Abstract

The oscillations of a slowly rotating star have long been classified into spheroidal and toroidal modes. The spheroidal modes include the well-known five-min acoustic modes used in helioseismology. In this presentation we report observations of the Sun’s toroidal modes, for which the restoring force is the Coriolis force and whose periods are on the order of the solar rotation period. By comparing the observations with the normal modes of a differentially rotating spherical shell, we are able to identify many of the observed modes. These include high-latitude modes, critical-latitude modes, and the equatorial Rossby modes. In the model, the high-latitude modes are baroclinically unstable due to the latitudinal entropy gradient. The frequencies of the modes and their surface eigenfunctions inform us about important properties of the deep convection zone, such as the superadiabaticity and the turbulent viscosity. The interpretation of the mode amplitudes requires some care and detailed modeling, in particular because the mode spectrum is dense and the mode amplitudes are observed to be sensitive to the solar-cycle magnetic field.