The Sun in transition? Persistence of near-surface structural changes through Cycle 24

We examine the frequency shifts in low-degree helioseismic modes from the Birmingham Solar-Oscillations Network covering the period from 1985 to 2016, and compare them with a number of global activity proxies well as a latitudinally resolved magnetic index. As well as looking at frequency shifts in different frequency bands, we look at a parametrization of the shift as a cubic function of frequency. While the shifts in the medium- and high-frequency bands are very well correlated with all of the activity indices (with the best correlation being with the 10.7-cm radio flux), we confirm earlier findings that there appears to have been a change in the frequency response to activity during solar Cycle 23, and the low-frequency shifts are less correlated with activity in the last two cycles than they were in Cycle 22. At the same time, the more recent cycles show a slight increase in their sensitivity to activity levels at medium and higher frequencies, perhaps because a greater proportion of activity is composed of weaker or more ephemeral regions. This lends weight to the speculation that a fundamental change in the nature of the solar dynamo may be in progress.