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We develop a theory of buoyancy instabilities of the electron-ion plasma withthe heat flux based on not the MHD equations, but using the multicomponentplasma approach. We investigate a geometry in which the background magneticfield, gravity, and stratification are directed along one axis. Nosimplifications usual for the MHD-approach in studying these instabilities areused. The background electron thermal flux and collisions between electrons andions are included. We derive the simple dispersion relation, which shows thatthe thermal flux perturbation generally stabilizes an instability. There is anarrow region of the temperature gradient, where an instability is possible.This result contradicts to a conclusion obtained in the MHD-approach. We showthat the reason of this contradiction is the simplified assumptions used in theMHD analysis of buoyancy instabilities and the role of the longitudinalelectric field perturbation, which is not captured by the MHD equations. Ourdispersion relation also shows that a medium with the electron thermal flux canbe unstable, if the temperature gradient of ions and electrons have theopposite signs. The results obtained can be applied to ICM and clusters ofgalaxies.

Multicomponent theory of buoyancy instabilities in magnetized astrophysical plasmas: MHD analysis revisited