The Centennial Evolution of Solar Chromospheric Rotation

TL;DR

This study analyzes a century-long Ca II K plage-area time series to trace the global, latitude-averaged chromospheric rotation. Using a Morlet continuous wavelet transform, it reveals a dominant synodic rotation period of $P\approx26.62$ days and a long-term, quadratic trend—decelerating through solar cycles $15$–$19$ and accelerating from $19$–$24$—which covaries negatively with solar magnetic activity. Autocorrelation uncovers significant periodicities at $3.2$, $5.7$, $7.7$, $10.3$, and $12.3$ years, while cross-correlation with sunspot numbers shows a complex, multi-scale modulation, including a lead of about $6.1$ years. These results imply chromospheric rotation is strongly influenced by multi-scale magnetic activity and small-scale magnetic concentrations, offering centennial-scale insight into solar dynamo processes and rotation of the solar atmosphere.

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 years, 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 years, 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.

Figures (6)

• Figure 2: The composite time series of the plage areas (in disk fraction units) from 1907 February 1 to 2023 December 31.
• Figure 3: Continuous wavelet power spectra of the composite time series of the plage areas from 1907 February 1 to 2023 December 31, arranged in chronological order from top to bottom panel. In each panel, the 99% confidence level is indicated by black contours. To clearly exhibit the rotation period and its temporal evolution, only the power spectra on the timescales of 16 to 45 days are shown. Right column of the bottom panel: global wavelet power spectrum (solid line) and the corresponding 99% confidence level (dashed line) for the composite time series of the plage areas during the entire time interval considered.
• Figure 4: Top panel: the 1-year smoothed sunspot numbers from 1907 February 1 to 2023 December 31 (solid black line), and the quadratic polynomial fit (solid red line). Bottom panel: temporal variation of rotation period of the plage area from 1907 February 1 to 2023 December 31 (solid blue line), and the quadratic polynomial fit (solid magenta line).
• Figure 5: Top panel: the dependence of RPLs for plage area on the phase of the solar cycle relative to the nearest preceding sunspot minimum. Bottom panel: same as the top panel, but relative to sunspot maximum. In each panel, the blue lines indicate their corresponding standard errors.
• Figure 6: Autocorrelation coefficients of the RPLs for relative phase shifts ranging from 1 to 5000 days. The two horizontal dashed lines indicate the 99% confidence level.