On the Sun's General Magnetic Field

Abstract

The time of electromagnetic decay oLthe Sun's general magnetic field is shown to be 10 10 years, suggesting that the field may be a relic from a different primeval state. An alternative hypothesis, ascribing the field to thermal effects-due to convection in the rotating Sun, is shown to give a field of the correct sign, but less than io~ 7 times the observed field; and a modification of the hypothesis, supposing that the field due to thermal currents is magnified by dynamo action, is shown to demand over 10 18 years for the field to grow to its present size. Other possible explanations of the Sun's field are suggested. In an Appendix, the effect of the Sun's rotation on turbulence in its interior is briefly considered. I. Our knowledge of solar magnetic fields is almost entirely derived from the work of Hale and his collaborators at Mount Wilson. By observations of polarization at the wings of spectral lines, these workers identified a general magnetic field of the Sun, similar in many respects to the Earth's magnetic field.* The field-strength is of order ~ (say) 25 gauss ; the axis of the field is inclined to the axis of rotation of the Sun at about 6°, and rotates round it in about 31J days. Different spectral lines indicate different intensities for the field; weak lines suggest larger fields than strong lines. Since the latter probably originate at greater heights in the Sun's atmosphere than the former, these results were interpreted as implying that the field falls off rapidly with increasing height. The average observed separation of the Zeeman components is of the same order as the probable error of a single measurement. Because of its smallness, its reality is sometimes doubted. The present author sees no reason to share these doubts. To ensure that personal bias should not colour the results, the original observer measuring the Zeeman displacements was kept in ignorance of the hemisphere and latitude of the observations. Thus if he were prone to imagine a non-existent phenomenon, he would give it sometimes one sign, sometimes the other, and in the aggregate his estimates would cancel out. Since, in spite of this, regular results were obtained, the phenomenon being observed was clearly not imaginary. In fact, though all observers have not been able to identify real Zeeman displacements, all those who have been able to identify them have found effects with the same sign. Moreover, no convincing explanation of the observations has been given, other than the existence of a magnetic field. The phenomenon requiring explanation is a difference in polarization between the two wings of a spectral line. Effects producing a pure displacement or pure broadening of the line cannot explain the polarization; no one has yet suggested a satisfactory alternative to the explanation in terms of a magnetic field. We may, however, doubt whether the field does fall off rapidly with increasing height, as originally suggested. Though all observers have agreed regarding the sign * Hale, Ap.