Principles of Measurement of Magnetic Fields of Celestial Bodies

Abstract

Magnetic lines of force pervade all parts of the universe. From the Earth to the Sun and from stars to galaxies, thousands of millions of heavenly objects possess magnetic fields, which may differ significantly in magnitude and other characteristics. Measurements of the strengths, directions, distributions and variations of so many magnetic fields have become one of the main tasks of contemporary astrophysical observations. After several decades of hard work, astronomers have mastered a series of methods for such measurements. For meteorites, moonrock, solar wind particles and some other samples of astronomical substance which may be captured directly, the magnetic field can be measured in the terrestrial laboratory. For the Moon and some planets, magnetic fields can be observed in situ by means of space probes. However, for the overwhelming majority of celestial objects, magnetic fields are measured only indirectly, e.g. through their influence on the electromagnetic radiation. This influence may be exhibited in the Zeeman effect, Paschen-Back effect, Hanle effect, Faraday rotation, etc. Among them the Zeeman effect is the most important. At present, the bulk of observational data of cosmical magnetic fields has been obtained by means of this effect.

Thanks to Zeeman’s discovery of the effect of magnetism on radiation, it appeared that the detection of such a magnetic field should offer no great difficulty, provided that it were sufficiently intense. G. E. Hale, On the possible existence of a magnetic field in sunspots, Astrophys. J. 28 (1908), 315.