This paper derives for the first time, the consequences of quasi-adiabatic ion motion in the magnetospheric tail, which is in fact a chaotic scattering of the ions due to their essentially nonadiabatic behavior. This is caused by the nonlinearity of their equations of motion in the strongly curved tail magnetic field and results from separatrix traversals in the phase space. In this paper it is shown how the continuous violation of adiabaticity of the ion motion in the Earth's magnetotail leads to a redistribution of ions between the central plasma hseet (CPS) and thje plasma sheet boundary layer (PSBL). It is also shown that, due to the mechanism described in this paper, ions are accelerated to the PSBL even without any assumption of the existence of an electric field caused by reconnection with an x line. For the CPs ion distribution this acceleration also results in the depletion of some domains in velocity space that can lead to substantial non-Maxwellian features. ¿1991 American Geophysical Union