Solar Magnetism

the aggregate of the phenomena associated with the existence of a magnetic field on the sun. Various magnetic fields are distinguished: the magnetic fields of sunspots, the magnetic fields of active regions outside the sunspots, and the general magnetic field of the sun.

A magnetic field on the sun was first discovered by the American astronomer G. Hale in 1908 from the splitting of absorption lines in the spectra of sunspots (seeZEEMAN EFFECT). Strong magnetic fields are usually measured through the use of a circular-polarization analyzer, which permits the Zeeman components of a line to be observed separately. For a weak magnetic field, measurements with a solar magnetograph are most accurate.

Solar magnetism may be the cause of the heating of the upper solar atmosphere, the acceleration of particles, and their ejection into interplanetary space. A determining role is played by solar magnetism in many phenomena of solar activity, for example, in solar flares. Weak magnetic fields are associated with regions of elevated brightness where gas is heated. Local intensification of the magnetic field above 1,400 oersteds (Oe), however, leads to cooling of the gas and to the formation of sunspots. Sunspots have the strongest magnetic fields (up to 5,000 Oe). These fields obey certain laws governing the change in their polarity with the solar-activity cycle; the length of the magnetic cycle is two 11-year cycles of solar activity, that is, approximately 22 years. The interaction of magnetic fields in sunspot groups is apparently responsible for solar flares. Outside the active regions, weak magnetic fields called background fields are observed. Together with the active regions, they basically determine the structure of the solar corona and the interplanetary medium.

At heliocentric latitudes above 55°, the general magnetic field, which is similar to a dipole field, is measured. Temporary fluctuations are characteristic of the general magnetic field; in some years its distribution with respect to latitude differs sharply from a dipole distribution. The magnetic field has been shown to change sign at the poles at times of maximum solar activity. The Soviet astronomer A. B. Sevemyi has studied the fine structure and statistical character of the general magnetic field. It is concentrated in individual structural elements that have different dimensions and a bipolar magnetic field with a strength of up to approximately 20 Oe. The strength of the averaged general magnetic field is 1–5 Oe.

The overall magnetic field of the entire sun as a star varies with a period of about 27–28 days and an amplitude of approximately 1 Oe. The field usually has two or four sectors of alternating polarity that coincide with the sector structure of the interplanetary magnetic field. The nature of solar magnetism has not yet been thoroughly investigated.