Sensitivity of low-degree solar p modes to active and ephemeral regions: frequency shifts back to the Maunder minimum

Abstract

We explore the sensitivity of the frequencies of low-degree solar p-modes to near-surface magnetic flux on different spatial scales and strengths, specifically to active regions with strong magnetic fields and ephemeral regions with weak magnetic fields. We also use model reconstructions from the literature to calculate average frequency offsets back to the end of the Maunder minimum. We find that the p-mode frequencies are at least three times less sensitive (at 95 % confidence) to the ephemeral-region field than they are to the active-region field. Frequency shifts between activity cycle minima and maxima are controlled predominantly by the change of active region flux. Frequency shifts at cycle minima (with respect to a magnetically quiet Sun) are determined largely by the ephemeral flux, and are estimated to have been 0.1 µHz or less over the last few minima. We conclude that at epochs of cycle minimum, frequency shifts due to near-surface magnetic activity are negligible compared to the offsets between observed and model frequencies that arise from inaccurate modelling of the near-surface layers (the so-called surface term). The implication is that this will be the case for other Sun-like stars with similar activity, which has implications for asteroseismic modelling of stars.

Unexpected solar-cycle variation of acoustic mode power in Sun-as-a-star observations

“…One possibility worth considering is that different kinds of surface magnetic activity are driving the changes in frequencies and in mode power. To investigate this, we followed the analysis of Chaplin et al (2019), who divided the magnetic flux observed at the Wilcox Solar Observatory (WSO) into "strong" (> 15 G magnetic field strength in a pixel, considered as a proxy for the field in active regions) and "weak" (< 15 G, considered as a proxy for the flux in ephemeral regions, plage, etc) components. They found that the frequency variation outside periods of solar minimum was dominated by the strong-flux component, while the weak-flux component was not well correlated with the frequency variations even though it makes up the majority of the flux except around solar maximum.…”

Impact of magnetic activity on inferred stellar properties of main sequence Sun-like stars

“…Therefore by using ratios when fitting stellar models, our pristine separation ratios are equivalent to what we would observe on a field-free star, and also what we would expect to observe at minimum levels of stellar activity. Chaplin et al (2019) showed that minimum-epoch solar p modes should have frequencies very close to field-free case.…”

The next generation Birmingham Solar Oscillations Network (BiSON) spectrophotometer: a new miniaturized instrument for helioseismology

“…The Birmingham Solar Oscillations Network (BiSON) observes acoustic oscillations of the Sun-as-a-star, via a network of groundbased automated telescopes (Brookes 1974;Brookes et al 1978;Hale et al 2016;Hale 2019). The observed modes are of low angular degree, and provide data for characterising the solar activity cycle at the "whole Sun" level (see, e.g., Chaplin et al 2019;Howe et al 2022), and the structure and dynamics of the deep solar interior. The network is composed of six sites -Mount Wilson, Los Angeles, USA; Las Campanas, Chile; Izaña, Tenerife, Canary Islands; Sutherland, South Africa; Carnarvon, Western Australia; and Narrabri, New South Wales, Australia.…”