Trapping and excitation of modes in the magnetotail

A slab model of the magnetotail lobes is studied, and the trapping and exci tation of fast magnetoacoustic modes is considered. The tail lobe is taken to be uniform with the magnetic-field stretching parallel to the flow in th e magnetosheath. The modes are assumed to be trapped between free magnetopa use boundaries. The ideal magnetohydrodynamic (MHD) equations are used to s tudy the effects of linear perturbations to this equilibrium state. It is f ound that the plasma parameters in this region mean that fast magnetoacoust ic waves are unlikely to be Kelvin-Helmholtz (KH) unstable for realistic fl ow speeds, and in fact these modes are likely to be leaky, losing energy ac ross the magnetopause boundary. However, the decay rates of modes propagati ng both with and against the magnetosheath flow are found to be small for r easonable plasma parameters and this boundary may be reasonably assumed to be reflecting for the purposes of wave guide theory. Slow modes are found t o have very small growth rates and since in the magnetotail lobes the plasm a beta is very low, they are not expected to be important. (C) 2000 America n Institute of Physics. [S1070-664X(00)02104-2].