Rotation of Solar Chromosphere: A Global Perspective of Mg <scp>II</scp> Index during the Decay Phase of the Modern Maximum

Abstract Rotation is one of the fundamental characteristics of the Sun, and it plays a crucial role in understanding the magnetic activities and dynamo processes. To reveal the variability patterns of the global rotation of the solar chromosphere, the Bremen composite Mg II index covering more than four solar cycles during the time interval from 1979 to 2024 is utilized in the present work. Our main findings are as follows: (1) The sidereal rotation period of the solar chromosphere varies between 22.63 and 27.10 days, with an average value of 25.13 days during the studied time interval. (2) From a global point of view, the period lengths of chromospheric rotation exhibit an obvious decreasing trend, which may be associated with the structural changes in magnetic fields and solar atmosphere in the declining phase of the most recent Gleissberg cycle. (3) The time-varying period lengths of chromospheric rotation display significant quasiperiodicities of 8.38 and 19.84 yr, indicating that chromospheric rotation might be related to the Schwabe cycle and Hale cycle. (4) There exists a moderate positive correlation between chromospheric rotation period and sunspot number. Chromospheric rotation is modulated by the solar activity cycle, further supporting its association with the Schwabe cycle. The possible physical mechanisms underlying the analysis results are discussed.