  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.
