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.

不同大气层中太阳磁活动指数的时空变化对于理解和揭示太阳磁场的动态过程具有重要意义。在本文中，太阳 H \(\alpha \)的中周期周期（位于 50-500 天）通过 Lomb-Scarge 周期图技术和蒙特卡罗显着性检验研究了太阳周期 21-24 期间的耀斑指数。主要结果如下：（1）强振幅太阳周期（21和22）中周期周期数较少，而弱振幅太阳周期（23和24）中周期周期数较多；(2) 120-180天范围内的Rieger型周期（中心周期为154天）及其二、三次谐波（52天和73天左右）存在于所有太阳周期中；(3) 在某些太阳活动周期中也发现了其他一些中周期周期，包括27天自转周期、十周自转周期和年变信号。时频分析技术揭示了对这些中频周期性的可能解释，