Three peaks near 27 days in a high-resolution spectrum of the international magnetic character figure,Ci

A high-resolution power spectrum of 72 years of daily values of the international magnetic character figure, Ci, shows two subsidiary peaks on either side of the well-known 27-day period. The larger of the two subsidiary peaks is situated close to the lunar synodic period of 29.53 days. The origin of the subsidiary peaks is examined. A lunar association is rejected as a significant causal agent, along with various errors and bias. It is concluded that the subsidiary peaks represent some real phenomenon and are probably produced by solar activity, although perhaps indirectly.