STATISTICAL ANALYSIS OF SOLAR ACTIVITY VARIATIONS USING SUNSPOT NUMBER DURING THE RISING PHASE OF SOLAR CYCLE 25

The 25th since 1755, Solar Cycle 25 is the current solar cycle when vast recording of solar sunspot activity began. Solar Cycle 25 began at the December 2019 solar minimum and subsequently developed into a substantially stronger cycle than the initial 2019 prediction. Our study quantifies the temporal evolution of solar activity using the International Sunspot Number (SSN) distributed by the World Data Centre–Sunspot Index and Long-term Solar Observations (WDC-SILSO) at the Royal Observatory of Belgium. Monthly mean, 13-month smoothed monthly, and yearly mean SSN Version 2.0 data are analysed. The cycle minimum is represented by a 13-month smoothed SSN of 1.8 in December 2019. The smoothed series rises from 1.8 in December 2019 to a confirmed maximum of 160.9 in October 2024,after which it declines to 99.8 by February 2026, the latest cantered smoothed value available in the SILSO series at the time of analysis. The annual mean SSN increases from 3.6 in 2019 to 154.7 in 2024 and decreases to 123.2 in 2025. Linear regression over 2019–2024 gives a slope of 33.12 SSN units yr−1 and r = 0.975 (p = 0.0009), indicating a strong monotonic rise in annual activity during the minimum-to-maximum interval. The Cycle 25 smoothed maximum is 38.2% higher than the Cycle 24 maximum of 116.4. The results demonstrate that Cycle 25 was not a simple repeat of the weak Cycle 24 pattern: its rise was rapid after 2021 and its maximum was considerably stronger. The study also emphasizes the difference between short-term monthly variability and the physically meaningful smoothed cycle envelope.