The 27-day variation in the Mg II index of solar activity

The 27-day variation attributed to the solar rotation is one of the most prominent periodicities of solar variations on short-term time scales. As a proxy of solar extreme ultraviolet (EUV) irradiance, the Mg II core-to-wing ratio is a measure of solar chromospheric variability, and has drawn some attention from solar physicists and geophysicists. In this present paper, the authors make use of wavelet technique to analyze the solar Mg II index series during 1978-2007, and obtain the obvious 27-day variation in solar activity. The time-variable characteristics exist in this periodic component, meanwhile, an obvious connection between the 27-day and 13-day variation is found at the end of this work.