  We analyze intermediate degree p-mode eigenfrequencies measured by GONG and
MDI/SOHO over a solar cycle to study the source of their variability. We carry
out a correlation analysis between the change in frequencies and several
measures of the Sun's magnetic activity that are sensitive to changes at
different levels in the solar atmosphere. The observations span a period of
about 12 years starting from mid-1996 (the minimum of cycle 23) to early-2008
(near minimum of cycle 24), corresponding to a nearly complete solar activity
cycle. We demonstrate that the frequencies do vary in phase with the solar
activity indices, however the degree of correlation differs from phase to phase
of the cycle. During the rising and declining phases, the mode frequency shifts
are strongly correlated with the activity proxies whereas during the
high-activity period, the shifts have significantly lower correlation with all
activity proxies, except for the 10.7-cm radio flux. In particular, the proxies
that are only influenced by the variation of the strong component of the
magnetic field in the photosphere have a much lower correlation at the
high-activity period. On the other hand, the shifts are better correlated with
the proxies sensitive to changes in the weak component of the magnetic field.
Our correlation analysis suggests that more than 90% of the variation in the
oscillation frequencies in all activity phases can be explained by changes in
both components of the magnetic field. Further, the slopes obtained from the
linear regression analysis also differ from phase to phase and show a strong
correlation with the correlation coefficients between frequency shifts and
solar activity.
