  Observations show that the solar p-mode frequencies change with the solar
cycle. The horizontal-phase-velocity dependence of the relative frequency
change, scaled by mode mass, provides depth information on the perturbation in
the solar interior. We find that the smoothed scaled relative frequency change
varies along the solar cycle for horizontal phase velocities higher than a
critical value, which corresponds to a depth near the base of the convection
zone. This phenomenon suggests that the physical conditions in a region near
the base of the convection zone change with the solar cycle.
