Exploring the Dominant Process of Solar Toroidal Magnetic Flux Loss

Abstract As the solar magnetic cycle evolves, subsurface toroidal magnetic flux is systematically generated and lost, and this work aims to identify the dominant process behind the flux loss. By employing a data-driven dynamo model and holding surface magnetic flux transport identical across cycles 12–21, we conducted numerical experiments to isolate and assess the loss of subsurface toroidal flux, and then compared the results with two observational constraints: the butterfly diagram and the observed correlation between the polar field at cycle minimum and the strength of the next cycle. We found that under weak bulk diffusivity, the loss of the previous cycle’s toroidal flux is dominated by cancellation by newly generated flux, causing the new cycle’s actual flux to differ from its generated value and thereby preventing the simulation of the observed polar field–cycle strength correlation. When diffusivity is increased to a level where it dominates flux loss, residual flux is more effectively removed, restoring the polar field–cycle strength correlation; yet operating in the diffusion-dominated regime suppresses the formation of the butterfly diagram. In contrast, active region emergence acts as an efficient mechanism for removing residual flux, and when it dominates the flux loss, both the correlation and the butterfly diagram are successfully reproduced. Thus, we conclude that active region emergence dominates the subsurface toroidal flux loss.