The midrange periodicities of solar Ha flare index during Solar Cycles 21-24

The temporal and spatial variations of solar magnetic activity indices in the various atmospheric layers are important for understanding and revealing the dynamic processes of magnetic field of the Sun. In this paper, the midrange periodicities (located at 50-500 days) of the solar H alpha flare index during Solar Cycles 21-24 were studied by the Lomb-Scarge periodogram technique and the Monte Carlo significance test. The main results are as follows: (1) a lesser number of midrange periodicities is found in the strong-amplitude solar cycles (21 and 22) while the weak-amplitude solar cycles (23 and 24) show a greater number of midrange periodicities; (2) the Rieger-type periodicity in the range 120-180 days (the central periodicity is 154 days) and its two and three subharmonics (around 52 and 73 days) exist in all solar cycles; (3) some other midrange periodicities, including the 27-day rotation period, the ten-rotation periodicity, and the annual-variation signal, are also found in certain solar cycles. Possible explanations of these midrange periodicities, revealed by the time-frequency analysis technique, are presented on the basis of the periodic emergences of magnetic flux.