Numerical simulations of oscillations for axisymmetric solar backgrounds with differential rotation and gravity

TL;DR

This paper implements and evaluates a hybridizable discontinuous Galerkin method for simulating solar oscillations in realistic, gravity-influenced, differentially rotating backgrounds, improving modeling accuracy for helioseismology.

Contribution

It introduces a robust HDG-based approach for modeling stellar oscillations with gravity and differential rotation, addressing stability and accuracy issues in complex solar models.

Findings

Numerical power spectra match observed solar rotation effects.

Stabilization is crucial for accurate solutions, especially near regions of elliptic or hyperbolic behavior.

The method effectively captures gravity's influence on wave propagation.

Abstract

Local helioseismology comprises of imaging and inversion techniques employed to reconstruct the dynamic and interior of the Sun from correlations of oscillations observed on the surface, all of which require modeling solar oscillations and computing Green's kernels. In this context, we implement and investigate the robustness of the Hybridizable Discontinuous Galerkin (HDG) method in solving the equation modeling stellar oscillations for realistic solar backgrounds containing gravity and differential rotation. While a common choice for modeling stellar oscillations is the Galbrun's equation, our working equations are derived from an equivalent variant, involving less regularity in its coefficients, working with Lagrangian displacement and pressure perturbation as unknowns. Under differential rotation and axisymmetric assumption, the system is solved in azimuthal decomposition with the HDG method. Compared to no-gravity approximations, the mathematical nature of the wave operator is now linked to the profile of the solar buoyancy frequency N which encodes gravity, and leads to distinction into regions of elliptic or hyperbolic behavior of the wave operator at zero attenuation. While small attenuation is systematically included to guarantee theoretical well-posedness, the above phenomenon affects the numerical solutions in terms of amplitude and oscillation pattern, and requires a judicious choice of stabilization. We investigate the stabilization of the HDG discretization scheme, and demonstrate its importance to ensure the accuracy of numerical results, which is shown to depend on frequencies relative to N, and on the position of the Dirac source. As validations, the numerical power spectra reproduce accurately the observed effects of the solar rotation on acoustic waves.