Changes in the sensitivity of solar p-mode frequency shifts to activity over three solar cycles

TL;DR

This study compares solar p-mode frequency shifts across three solar cycles using multiple data sources, revealing cycle-dependent differences likely linked to changes in the Sun's magnetic properties.

Contribution

It provides the first comprehensive comparison of medium-degree p-mode responses to solar activity over multiple cycles using diverse helioseismic datasets.

Findings

Cycle 24 shows ~10% larger frequency shifts than Cycle 23 for the same activity level.

Medium-degree modes exhibit systematic differences between cycles.

Results suggest changes in the Sun's magnetic properties affect p-mode frequency shifts.

Abstract

Low-degree solar p-mode observations from the long-lived Birmingham Solar Oscillations Network (BiSON) stretch back further than any other single helioseismic data set. Results from BiSON have suggested that the response of the mode frequency to solar activity levels may be different in different cycles. In order to check whether such changes can also be seen at higher degrees, we compare the response of medium-degree solar p-modes to activity levels across three solar cycles using data from Big Bear Solar Observatory (BBSO), Global Oscillation Network Group (GONG), Michelson Doppler Imager (MDI) and Helioseismic and Magnetic Imager (HMI), by examining the shifts in the mode frequencies and their sensitivity to solar activity levels. We compare these shifts and sensitivities with those from radial modes from BiSON. We find that the medium-degree data show small but significant systematic differences between the cycles, with solar cycle 24 showing a frequency shift about 10 per cent larger than cycle 23 for the same change in activity as determined by the 10.7 cm radio flux. This may support the idea that there have been changes in the magnetic properties of the shallow subsurface layers of the Sun that have the strongest influence on the frequency shifts.