Compositional convection in the presence of a magnetic field. Ii. Cartesian plume

The analysis of part I, dealing with the morphological instability of a single interface in a fluid of infinite extent, is extended to the case of a Cartesian plume of compositionally buoyant fluid, of thickness 2x₀, enclosed between two vertical interfaces. The problem depends on six dimensionless parameters: the Prandtl number, ó; the magnetic Prandtl number, ś_m; the Chandrasekhar number, Q_c; the Reynolds number, Re; the ratio, Bv, of vertical to horizontal components of the ambient magnetic field and the dimensionless plume thickness. Attention is focused on the preferred mode of instability, which occurs in the limit Re«/1 for all values of the parameters. This mode can be either sinuous or varicose with the wavenumber vector either vertical or oblique, comprising four types. The regions of preference of these four modes are represented in regime diagrams in the (x0, ó) plane for different values of ó_m, Q_c, B_v. These regions are strongly dependent on the field inclination and field strength and, to a lesser extent, on magnetic diffusion. The overall maximum growth rate for any prescribed set of the parameters ó_m, Q_c, B_v, occurs when 1.3«x₀ «1.7, and is sinuous for small ó and varicose for large ó. The magnetic field can enhance instability for a certain range of thickness of the plume. The enhancement of instability is due to the interaction of the field with viscous diffusion resulting in a reverse role for viscosity. The dependence of the helicity and α-effect on the parameters is also discussed.