Lifetime of ring current particles due to Coulomb collisions in the plasmasphere

Ring current ions and electrons in the trapped belt are scattered and slowed down to Coulomb interactions with the thermal plasma in the plasmasphere and are eventually removed from trapped orbits. An expression for bounce-averged Coulomb lifetimes is derived using methods developed by Wentworth et al. (1959) but with the improvements of considering a more realistic representation of the thermal plasma distribution, as a Maxwellian rather than a delta function, and considering heavy ions in the ring current and thermal plasmas. Bounce-average Coulomb lifetimes for the major ring current ions are presented and are compared to Coulomb lifetimes in the literature. Coulomb lifetimes for ring current ions with energies less than ∼ 10 keV are longer than those predicted using previously derived expressions; however, they are comparable to, or shorter than, charge exchange lifetimes at these energies. For realistic thermal plasma densities in the outer plasmasphere, Coulomb lifetimes for ring current heavy ions can be comparable to charge exchange lifetimes at energies near the peak of the ring current differential number density. These results suggest that Coulomb decay is an important ring current loss process for tens of keV ions and might explain the discrepancy noted by Kistler et al. (1989) between model ring current distributions and the ring current fluxes measured by AMPTE/CCE during magnetically active conditions