A Conjecture Concerning the Rieger and Quasi-Biennial Solar   Periodicities
  It has been established that the Rieger periodicity of approximately 153 days is part of a complex of periodicities, all multiples of a basic period of approximately 25.5 days. However, it has not been clear why the sixth subharmonic of this periodicity should be preferentially manifested. We here note that if the Sun contains two rotating elements, with different periods and different axes, a special situation will arise if the two periods have a lowest common multiple, for in this case the relative configuration of the two rotators would repeat exactly at that (LCM) period. This chain of thought leads us to suspect that the Sun contains a second rotating element with rotational period in the range 21 - 22.5 days. 
P. A. Sturrock (Center for Space Science and Astrophysics, Stanford   University)