Activity-Cycle Variations of Convection Scales in Subsurface Layers of the Sun

TL;DR

This study investigates how large-scale convection patterns in the Sun's subsurface layers vary with the solar activity cycle, revealing stable supergranulation scales and increasing giant-cell scales during activity maxima.

Contribution

It provides new insights into the solar convection scale variations over Solar Cycles 24 and 25 using helioseismic data.

Findings

Supergranulation scales remain stable over the cycle.

Giant-cell scales increase during activity maxima.

Power of giant-cell scales correlates with solar activity levels.

Abstract

We use subsurface-flow velocity maps inferred by time--distance helioseismology from Doppler measurements with the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory (SDO) to investigate variations of large-scale convection during Solar Cycles 24 and 25 in the 19-Mm-deep layer. The spatial power spectra of the horizontal-flow divergence reveal well-defined characteristic scales of solar supergranulation in the upper 4 Mm layer, while the giant-cell scale is prominent below levels of d ~ 8 Mm. We find that the characteristic scales of supergranulation remain stable while the giant scales increase during the periods of the 11-year activity cycle maxima. The power of the giant-cell scales increases with the enhancement of solar activity. This may be due to large-scale flows around active regions and, presumably, solar-cycle variations of the convection-zone stratification.