A general model of particle acceleration by plasma waves coupled with
adiabatic radial diffusion in a planetary magnetic field is developed.
The model assumes that a spectrum of lower hybrid waves is present to
resonantly accelerate ions transverse to the magnetic field. The stea
dy state Green's function for the combined radial diffusion and wave a
cceleration equation is found in terms of a series expansion. The resu
lts provide a rigorous demonstration of how a quasi-Maxwellian distrib
ution function is formed in the absence of particle collisions and elu
cidate the nature of turbulent heating of magnetospheric plasmas. The
solution is applied to the magnetosphere of Neptune for which a number
of examples are given illustrating how the spectrum of pickup N+ ions
from Triton evolves.