  In weakly magnetized, dilute plasmas in which thermal conduction along
magnetic field lines is important, the usual convective stability criterion is
modified. Instead of depending on entropy gradients, instability occurs for
small wavenumbers when (dP/dz)(dln T/dz) > 0, which we refer to as the Balbus
criterion. We refer to the convective instability that results in this regime
as the magnetothermal instability (MTI). We use numerical MHD simulations which
include anisotropic electron heat conduction to follow the growth and
saturation of the MTI in two-dimensional, plane parallel atmospheres that are
unstable according to the Balbus criterion. The linear growth rates measured in
the simulations agree with the weak field dispersion relation. We investigate
the effect of strong fields and isotropic conduction on the linear properties
and nonlinear regime of the MTI. In the nonlinear regime, the instability
saturates and convection decays away, when the atmosphere becomes isothermal.
Sustained convective turbulence can be driven if there is a fixed temperature
difference between the top and bottom edges of the simulation domain, and if
isotropic conduction is used to create convectively stable layers that prevent
the formation of unresolved, thermal boundary layers. The largest component of
the time-averaged heat flux is due to advective motions. These results have
implications for a variety of astrophysical systems, such as the temperature
profile of hot gas in galaxy clusters, and the structure of radiatively
inefficient accretion flows.
