A. van Maanen

The General Magnetic Field of the Sun. Apparent Variation of Field-Strength with Level in the Solar Atmosphere

The preliminary results of a study of the Zeeman effect due to the general magnetic field of the sun have been given in a previous paper. With the aid of suitable polarizing apparatus, used in conjunction with the 75-foot spectrograph of the 150-foot tower telescope, four lines in the third-order spectrum of an excellent Michelson grating were found to show displacements corresponding in sign and agreeing closely in magnitude with theoretical values calculated for a uniformly magnetized sphere. The extreme minuteness of the displacements, usually less than a thousandth of an angstrom, led us to defer final acceptance of the provisional conclusions until they could be rigorously tested by additional measures. The present paper contains results which amply confirm those previously published, and reveal a relation between the intensity of the sun’s general field and the character of the spectral lines used for its determination. The stronger fines give smaller values of the field-strength, and, since the intensity of a fine depends upon the level in the solar atmosphere at which it originates, it is natural to interpret these differences in field-strength as a consequence of differences in level. In the original paper only four fines were shown to have displacements attributable to the general field of the sun. A number of others, mainly stronger fines known from laboratory investigations to have large Zeeman separations, showed no corresponding solar displacements. Fortunately an explanation of this apparent contradiction was offered in the circumstance that the displaced fines probably originate at a low level in the solar atmosphere, while 1 Contributions from the Mount Wilson Solar Observatory, No. 148. 2 Hale, Mt. Wilson Contr., No. 71; Astrophysical Journal, 38, 27, 1913. See also Hale, Terrestrial Magnetism, 17, 173, 1912. 206