Effects of Magnetic Fields on Solar Inertial Modes in Global Nonlinear Simulations

Abstract

Solar inertial modes are increasingly viewed as probes of the solar interior, with recent helioseismic evidence indicating that these modes may be sensitive to the internal magnetic field. We investigate the role of magnetic fields on inertial mode excitation and dynamics using global nonlinear MHD simulations with differential rotation forced toward a helioseismically derived profile. Initial poloidal fields of varying strengths and topologies self-consistently generate different toroidal field configurations through the $Ω$-effect. In all cases, baroclinic instability excites high-latitude inertial modes, with weak fields enhancing its growth rate above the hydrodynamic value. For stronger fields, the response depends on topology and depth. As a consequence, the polar vortex emerges earlier and modifies its relative amplitude, handedness, and hemispheric asymmetry. Only magnetic topologies exhibiting toroidal bands at the upper convection zone develop a second, magneto-shear ("edge") instability at the equatorward edges of these bands. Powered mainly by the toroidal field, this instability sets in earlier and grows faster for stronger initial fields. It excites equatorial modes absent in the hydrodynamic simulation, with fast magneto-Rossby signatures approaching the classical Rossby dispersion relation at tachocline depths. Alfvénic signatures and tentative slow magneto-Rossby branches also appear. Inertial-mode excitation and frequencies thus depend on magnetic topology and spatial distribution, not merely field strength.