On the identification of plasma sheet flapping waves observed by Cluster

Runov et al. [2003] and Sergeev et al. [2003, 2004] have reported on low-frequency oscillations of the plasma sheet generated by some impulsive source in the center of the magnetospheric tail and propagating toward the flanks, with velocities that range from a few tens to a few hundreds of km/s. To interpret the finding, a number of wave modes have been invoked and then discarded, for either the group velocities or propagation directions were inconsistent with the observations. In the present paper we examine the MHD ballooning-type waves first described by Safargaleev and Maltsev [1986] who termed them internal "gravitational'' waves, as a possible candidate to match the observed flapping motions. The role of gravity is played by the centrifugal force, acting on hot plasma in a curved magnetic field. The corresponding dispersion relation indicates propagation in the positive/negative azimuthal direction with a group velocity dependent on the wave number across the magnetic field, half-thickness of the current sheet, and thermal velocity of ions in the neutral sheet. The calculated group velocity ranges from 40 to 400 km/s, being consistent with the observations.