Interactions between tidal flows and magnetic fields in stellar/planetary convective envelopes

TL;DR

This study explores how magnetic fields influence tidal responses and dissipation in stellar and planetary convective envelopes through nonlinear MHD simulations, revealing different behaviors for weak and strong magnetic fields.

Contribution

First nonlinear MHD simulations of tidally-excited waves in magnetized convective envelopes, showing magnetic field strength impacts on tidal dissipation and flow structures.

Findings

Weak magnetic fields allow differential rotation and modify tidal dissipation.

Strong magnetic fields inhibit zonal flows and induce magnetic instabilities.

Magnetic field strength regimes are relevant for low-mass stars.

Abstract

Stars and gaseous planets are magnetised objects but the influence of magnetic fields on their tidal responses and dissipation rates has not been well explored. We present the first exploratory nonlinear magnetohydrodynamic (MHD) simulations of tidally-excited waves in incompressible convective envelopes harbouring an initial dipolar magnetic field. Simulations with weak magnetic fields exhibit tidally-generated differential rotation in the form of zonal flows (like in the purely hydrodynamic case) that can modify tidal dissipation rates from prior linear predictions. Moreover, tidal waves and zonal flows affect the amplitude and structure of the magnetic field, notably through creation of toroidal fields via the $\Omega$-effect. In contrast, simulations with strong magnetic fields feature severely inhibited zonal flows, due to large-scale magnetic stresses, excitation of torsional waves, or magnetic instabilities. We predict that the different regimes observed for weak and strong magnetic fields may be both relevant for low-mass stars when using turbulent values of the magnetic Prandtl number.