Spatial Scales and Time Variation of Solar Subsurface Convection

TL;DR

This study analyzes the spatial and temporal variation of solar subsurface convection across different depths using helioseismological data, revealing scale changes and magnetic influences over the solar cycle.

Contribution

It provides a detailed spectral analysis of subsurface flow scales at various depths, highlighting the evolution of convective structures and their relation to solar magnetic activity.

Findings

Flow scales increase with depth, from supergranulation to giant cells.

Large-scale convective motions are present in the near-surface shear layer.

Convective flow power varies with the solar cycle, showing depth-dependent correlations.

Abstract

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 (HMI) onboard Solar Dynamics Observatory (SDO) 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 shallow layers and narrows as the depth increases. The horizontal flow scales rapidly increase 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 in 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 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.