Evidence for an oblique magnetic solar rotator

Abstract

The observation by Claverie et al.¹. of fine structure in the peaks in the power spectrum of low-degree 5-min solar oscillations has been interpreted as being a result of rotational splitting. Claverie et al.¹ claim that their measurements imply that an appropriately weighted average Ω̄ of the interior angular velocity Ω(r) of the Sun is about twice the value of Ω at the surface. At first sight their claim looks doubtful, because all 2l + 1 components of the set of modes of degree l appear in the spectrum, whereas only l + 1 of them should be detectable. However, Isaak² has recently speculated that the additional components are produced by an intense rotating magnetic core, such as had been postulated by Dicke^3,4 to account for the 12.2-day periodic component in the Princeton oblateness data^5–9. Isaak² pointed out that the mean period 2π/Ω≃15 days of the solar interior that was inferred from the Birmingham data¹ is consistent with a central core rotating with the 12.2-day period and an outer envelope rotating with the photosphere. Moreover, his rough estimate of a few megagauss for the r.m.s. magnetic field that is required to support his conjecture is within about a factor 10 of that required by Dicke³. Thence he concluded that the Birmingham data¹ provide the first clear empirical evidence for an intense internal solar magnetic field. Here I examine this evidence in more detail, and show that the conclusion is premature. If the magnetic core does exist, the 5-min oscillations provide no clear evidence that it is rotating rapidly. Furthermore, unless one accepts a contrived magnetic-field configuration, an explanation for the 2l + 1 components of the multiplets is still lacking.