Revisiting 154-day periodicity in the occurrence of hard flares. A   planetary influence?
  Rieger et al (1984) reported observations of a 154 day periodicity in flares during solar cycle 21. This paper discusses the observations in the light of a simple empirical planetary model of sunspot emergence. The planetary model predicts sunspot emergence when Mercury and Earth approach conjunction and Mercury approaches the Sun. We show that the reported times of flare activity are coherent with the planetary model. While the base period of the model is 170 days, the average model period, over the interval of flare recordings, is 157 days due to a 180 degree phase change in the planetary forcing near the middle of the record interval. We conclude that the periodicity at 154 days arises when the phase change in planetary forcing and the resulting progressive phase change in total sunspot area emergence and flare occurrence shifts the major peak in the flare spectrum from the planetary forcing period, 170 days, to 154 days. 
Ian Edmonds
astro-ph.SR