With nearly 5 years of observations of medium-degree acoustic modes from the GONG network and the MDI instrument aboard SOHO, we are able to use helioseismology to probe the dynamics of the convection zone in unprecedented detail.
We have analyzed 43 overlapping 108-day periods of GONG data, starting on dates 36 days apart, and 18 non-overlapping 72-day periods of MDI data. Together, these data sets cover the period from May 1995 to April 2000, encompassing the end of the previous solar cycle and most of the rising phase of the current one. Two two-dimensional inversion techniques -- Regularized Least Squares (RLS) and Optimally Localized Averages (OLA) were used to infer the rotation profiles. These observations now reveal the substantial (60 Mm) penetration into the convection zone of the banded zonal flows associated with the torsional oscillation. Furthermore, the data show periodic variations in rotation rate at and below the base of the convection zone with a surprising 1.3 year period. These discoveries further underscore the complexity of the dynamics within the convection zone, and offer new challenges for theoretical models. For more details, see R. Howe et al. 2000, Science 287 2456, and R. Howe et al. 2000, Astrophysical Journal 533 L163.