The seismic diversity of four successive solar cycle minima as observed by the Birmingham Solar-Oscillations Network (BiSON)

TL;DR

This work leverages BiSON's long, Sun-as-a-star, low-degree p-mode data to compare four successive solar minima and to quantify near-surface structural changes in the Sun. By analyzing the HeII acoustic glitch, comparing observed frequency differences to model perturbations, and applying a constrained pseudo-inversion, the authors find small but marginally significant differences, notably a larger HeII signature and higher near-surface sound speed in the 23/24 minimum (0.93–0.97 $R_{igodot}$) likely linked to lower magnetic flux. The study confirms that minute magnetic-activity variations between minima can imprint measurable structural differences, and demonstrates a practical framework for similar asteroseismic analyses in other solar-type stars. These results pave the way for applying such diagnostics to future high-precision datasets, including PLATO, to study stellar magnetic cycles via asteroseismology.

Abstract

We have used data collected by the Birmingham Solar-Oscillations Network (BiSON) to perform a helioseismic diagnosis of changes to the Sun's internal structure between four successive solar cycle minima, beginning with the minimum at the end of cycle 21 and ending with the recent minimum at the beginning of cycle 25. The unique duration of the BiSON database makes such a study possible. We used the low-degree BiSON p-mode frequencies to constrain structural changes between minima in the layers above $\approx 0.9 R_{\odot}$. We accomplished this by examining variations in the HeII ionisation zone signature; and by inverting the frequency differences to infer changes in the sound speed. Additionally, we employed frequency differences between various solar models that had subtle modifications to their internal structures to facilitate analysis of the observations. We find evidence for small, but marginally significant, changes in structure between different minima. The HeII signature was larger, and the sound speed in the range $\approx 0.93$ to $0.97 R_{\odot}$ was slightly higher, during the cycle 23/24 minimum, than during the other minima. The cycle 23/24 minimum was the deepest, as measured by proxies of global solar activity. These findings are consistent with magnetic flux levels having been lower in this minimum than the others, resulting in a higher gas pressure, higher temperatures, and higher sound speed. Our results demonstrate the potential of using asteroseismic data to perform similar analyses on other solar-type stars.

Figures (13)

• Figure 1: Plots of the 10.7-cm radio flux spanning the four cycle minima, with daily data in grey and 2-yr boxcar averages in blue. The vertical dotted lines mark the centres of selected 2-yr periods, where the boxcar averages take their minimum values, with the start and end dates of the periods denoted by the vertical dashed lines.
• Figure 2: The average shift in the low-degree mode frequencies with respect to the temporal mean, plotted as a function of time. The coloured bands demarcate the different solar cycles, while the red vertical lines mark the minimum epoch periods flagged in Fig. \ref{['fig:minima1']}.
• Figure 3: The frequency differences for each overtone $n$ and angular degree $l$ between the three minima indicated in the different panels, and those of the Cycle 23/24 minimum. The error bars come from propagating the uncertainties on the frequencies used to construct each difference.
• Figure 4: The second differences of the frequencies of the different minima showing the signatures of the acoustic glitches caused by the base of the convection zone and the HeII ionization zone. The different colours and type of points are for the different minima as indicated in the legend. We also show the fit to the data for the 22/23 minimum as the dot-dashed line. We do not show the fits to the other sets to avoid making the figure crowded and hence, unclear.
• Figure 5: Panel (a): The frequency differences of $l=0$--$3$ modes between standard solar model BSB(GS98)BSB and standard solar models Model SmodelS, BP04bp04, and MB22phot mb22. Panel (b): The same as above, but with a fit to the first five Legendre polynomials (i.e., $P_i, i=0,4$) subtracted out. The colours are for the same model pairs as in Panel (a). The grey points with the error-bars are the differences shown in Panel (b) of Fig. \ref{['fig:freqdif']}.