mid-latitude active-region belts to the Sun's poles. By sunspot minimum, when the active regions have largely disintegrated, submerged or annihilated each other, the continued poleward transport of their debris may form a global dipole, like the phoenix rising from its ashes. Because the Sun's polar field is created from the following polarity of decaying active regions, they reverse the overall dipole polarity at sunspot minimum, so the north and south pole switch magnetic direction or polarity, tte Earth's dipole magnetic field also reverses itself, but at much longer intervals of about a million years. When the Sun's magnetic flip is taken into account, we see that it takes two activity cycles, or about 22 years, for the overall magnetic polarity to get back where it started.

By the time that sunspot minimum occurs, most of the magnetic flux that emerged in former active regions has been obliterated. It's as if the internal magnetism has been wound so tight that it snaps, like an over-wound watch spring, and no more sunspots can form. Relatively small amounts of magnetism remain, as leftover flux that originated in active regions that are no longer there; it is this flux that has been gradually dispersed over a much wider range of latitudes to form the Sun's global dipole. tte internal magnetism has then readjusted to a poloidal form, and the magnetic cycle begins again.

ttus, the dynamo theory seems to explain all of the repetitive aspects of the Sun's magnetism, including the periodic variation in the number of sunspots, their cyclic migration toward the equator (the butterfly diagram), the roughly east-west orientation, location and polarity (Hale's law of polarity) of bipolar sunspot pairs, and the periodic reversal of the overall global dipole. However, many details of the theory are uncertain or incomplete, and so far no dynamo model has succeeded in accounting for all of the magnetic observations.

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