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

TL;DR

The paper investigates how large-scale solar convection scales in subsurface layers respond to the 11-year solar cycle. It uses time-distance helioseismology data from SDO/HMI to compute the divergence spectrum of horizontal flows across depths, deriving a center-of-mass harmonic degree and a corresponding wavelength to characterize convective scales, with noise thresholding and filtering to isolate cycle signals. The main finding is that the upper-layer supergranulation scales remain stable across the cycle while the deeper giant-cell scales increase with solar activity; the correlation between the characteristic scale and sunspot number shows two depth peaks near partial ionization zones, and long-wavelength power is enhanced at depths up to about 12 Mm during maxima. This implies cycle-dependent modulation of convection tied to ionization-zone stratification and active-region flows, with implications for solar dynamo processes and interpretation of subsurface flow structures.

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.

Figures (8)

• Figure 1: Left column: depth variation of a typical 45-day averaged power spectrum of the divergence field of horizontal velocity, $p_{\ell m}$, obtained for the period of low activity (9 November to 24 December 2019). The depth values and the averaging interval are indicated at the top of each panel. Right column: power-spectrum differences, $\Delta p_{\ell m}$, between the periods of high activity (9 January to 23 February 2014) and low activity (9 November to 24 December 2019) for the same depths.
• Figure 2: Correlation of the integrated spectral power of convection, $p_\mathrm{tot}$, with the sunspot number at different depths.
• Figure 3: Time variation of the wavelength, $\lambda_\mathrm{c}$, corresponding to the Butterworth-filtered variation of the weighted mean degree of the horizontal-velocity-divergence spectrum (heavy curves). The light curves represent the variation of the sunspot number. The depth values are indicated at the top of each panel.
• Figure 4: Correlation of the characteristic wavelength $\lambda_\mathrm c$ with the sunspot number at different depths.
• Figure 5: Depth variation of the weighted mean spherical-harmonic degree, $l_\mathrm c$ (left), and characteristic wavelength, $\lambda_\mathrm c$, inferred from $l_\mathrm c$ (right), both averaged over 360 days. Solid curves: high-activity periods centered at February 2014 (black) and immediately preceding August 2024 (red); dashed curves: low-activity periods centered at October 2019 (black) and starting from May 2010 (blue). The error bars are plotted in the same color as the corresponding mean-value curves, with error-bar endcaps being longer for the averaging period centered on February 2014.