Abstract
The oscillations of a slowly rotating star have long been classified into spheroidal and toroidal modes. The spheroidal modes include the well-known 5-min acoustic modes used in helioseismology. Here 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 are the high-latitude inertial modes, the critical-latitude inertial modes, and the equatorial Rossby modes. In the model, the high-latitude and critical-latitude modes have maximum kinetic energy density at the base of the convection zone, and the high-latitude modes are baroclinically unstable due to the latitudinal entropy gradient. As a first application of inertial-mode helioseismology, we constrain the superadiabaticity and the turbulent viscosity in the deep convection zone.
Highlights
The free oscillations of a nonrotating spherical star have zero radial vorticity and are called spheroidal modes: they are the pressure (p), surface-gravity ( f ), and gravity (g) modes
The solar g modes would have important diagnostic potential regarding the radiative interior of the Sun; they evanesce in the convection zone and their amplitudes at the surface are exceedingly small (García et al 2007; Alvan et al 2015)
We use helioseismic maps of horizontal flows near the solar surface provided by the Stanford ring-diagram pipeline (Bogart et al 2011a,b) applied to continuous high-resolution observations from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) for the period from 1 May 2010 to 6 September 2020
Summary
The free oscillations of a nonrotating spherical star have zero radial vorticity and are called spheroidal modes: they are the pressure (p), surface-gravity ( f ), and gravity (g) modes. Quasitoroidal modes that resemble classical Rossby modes, known as r modes, are predicted (Papaloizou & Pringle 1978) They owe their existence to the Coriolis force, have frequencies on the order of the rotation frequency, and propagate in the retrograde direction. We report observations of a rich spectrum of inertial modes of the Sun over a wide range of latitudes, and we show they can be used to directly probe the superadiabaticity and turbulent viscosity in the deep convection zone. The observational discovery and the identification of the quasitoroidal normal modes of the Sun
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