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 2
x
0
, enclosed between two vertical interfaces. The problem depends on six dimensionless parameters: the Prandtl number,
σ
; the magnetic Prandtl number,
σ
m
; the Chandrasekhar number,
Q
c
; the Reynolds number,
Re
; the ratio,
B
v
, 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 (
x
0
,
σ
) plane for different values of
σ
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
σ
m
,
Q
c
,
B
v
, occurs when 1.3<
x
0
<1.7, and is sinuous for small
σ
and varicose for large
σ
. 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.
Turbulent magnetic Prandtl number and magnetic diffusivity
quenching from simulations
