  We examine continuous measurements of the high-degree acoustic mode
frequencies of the Sun covering the period from 2001 July to June 2014. These
are obtained through the ring-diagram technique applied to the full-disk
Doppler observations made by the Global Oscillation Network Group (GONG). The
frequency shifts in the degree range of 180-1200 are correlated with different
proxies of solar activity e.g. 10.7 cm radio flux, the International Sunspot
Number and the strength of the local magnetic field. In general, a good
agreement is found between the shifts and activity indices, and the correlation
coefficients are found to be comparable with intermediate degree mode
frequencies. Analyzing the frequency shifts separately for the two cycles, we
find that cycle 24 is weaker than cycle 23. Since the magnetic activity is
known to be different in the two hemisphere, for the first time, we compute the
frequency shifts over the two hemispheres separately and find that the shifts
also display hemispheric asymmetry; the amplitude of shifts in the northern
hemisphere peaked during late 2011, more than two years earlier than the south.
We further correlate the hemispheric frequency shifts with the hemispheric
sunspot number and mean magnetic activity index. Since the frequency shifts and
the hemispheric activity indices are found to be significantly correlated, we
suggest that the shifts be used as an indicator of hemispheric activity since
not many indices are measured over the two hemispheres separately. We also
investigate the variation at different latitudinal bands and conclude that the
shifts in active latitudes correlate well with the local magnetic activity
index.
