Spatial Scales and Time Variation of Solar Subsurface Convection

Spectral analysis of the spatial structure of solar subphotospheric convection is carried out for subsurface flow maps constructed using the time-distance helioseismological technique. The source data are obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory from 2010 May to 2020 September. A spherical harmonic transform is applied to the horizontal velocity divergence field at depths from 0 to 19 Mm. The range of flow scales is fairly broad in the shallow layers and narrows as the depth increases. The horizontal flow scales increase rapidly with depth, from supergranulation to giant-cell values, and indicate the existence of large-scale convective motions in the near-surface shear layer. The results can naturally be interpreted in terms of a superposition of differently scaled flows localized at different depth intervals. There is some tendency toward the emergence of meridionally elongated (banana-shaped) convection structures in the deep layers. The total power of the convective flows is anticorrelated with the sunspot number variation over the solar activity cycle in shallow subsurface layers, and positively correlated at larger depths, which is suggestive of the depth redistribution of the convective flow energy due to the action of magnetic fields.