The threshold and growth rate of conventional stimulated Brillouin scattering are first calculated for a uniform magnetoplasma. Approximate calculations of the threshold and growth rate are then carried out for a new instability which could be termed thermally stimulated Brillouin scattering; in this instability the nonlinear Lorentz force (ponderomotive force) is replaced by a pressure force due to differential heating in the interference pattern of the pump wave and the generated electromagnetic wave. The threshold for this new instability in a uniform medium is much lower than for conventional Brillouin scattering if the low-frequency wavelength measured along the magnetic field is much longer than the electron mean free path. The threshold is then calculated for the purely growing version of the new instability numerically, taking both the inhomogeneity of the medium and the effect of the geomagnetic field on wave propagation into account. The growth rate is also calculated and it is concluded that this new instability is probably responsible for the generation of large-scale field-aligned irregularities associated with artificial spread-F produced by ionospheric heating.