The Centennial Evolution of Solar Chromospheric Rotation

Abstract Rotation is a prominent feature of the Sun, and it plays a crucial role in the generation and dynamic evolution of solar magnetic fields. The daily composite time series of Ca II K plage areas from 1907 February 1 to 2023 December 31 is used to analyze its periodicity and examine the temporal variation of its rotation period lengths (RPLs) using continuous wavelet transform. Wavelet analysis reveals that over a time span of more than a century, chromospheric rotation exhibits a dominant synodic period of approximately 26.62 days, with complex temporal variations. The long-term trend of chromospheric rotation is well-characterized by a statistically significant quadratic polynomial, showing a gradual deceleration from solar cycles 15 to 19, followed by a gradual acceleration from cycles 19 to 24. The RPLs exhibit a negative correlation between the rotation rate of the chromosphere and solar magnetic activity. Their behavior follows a distinct pattern within a Schwabe cycle: the rotation period progressively lengthens during the initial approximately 3 yr, then maintains a relatively long value from year 3 to approximately 7.5, and finally shortens during the declining phase, returning to a minimum near the subsequent solar minimum. The variations of chromospheric RPLs show significant periods of 3.2, 5.7, 7.7, 10.3, and 12.3 yr, with cross-correlation analysis pointing to a complex relationship with solar activity. The possible mechanisms for the temporal variation of the chromospheric rotation are discussed.