Probing Depth Variations of Solar Inertial Modes through Normal Mode Coupling

Abstract

Recently discovered inertial waves, observed on the solar surface, likely extend to the deeper layers of the Sun. Utilizing helioseismic techniques, we explore these motions, allowing us to discern inertial-mode eigenfunctions in both radial and latitudinal orientations. We analyze $8$ years of space-based observations ($2010 - 2017$) taken by the Helioseismic and Magnetic Imager (HMI) onboard the Solar dynamic observatory (SDO) using normal-mode coupling. Coupling between same and different-degree acoustic modes and different frequency bins are measured in order to capture the various length scales of inertial modes. We detect inertial modes at high latitude with azimuthal order $t=1$ and frequency $\sim -80$ nHz. This mode is present in the entire convection zone. The presence of Rossby modes may be seen down to a depth of $\sim 0.83R_\odot$ and the Rossby signal is indistinguishable from noise below that depth for high azimuthal order. We find that the amplitudes of these modes increase with depth down to around $0.92 R_\odot$ and decrease below that depth. We find that the latitudinal eigenfunctions of Rossby modes deviate from sectoral spherical harmonics if we use a similar approach as adopted in earlier studies. We found that spatial leakage and even pure noise in the measurements of non-sectoral components can also explain the above-mentioned characteristics of the latitudinal eigenfunctions. This realization underscores the necessity for careful interpretation when considering the latitudinal eigenfunctions of Rossby modes. Exploring the depth-dependent characteristics of these modes will enable us to capture interior dynamics distinctly, separate from p-mode seismology.