The Sun’s seismic radius, measured from the frequencies of f modes, is determined using both MDI and HMI data, covering a total of 21 years. It is found that the seismic radius is reduced by 1-2 km during the maxima, but the largest change of the radius happens at about 5 Mm beneath the surface.
Waves of magnetic-field variations were observed associated with the sunquake waves that were excited by the X9.3 flare on 2017 September 6. The nature and cause of the magnetic waves are discussed after the phase relations and power distributions of the magnetic waves and Doppler-observed sunquake waves are investigated.
Ring-diagram analysis is applied on the HMI-observed sunspots of about 3 years. The attenuation of wave amplitudes near sunspots, rotational speed of sunspots, and subsurface flows around sunspots are discussed.
Meridional flows during the solar minimum and maximum years are derived using 14 years of SOHO/MDI data. The flows changed significantly from the minimum to the maximum, and major changes were associated with the active latitudes.
It is demonstrated that when taking into account of the radial inhomogeneity of the Coriolis number, the solar-like differential rotation and the double-cell meridional circulation can both be reproduced by the mean-field model.
The systematic Center-to-Limb effect in time-distance helioseismic measurements is found to be significantly frequency dependent. The dependence further varies with disk-centric distance but not with travel distance.
A more comprehensive time-distance helioseismic method is developed to derive the Sun’s meridional circulation, and a 3-layer flow structure is found through the convection zone.
Helioseismic far-side images are compared with the STEREO-AIA EUV observations, and a reliability of the helioseismic images is assessed.
An X9.3 flare excited strong yet unusual sunquakes.
Rapid and irreversible changes in chromospheric magnetic field during a flare have been observed for the first time. They look surprisingly different from their photospheric counterpart.