Numerical Study of Magnetic Influences on Plume-Driven Convection in the Sun

The thermal convection in solar-type stars is thought to be driven by a local entropy gradient. Such a theory predicts the existence of convection-zone-scale convection (giant cells). However, giant cells have not yet been observationally confirmed in the Sun. This discrepancy between theory and observation is known as the convection conundrum, suggesting the need to revise current solar and stellar modeling.

As a possible solution to this problem, a hypothesis that has attracted attention in recent years proposes that convection in the Sun is driven not by local entropy gradients, but by strong downflows (plumes) generated near the solar surface, which is known as plume-driven convection. In this scenario, the entropy gradient in the bulk of the convection zone approaches zero, suppressing the driving of giant cells.

Several previous studies have performed numerical simulations of plume-driven convection. However, all of these studies neglected magnetic fields, leading to models that do not fully reflect the conditions in the convection zones of real stars, where self-consistent dynamo generated magnetic fields are known to play a key dynamical role. In this study, we investigated the effects of such dynamo-driven magnetic fields on plume-driven convection by conducting high performance numerical simulations in a Cartesian box.

The results show that the presence of magnetic fields tends to make the computational domain more convectively stable and reduces the kinetic energy of large-scale convection. In particular, the kinetic energy spectrum at large scales is reduced by approximately 10% compared to the non-magnetic case. These results provide support for the plume-driven convection hypothesis.