Helioseismic Inferences of Large-scale Dynamics in the Sun

Recently, inertial oscillations have been discovered on the solar surface, suggesting their existence in the deep layers of the Sun. In this dissertation, we have developed a novel helioseismic technique based on modeling and analysis of coupling of the normal acoustic modes to characterize these oscillations. The solar acoustic modes exhibit sensitivity to the motions induced by inertial oscillations. We can capture the diverse length scales of inertial oscillations by measuring the coupling between acoustic modes of the same and different degrees. To study the temporal variability, we analyze the coupling between different frequency bins of acoustic modes. Our findings show that a two-year data set of Doppler velocity measurements from the SDO/HMI is sufficient to detect the inertial oscillations in the solar convection zone. This discovery holds promising potential for studying their temporal evolution in relation to the solar cycle. Furthermore, we conducted a comprehensive analysis utilizing a dataset spanning 20 years from the SOHO/MDI and SDO/HMI instruments to characterize the frequencies, line widths, and amplitudes of Rossby waves. Our research reveals that these waves follow a dispersion relation derived for a uniformly rotating solar model. We successfully detected Rossby modes ranging from azimuthal order m=1 to m=16, except the m=2 mode. The amplitudes of these modes varied between 1 and 2 m/s, with the mode power peaking at azimuthal order m=8. Additionally, we established an upper bound of 0.2 m/s for the sectoral m=2 mode, which was not detected in our measurements. The presence of Rossby waves can be observed down to a depth of 0.83 solar radii, beyond which the signal becomes indistinguishable from background noise. We find that their amplitude increases with depth down to around 0.92 solar radii and decreases below that depth.

Expanding our analysis, we confirmed the detection of inertial modes at high latitudes with an azimuthal order of m=1 and a frequency of approximately 80 nHz in a co-rotating frame. This mode is observed throughout the entire convection zone. Notably, these inertial modes hold potential diagnostic capability for imaging the Sun’s internal dynamics, in addition to the classical p-mode seismology.