Connection between Sub-surface Layers and Surface Magnetic Activity over Multiple Solar Cycles

Abstract

We investigate spatio-temporal evolution of high-degree acoustic mode frequencies of the Sun and the surface magnetic activity, over the course of multiple solar cycles, to improve our understanding of the connection between the solar interior and atmosphere. We focus on high-degree modes due to their ability to characterize conditions in the shear layer just below the solar surface. Using the full-disk Doppler observations made by the Global Oscillation Network Group (GONG), mode frequencies covering the period from July 2001 to December 2021 are computed through the local helioseismic technique of ring diagrams. Considering 10.7 cm radio flux measurements, the sunspot number, and the local magnetic activity index as solar activity proxies, we note strong correlation between the frequency shifts and each activity index. We further investigate the hemispheric asymmetry in frequency shifts and magnetic activity and find that both the activity and frequencies in the descending phase of cycle 23 were dominant in the southern hemisphere, while in cycle 24 these quantities fluctuated between northern and southern hemispheres. Analyzing the frequency shifts at different latitudes with the progression of solar cycles, we observe that the shifts at mid-latitudes are dominant in the southern hemisphere during the maximum period of solar activity in cycle 24 but the values overlap as the cycle advances towards the minimum activity period. The frequency shifts at the beginning of cycle 25 are found to be dominant in the southern hemisphere following magnetic activity. The analysis presents additional evidence that the variability in oscillation frequencies are caused by both strong and weak magnetic fields.