Interaction of solar inertial modes with turbulent convection

A 2D model for the excitation of linearly stable modes

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: philidet@mps.mpg.de
² Institut für Astrophysik, Georg-August-Universität Göttingen, 37077 Göttingen, Germany

Received: 12 December 2022
Accepted: 28 March 2023

Abstract

Context. Inertial modes have been observed on the Sun at low longitudinal wavenumbers. These modes probe the dynamics and structure of the solar convective zone down to the tachocline. While linear analysis allows the complex eigenfrequencies and eigenfunctions of these modes to be computed, it gives no information about their excitation nor about their amplitudes.

Aims. We tested the hypothesis that solar inertial modes are stochastically excited by the turbulent motions entailed by convection. Unlike the acoustic modes, which are excited by vertical turbulent motions, the inertial modes are excited by the radial vorticity of the turbulent field.

Methods. We have developed a theoretical formalism where the turbulent velocity fluctuations provide the mechanical work necessary to excite the modes. The modes are described by means of a 2D linear wave equation with a source term, under the β plane approximation. This wave equation restrained to a spherical surface is relevant for the quasi-toroidal inertial modes that are observed on the Sun. Latitudinal differential rotation is included in the form of a parabolic profile that approximates the solar differential rotation at low and mid latitudes. The turbulent vorticity field underlying the source term is treated as an input to the model and is constrained by observations of the solar surface. The solution to the linear inhomogeneous wave equation is written in terms of a Green function, which is computed numerically.

Results. We obtain synthetic power spectra for the wave’s latitudinal velocity, longitudinal velocity, and radial vorticity, with azimuthal orders between 1 and 20. The synthetic power spectra contain the classical equatorial Rossby modes, as well as a rich spectrum of additional modes. The mode amplitudes are found to be of the same order of magnitude as observed on the Sun (∼1 m s⁻¹). There is a qualitative transition between low and high azimuthal orders: the power spectra for m ≲ 5 show modes that are clearly resolved in frequency space, while the power spectra for m ≳ 5 display regions of excess power that consist of many overlapping modes.

Conclusions. The general agreement between the predicted and observed inertial mode amplitudes supports the assumption of stochastic excitation by turbulent convection. Our work shows that the power spectra are not easily separable into individual modes, thus complicating the interpretation of the observations.