Solar magnetic field

(astrophysics) The magnetic field that pervades the ionized and highly conducting gas composing the sun.

The magnetic field rooted in the Sun and extending out past the planets into the solar system. The field at the Sun's surface is detected remotely by its effect (the Zeeman effect) on atoms whose radiation is observed from Earth. This technique was first applied in 1908 by G. E. Hale to detect the fields in sunspots. In 1952, H. D. Babcock and his son H. W. Babcock used a scanning technique to make the first magnetic maps of the entire visible disk of the Sun. Their daily “magnetograms” soon revealed a variety of magnetic features, including bipolar regions associated with sunspot groups, and unipolar regions whose fields extend far from the Sun and are responsible for recurrent geomagnetic activity at Earth. The field strengths range from a few gauss (a few hundred microtesla) in quiet areas to 3500 gauss (0.35 tesla) in sunspots. In 1962, the Mariner 2 spacecraft, en route to Venus, made the first on-site sampling of the extended solar field in space. The average field strength was only 50 microgauss (5 nanotesla), reflecting the rapid (inverse square) fall-off of field strength with distance from the Sun. See also Zeeman effect.

Solar magnetic fields are related to the 11-year variation in the occurrence of sunspots. As the new sunspot cycle begins, concentrations of bipolar flux break through the Sun's surface in each hemisphere, beginning at about 40° latitude and gradually progressing toward the solar equator over the next few years. The bipolar regions are oriented approximately east-west with their leading polarities (in the sense of solar rotation) all positive in the northern hemisphere and negative in the southern hemisphere during a given sunspot cycle (Hale's law). They are tilted slightly so that their trailing polarities are closer to the Sun's poles and the leading polarities are closer to the equator. This small but systematic effect ultimately leads to the formation of unipolar regions at the Sun's poles, positive in one hemisphere and negative in the other.

It is presently believed that convection and differential rotation are responsible for the eruption of bipolar magnetic regions. See also Magnetism; Sun.