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Helioseismic measurements of the sun's meridional circulation and sunquakes

In this thesis, we use helioseismic methods to study two separate topics, the Sun's meridional circulation and sunquake events. The Sun's meridional circulation is a key component of solar dynamo and interior dynamics, playing an important role in transporting magnetic flux and redistributing angular momentum. A profile of the meridional circulation has long been sought, but results from previous studies were not fully consistent, due to a systematic center- to-limb (CtoL) effect in helioseismic measurements that complicates the inference of meridional circulation in the deep interior. In the first part of this thesis, we measure the Sun's meridional circulation and its temporal evolution using 8 years of SDO/HMI Doppler-velocity observations, with a new CtoL-effect-removal method that we have developed in time-distance helioseismology. The long-time-averaged meridional circulation profile is found to have a three-layer flow structure: an equatorward flow is sandwiched between two poleward flow zones above and beneath it, indicating a double-cell circulation in each hemisphere. Moreover, the 3-layer flow pattern is more significant when the Sun's magnetic activity level is low, while significant changes are found in the flow structure during the active phase of the solar cycle. Besides, we also study the observational properties of the CtoL effect in the measured travel time of helioseismic waves. The CtoL effect is found isotropic relative to the azimuthal angle around the solar disk center. It also exhibits a significant frequency dependence -- it reverses sign at a frequency around 5.4 mHz, and is strongest at around 4.0 mHz. The tendency of frequency dependence varies with disk-centric distance but not with the waves' travel distance. In the second part this thesis, we focus on sunquakes. Sunquakes are helioseismic power enhancements initiated by solar flares, but not all flares generate sunquakes. It is curious why some flares cause sunquakes while others do not. Here we propose a hypothesis to explain the disproportionate occurrence of sunquakes: during a flare's impulsive phase when the flare's impact acts upon the photosphere, a sunquake tends to occur if the background oscillation at the flare's footpoint happens to oscillate downward, in the same direction with the impulse from above. To verify this hypothesis, we survey 60 strong flares in Solar Cycle 24 to search for sunquakes, by reconstructing the oscillatory velocity in the flare sites using a helioseismic holography method. A total of 24 flares are found to be sunquake active, giving a total of 41 sunquakes. It is found that in 3 − 5 mHz frequency band, 25 out of 31 (81%) sun- quakes show net downward oscillatory velocities during the flares' impulsive phases, and in 5 − 7 mHz frequency band, 33 out of 38 (87%) sunquakes show net downward velocities. These results support our hypothesis that a sunquake more likely occurs when a flare impacts a photospheric area that happens to have a downward background oscillation.