Gravitational waves in magnetized relativistic plasmas

We study the propagation of gravitational waves (gw) in a uniformly magnetized plasma at arbitrary angles to the magnetic field. No a priori assumptions are made about the temperature, and we consider both a plasma at rest and a plasma flowing out at ultra-relativistic velocities. In the 3+1 orthonormal tetrad description, we find that all three fundamental low-frequency plasma wave modes are excited by the gw. Alfvén waves are excited by a × polarized gw, whereas the slow and fast magneto-acoustic modes couple to the + polarization. The slow mode, however, doesn’t interact coherently with the gw. The most relevant wave mode is the fast magneto-acoustic mode which in a strongly magnetized plasma has a vanishingly small phase lag with respect to the gw allowing for coherent interaction over large length scales. When the background magnetic field is almost, but not entirely, parallel to the gw’s direction of propagation even the Alfvén waves grow to first order in the gw amplitude. Finally, we calculate the growth of the magneto-acoustic waves and the damping of the gw.