High Energy Plasmas, General Relativity and Collective Modes in the Vicinity of Black Holes*

Plasmas around black holes can take different equilibrium configurations$^1$ from those known from fluid theory as the vertical Lorentz compression due to plasma currents can overtake that of the gravitational force. In a disk with a ``seed'' magnetic field, axisymmetric modes as well as tri-dimensional spirals can be excited by the combined effects of the radial gradient of the plasma rotation frequency and of the plasma pressure gradient$^2$. The spirals' properties depend strongly on their vertical structure$^3$. Axisymmetric modes can produce vertical counter-flows of thermal energy and particles and be candidates for the origin of the winds emanating from disks in Active Galactic Nuclei (AGN's)$^2$. The excitation of radially localized density spirals corotating with the plasma near a black hole can provide an explanation for$^4$ the observed Quasi Periodic Oscillations (QPO's) of the X-ray emission from compact objects. Convective spiral modes$^3$ that are purely oscillatory in time and not localized radially can acquire their amplitudes from coupling to unstable modes and provide transport$^3$ of angular momentum toward the outer region of the disk structure.*Sponsored in part by the U.S. DOE. $^1$B. Coppi and F. Rousseau, Ap. J., $\bf{641}$, 458 (2006). $^2$B. Coppi, Europhys. Letters $\bf{82}$, 19001 (2008). $^3$B. Coppi, MIT/LNS Report 08/08, submitted to A$\&$A (2008). $^4$B. Coppi and P. Rebusco, Paper P5.154, E.P.S. Conf. Pl. Phys. (Crete, 2008).