Using 14 years of helioseismic and magnetic-field observations, this study shows that deeper subsurface outflows from active regions play an important role in transporting magnetic fluxes to the polar regions and regulating polar-field formation.
Helioseismic analysis of solar torsional oscillations reveals dynamo-wave–like signatures that originate near the tachocline and propagate through the convection zone, linking internal zonal flows to the surface magnetic cycle. Additional evidence from frequency-splitting coefficients and rotational shear variations shows strong solar-cycle correlations, supporting the tachocline as a primary site of solar dynamo action.
We report a point spread function (PSF) and deconvolution procedure to remove stray light from the Helioseismic and Magnetic Imager (HMI) data. The deconvolution uses a Richardson-Lucy algorithm and takes less than one second per full-disk image. In 2018, the HMI team began providing full-disk, stray-light-corrected data daily. The results, on average, show decreases in umbral continuum intensity, a doubling of the granulation intensity contrast, increases in the total field strength, most notably in plage by ∼1.4–2.5 the original value, and a partial correction for the convective blueshift.
Strong acoustic scatterers are found beneath sunspot umbrae and penumbrae using the phase-correlation holography method. These scatterers indicate the fragmentation of magnetic flux, as suggested by Eugene Parker in the 1970s.
Equatorial Rossby waves are detected using the HMI’s time-distance subsurface flow fields. It is also found that the power of the Rossby waves show a positive correlation with the sunspot number, while the frequency of the waves shows an anti-correlation with the sunspot number.
Spectral analysis of the spatial structure of solar subphotospheric convection is carried out for subsurface flow maps. It is found that the horizontal flow scales increase rapidly with depth, from supergranulation to giant-cell values. The total power of the convective flows is found to be anticorrelated with the sunspot number variation over the solar activity cycle in shallow subsurface layers and positively correlated at larger depths.
Forward modeling is applied to numerous global hydrodynamic solar models, and helioseismic measurements on the meridional circulation are made using the forward modeling results. Comparison against the observational measurements shows significant differences, indicating our insufficient knowledge on either the global hydrodynamic modeling or the helioseismic inversions.
Analysis on high-spectral resolution data shows that oscillations in the higher atmosphere lead those in the lower atmosphere by an order of 1 s when their frequencies are below about 3.0 mHz, and lags behind by about 1 s when their frequencies are above 3.0 mHz. These phase shifts in the evanescent waves pose great challenges to the interpretation of some local helioseismic measurements that involve data acquired at different atmospheric heights.
High-frequency inertial waves were detected inside the Sun, propagating retrograde relative to the solar rotation with a phase speed faster than equatorial Rossby waves. How these waves are generated is discussed but remains unclear.
A new method, which is to characterize the multiscale convective spectrum of the Sun using high-resolution line-of-sight Dopplergram images from HMI, is developed, enabling the authors to estimate the spectrum to the finest observable scales.