(May 28, 2026)
We use Sun-as-a-star helioseismology data, collected by the Birmingham Solar-Oscillations Network, to examine the relationship between the solar-cycle-induced frequency shifts of whole-Sun, low-angular degree solar p modes and well-known proxies of global solar activity. Changes in behaviour between the low-frequency modes and proxies, which in a previous study we found had occurred on the declining phase of Cycle 23, appear to have persisted into Cycle 25. More striking is a significant change in the relationship for higher-frequency modes, which the new Cycle 25 data now reveal. The observed mean frequency shifts in Cycle 25 are much stronger than one would expect for these modes based on the relationship between the frequencies and proxies seen in previous cycles, in particular Cycle 22. In sum, Cycle 25 is as strong as Cycles 22 and 23 when observed in this higher-frequency seismic band, in marked contrast to the relative sizes of the cycles seen in the global activity proxies, where Cycle 25 is noticeably weaker. (…)
The last few solar cycles have seen significant changes in overall levels of activity and differences in the evolution of magnetic fields at different solar latitudes (e.g. see D. H. Hathaway 2015; A. Norton et al. 2023). Cycle 24 was significantly weaker in well-known proxies of global solar activity than previous cycles, and marked a departure from the preceding so-called modern maximum epoch (I. G. Usoskin 2017). While the current Cycle 25 has peaked at higher activity, it did not return to pre-Cycle-24 levels.