Equatorial trapped plasmasphere ion distributions and transverse stochastic acceleration

Observations by the DE 1 and SCATHA satellites have revealed ion distribution functions in the equatorial plasmasphere which are sharply peaked at 90 deg pitch angle. The pitch angle anisotropy increases with increasing energy for the observed distributions. Also observed by SCATHA and earlier spacecraft are cyclotron harmonic emissions which are closely confined to the plasmasphere equator. It is demonstrated here that the observed highly anisotropic ion distribution at thermal energies can be explained, as the consequence of the nonlinear evolution of an initially Maxwellian distribution with a characteristic plasmasphere thermal plasma temperature by stochastic acceleration, using the observed levels of wave turbulence.