Is the Sun a Magnet?

It has been argued (Gough and McIntyre in Nature 394 , 755, 1998 ) that the only way for the radiative interior of the Sun to be rotating uniformly in the face of the differentially rotating convection zone is for it to be pervaded by a large-scale magnetic field, a field which is responsible also for the thinness of the tachocline. It is most likely that this field is the predominantly dipolar residual component of a tangled primordial field that was present in the interstellar medium from which the Sun condensed (Braithwaite and Spruit in Nature 431 , 819, 2004 ), and that advection by the meridional flow in the tachocline has caused the dipole axis to be inclined from the axis of rotation by about 60^∘ (Gough in Geophys. Astrophys. Fluid Dyn. , 106 , 429, 2012 ). It is suggested here that, notwithstanding its turbulent passage through the convection zone, a vestige of that field is transmitted by the solar wind to Earth, where it modulates the geomagnetic field in a periodic way. The field variation reflects the inner rotation of the Sun, and, unlike turbulent-dynamo-generated fields, must maintain phase. I report here a new look at an earlier analysis of the geomagnetic field by Svalgaard and Wilcox ( Solar Phys. 41 , 461, 1975 ), which reveals evidence for appropriate phase coherence, thereby adding support to the tachocline theory.