Using spacecraft data collected over three solar cycles, Neugebauer et al. [2000> found a persistent dependence of the solar wind speed and the radial component of the interplanetary magnetic field on solar longitude, defined in a coordinate system with a rotation period of 27.03 days. Here we investigate the solar source of this period using observations of the photospheric magnetic field. We study the lowest-order nonaxisymmetric modes of the solar field extracted from synoptic charts of the Wilcox Solar Observatory. We find there is a robust magnetic structure on the Sun, which rotates with the period found by Neugebauer et al. [2000> in the solar wind. This rotation is more rapid than the solar equatorial and core rotations. We discuss the association of this nonaxisymmetric mode with the magnetic field generated by the solar mean field dynamo. ¿ 2001 American Geophysical Union