The New Solar Minimum: How deep does the problem go?

TL;DR

This paper investigates the unusual depth and characteristics of the current solar minimum by analyzing seismic p mode frequencies from BiSON data, revealing internal solar activity patterns that differ from surface observations.

Contribution

It introduces a detailed analysis of low-degree p mode frequency shifts over three solar cycles, highlighting internal solar dynamics during the extended minimum not evident from surface activity proxies.

Findings

Significant differences between seismic and surface activity during the minimum.

Detection of quasi-biennial periodicity in p mode frequencies.

Internal solar activity patterns differ from surface observations during the minimum.

Abstract

Although there are now some tentative signs that the start of cycle 24 has begun there is still considerable interest in the somewhat unusual behaviour of the current solar minimum and the apparent delay in the true start of the next cycle. While this behaviour is easily tracked by observing the change in surface activity a question can also be asked about what is happening beneath the surface where the magnetic activity ultimately originates? In order to try to answer this question we can look at the behaviour of the frequencies of the Sun's natural seismic modes of oscillation - the p modes. These seismic frequencies also respond to changes in activity and are probes of conditions in the solar interior. The Birmingham Solar Oscillations Network (BiSON) has made measurements of low-degree (low-$\ell$) p mode frequencies over the last three solar cycles, and so is in a unique position to explore the current unusual and extended solar minimum. We compare the frequency shifts in the low-$\ell$ p-modes obtained from the BiSON data with the change in surface activity as measured by different proxies and show there are significant differences especially during the declining phase of solar cycle 23 and into the current minimum. We also observe quasi-biennial periodic behaviour in the p mode frequencies over the last 2 cycles that, unlike in the surface measurements, seems to be present at mid- and low-activity levels. Additionally we look at the frequency shifts of individual $\ell$ modes.