The Effects of Background Magnetic Fields on Astrophysical Fingering Convection
Double-diffusive convection at higher Prandtl numbers (Pr ~O(1) or larger) has been well studied in geophysical contexts, but detailed investigations of the low Prandtl number regimes (Pr << 1) which are relevant to most astrophysical scenarios have only recently become feasible. Since most low-Pr fluids in astrophysical scenarios are electrically conducting, it is possible that magnetic fields play a role in either enhancing or suppressing double-diffusive convection, but to date there have been no numerical investigations of such possibilities. Here we study the effects of both vertical (aligned with the gravitational axis) and horizontal background magnetic fields on the linear stability and nonlinear saturation of double-diffusive fingering, through a combination of theoretical work and direct numerical simulation (DNS). Both vertical and horizontal background magnetic fields are found to significantly enhance the fluid kinetic energy, vertical motion, and chemical flux relative to standard fingering convection, but the two cases differ considerably in their behavior. We focus mainly on the vertical case, finding that a vertical magnetic field suppresses the secondary shear instabilities between up- and down-flowing fingers such that saturation of the instability is delayed until significantly higher levels of vertical fluid motion are reached. This allows magnetized fingering convection to have significantly enhanced levels of turbulent mixing of chemical species with respect to the hydrodynamic case. Consequentially, magnetic effects offer a promising explanation of discrepancies between theoretical and observed mixing rates in low-mass red giant branch (RGB) stars.