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We develop a general theory of buoyancy instabilities in the electron-ionplasma with the electron heat flux based not upon MHD equations, but using amulticomponent plasma approach in which the momentum equation is solved foreach species. We investigate the geometry in which the background magneticfield is perpendicular to the gravity and stratification. General expressionsfor the perturbed velocities are given without any simplifications. Collisionsbetween electrons and ions are taken into account in the momentum equations ina general form, permitting us to consider both weakly and strongly collisionalobjects. However, the electron heat flux is assumed to be directed along themagnetic field that implies a weakly collisional case. Using simplificationsjustified for an investigation of buoyancy instabilities with the electronthermal flux, we derive simple dispersion relations both for collisionless andcollisional cases for arbitrary directions of the wave vector. Thecollisionless dispersion relation considerably differs from that obtained inthe MHD framework and is similar to the Schwarzschild's criterion. Thisdifference is connected with simplified assumptions used in the MHD analysis ofbuoyancy instabilities and with the role of the longitudinal electric fieldperturbation which is not captured by the ideal MHD equations. The resultsobtained can be applied to clusters of galaxies and other astrophysicalobjects.

Multicomponent theory of buoyancy instabilities in astrophysical plasma objects: The case of magnetic field perpendicular to gravity