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In page Solar flare:

"

In 1984, Erich Rieger and coworkers discovered an approximately 154-day period in the occurrence of gamma-ray emitting solar flares at least since the solar cycle 19.[2] The period has since been confirmed in most heliophysics data and the interplanetary magnetic field and is commonly known as the Rieger period. The period's resonance harmonics also have been reported from most data types in the heliosphere.[citation needed]