Helioseismic diagnostics of solar dynamics in the near-surface layers

This thesis proposes to develop new helioseismic diagnostics of solar near-surface flows. In a first part, we discuss the interaction of solar seismic waves (sound waves) with small-scale turbulent convection. The treatment of the effect of turbulent convection on waves is challenging because it involves all spatial and temporal scales. We consider several effective-medium approximations used to describe the propagation of acoustic waves through solar granulation and compare these approximations with numerical simulations. For large-amplitude perturbations, we find that the Keller approximation is best suited to estimate the effective wave speed and attenuation. While the temporal evolution of granulation may be ignored when estimating the effective wave speed, it must be taken into account in the computation of the attenuation. In addition, late arrival waves due to multiple scattering (coda waves) are seen in the simulations in the variance of the wave field. This work will help improve our understanding of the physics of the modes of solar oscillations, and contributes to the interpretation of global and local helioseismology observations. In a second part, we study near-surface local flows around solar active regions. These active-region inflows are important because they contribute to the observed solar-cycle changes in the longitudinal average of the solar meridional circulation. Using measurements of the inflows with a granulation tracking method and a model for their depth dependence, we solve the forward problem of time-distance helioseismology to estimate their contribution to the observed helioseismic travel times. In the granulation-tracking maps, the inflows contribute up to $\pm 7$ m/s to the surface meridional flow, which is about $50\%$ of its amplitude. We find however that the travel-time perturbations associated with near-surface active-region flows do not explain in full the solar-cycle variations observed in the seismic data. This work paves the way for correcting the travel times for the near-surface flows, in order to probe the cycle variations of the meridional flow at depth.2022-02-1