Statistical acceleration of interstellar pickup ions in corotating interaction regions

Observations from the SWICS instrument on Ulysses reveal that there is little evidence for the acceleration of interstellar pick-up ions at the forward and reverse shocks which surround Co-rotating Interactions Regions (CIR's) in the solar wind. Rather, the pick-up ions exhibit strong acceleration in regions within the CIR where there are increased variations in the magnitude of the magnetic field. A simple model is presented in which the pick-up ions are statistically accelerated by transit-time damping the magnitude variations. The model yields the observed distribution functions for the accelerated pick-up ions. It is the purpose of this paper to extend the study of Gloeckler et al. [1994] to encompass many different CIR events by analyzing data obtained with the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses [Gloeckler et al., 1992]. The measurements reported here were made between Febuary 19, 1992, and July 18, 1992, while Ulysses remained near the solar equatorial plane, at about 5.5 AU from the Sun and encountered 22 forward and reverse shocks surrounding CIR's. The presence of these CIR's resulted in substantial acceleration of pick-up protons.