Linhua Deng

This study systematically examines differential rotation of α- type (unipolar), β- type (bipolar), and βx- type (complex) sunspot groups (SGs) using 1986–2025 RGO/USAF/NOAA data across solar cycles (SCs) 22–25. Methods include daily displacement measurements for rotation rates, outlier removal, binomial fitting for differential rotation profiles, and comparisons across magnetic types, cycles, and hemispheres. Our key findings are as follows: βx- type SGs exhibit the most pronounced differential rotation and significant cycle-dependent variations, with stronger differential rotation in active cycles (22–23) and weaker in less active ones (24–25); α- and β- type SGs show similar stable profiles. A comparison of near-equatorial low-latitude (10°) rotation rates across SCs reveals that both α- and β- type SGs are ordered as follows: SC 25 > SC 24 > SC 23 > SC 22; in contrast, βx- type SGs exhibit a different sequence: SC 25 > SC 23 > SC 22 > SC 24. The rotational rates of βx- type SGs exhibit inconsistent hemispheric dominance across different cycles; the north–south hemispheric differences in rates between α- and β- type SGs are relatively weak.

Coronal bright points (CBPs) are ubiquitous in the Sun and represent a key class of solar magnetic activities. They offer valuable insights into unresolved problems in solar cycle studies, but their hemispheric distribution in different cycles and for different types has not been understood yet. Based on the observations of the Extreme-ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory mission, a total of more than 1.8 million CBPs are identified in full-disk images spanning the period 1996–2023, and the hemispheric distribution of bright and dim CBPs is studied in detail. Our main results are as follows: (1) Bright CBPs are significantly correlated with sunspot activity, whereas dim CBPs are in antiphase with the solar activity cycle, suggesting that bright and dim CBPs have different characteristics and sources. (2) Two types of CBPs both exhibit significant hemispherical asymmetry, implying that the north–south asymmetry of CBPs is a real observational phenomenon and not a random fluctuation. (3) Before and after the solar cycle maximum, the slope of the normalized and absolute asymmetry indices of both bright and dim CBPs changes the sign or value, which suggests that the hemispheric distribution of CBPs might be influenced by the reversal of magnetic field polarity. (4) The cumulative trend and difference of bright and dim CBPs both display an intercycle- and intracycle-dependent pattern. Our analysis results are beneficial for elucidating the underlying mechanism of hemispheric variation of small-scale magnetic structures in solar corona, and could offer a new perspective on the fundamental physical processes responsible for the global and local dynamo theories.

The solar chromosphere is a key interface layer for understanding magnetic energy transport and multiscale magnetic structures, yet its multi-scale periodic variations from days to a century remain poorly understood. The longest available composite time series of Ca II K plage areas from 1907 February 1 to 2023 December 31 is used to investigate these characteristics by applying the methods of ensemble empirical mode decomposition (EEMD) and continuous wavelet transform. The data are decomposed by EEMD into 14 intrinsic mode functions, which represent the inherent periodicities of the plages. Periodicity analysis reveals a wide range of statistically significant periods. The periods for short-term components at 2.64, 8.63, 26.62, 27.80, and 77.50 days are associated with magnetic flux emergence and solar rotation. The midterm periods of 154.99 days (Rieger period), 358.15 days (annual variation), and 2.26 yr (quasi-biennial oscillations) are modulated by different harmonics of magnetic Rossby waves at the base of the convection zone. The long-term periods of 10.32 and 11.25 yr correspond to the Schwabe cycle; the 21.48 yr period is associated with the magnetic polarity reversal cycle; and the 49.76 yr period is interpreted as the first subharmonic of the Gleissberg cycle. Overall, chromospheric variations on timescales from days to several decades are tightly modulated by solar magnetic activity, and the century-long ground-based Ca II K observations provide a valuable resource for understanding the long-term behavior of the Sun.

This work relies on multi-source observational data from the 1997-2025 solar activity cycles, extracting various solar activity features such as flares, coronal mass ejections, and radio bursts. It constructs a dataset containing approximately 2,000 solar eruption events, and classifies the samples into SEP event and NonSEP event categories to conduct prediction study. The XGBoost model is chosen as the base prediction model. To address the issues of local optima, over-fitting, and insufficient generalization ability when using traditional grid search for parameter tuning, the genetic algorithm(GA) is introduced to conduct global adaptive optimization for the core hyperparameters of the model. Experimental results show that after standard preprocessing and parameter optimization, this GA-XGBoost model can effectively mine the correlation between solar activity parameters and SEP events. Through SHAP explainability analysis, seven core influencing features are selected, and simplifying the features, the model still maintains a high prediction performance. Comparative experiments of multiple models confirm that compared with SVM, traditional XGBoost, and AdaBoost models, GA-XGBoost has significant advantages in accuracy, precision, recall rate, F1-score and various error evaluation indicators. It can effectively improve the prediction ability of SEP events, alleviate the prediction bias caused by the imbalance of sample categories, and provide efficient and feasible new ideas and technical support for space weather warning and SEP event prediction.

Predicting the probability of extreme solar events is critical to understanding how to mitigate risks associated with space weather disasters, as their occurrence can pose a threat to life and infrastructure on Earth or in near‐Earth orbit. In the present study, extreme value theory is applied to the daily modified coronal index (MCI) for the period 1 January 1939–30 April 2025 to assess the characteristics of the extreme solar activity events over a long time scale and the likelihood of their occurrence in the future. The shape parameters of the generalized extreme value distribution estimated using the block maxima approach and those of the generalized Pareto distribution estimated using the peaks‐over‐threshold approach are both negative within the 95% confidence interval, indicating that the extreme values of the daily MCI have an upper limit. The return level obtained from the peaks‐over‐threshold approach suggests that the 100‐year return level (200‐year and 500‐year return level) is below (close to or above) the observed maximum value of the daily MCI (9.165), implying that it is shown to be a 1‐in‐200 year event. These results indicate that solar green‐line corona has reached its upper limit during the past eight solar cycles and the likelihood of observing values significantly higher than this maximum value in the near future is low. These findings provide clues for avoiding or mitigating the problems that extreme solar activity may pose to human society.

Solar coronal rotation exhibits significant complexity, and its relationship with the 11 yr solar cycle remains poorly understood. Using 597 McIntosh synoptic maps (CR1601–CR2197), we construct the temporal evolution of coronal rotation by performing cross-correlation analysis of coronal hole areas between consecutive Carrington rotations. Results show coronal rotation rate decreases from the equator to mid-latitudes (45°–60°), then increases at high latitudes (>60°). Comparison across solar cycles 21–24 reveals that cycle 23 displays the highest equatorial rotation rate and the strongest differential rotation. Equatorial rotation rates in odd-numbered cycles are generally higher than in preceding even-numbered ones. The latitudinally averaged rotation rate reaches a minimum near solar maximum and recovers during the declining phase, likely due to the latitudinal migration of coronal holes. Hemispheric asymmetry analysis shows no significant north–south difference in rotation rates, contrasting sharply with the strong asymmetry in sunspot distributions. Time series analysis of rotational parameters A, B, and C reveals a significant negative correlation between A and B, and a positive correlation between A and C. Singular spectrum analysis identifies prominent 5.6 and 6.9 yr periods in all three, with only C showing a strong 10 yr period.

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.

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.