Comparison of Solar Cycle Variations of Solar<i>p</i>‐Mode Frequencies from GONG and MDI
Abstract
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 perturbations in the solar interior. Both GONG data and SOHO MDI data show that the horizontal phase velocity dependence of the scaled relative frequency change varies with solar cycle. Temporal variations of GONG and MDI frequency changes are qualitatively consistent despite some discrepancies. For both GONG and MDI, the smoothed scaled relative frequency change versus horizontal phase velocity is approximately constant at small horizontal phase velocities, but its value decreases for horizontal phase velocities higher than a critical value, as surface activities develop. This critical horizontal phase velocity corresponds to a depth near the base of the convection zone. The magnitude of the decrease in frequency change at large horizontal phase velocities approximately correlates with the sunspot number for both GONG and MDI, although the GONG magnitude is greater than the MDI one. If these signals are real, it suggests that the wave speed in a region near the base of the convection zone changes with the solar cycle. This change might relate to solar cycle variations of magnetic fields near the base of the convection zone.