Trapping and excitation of modes in the magnetotail

A slab model of the magnetotail lobes is studied, and the trapping and excitation of fast magnetoacoustic modes is considered. The tail lobe is taken to be uniform with the magnetic-field stretching parallel to the flow in the magnetosheath. The modes are assumed to be trapped between free magnetopause boundaries. The ideal magnetohydrodynamic (MHD) equations are used to study the effects of linear perturbations to this equilibrium state. It is found that the plasma parameters in this region mean that fast magnetoacoustic waves are unlikely to be Kelvin-Helmholtz (KH) unstable for realistic flow speeds, and in fact these modes are likely to be leaky, losing energy across the magnetopause boundary. However, the decay rates of modes propagating both with and against the magnetosheath flow are found to be small for reasonable plasma parameters and this boundary may be reasonably assumed to be reflecting for the purposes of wave guide theory. Slow modes are found to have very small growth rates and since in the magnetotail lobes the plasma beta is very low, they are not expected to be important. (C) 2000 American Institute of Physics. [S1070-664X(00)02104-2].