On Pitch-Angle Scattering Rates of Interstellar Pickup Ions as Determined by in Situ Measurement of Velocity Distributions

Abstract

Newly ionized interstellar atoms are acted upon by electromagnetic forces in the solar wind. Measurements of these pickup ions enable study of the transport processes controlling the evolution of charged particle populations in solar wind plasma. Data from the CELIAS instruments on board the SOHO spacecraft allow measurement of the velocity distribution of singly charged helium ions. These observations are compared to the predictions of a hemispheric model of pitch-angle diffusion. To justify the use of the hemispheric model we show here that a ``resonance gap,'' which hinders cross-hemispheric scattering of protons by Alfvén waves, can exist for the case of pickup helium scattered by outward-propagating waves. We find that the observed shape of helium pickup ion velocity distributions is consistent with the hemispheric model prediction. The parallel mean free path is found to vary with wave power from values of 0.1 to over 1 AU, here presented as a first measurement of the parallel mean free path as a function of magnetic wave power. Magnetic field data from the Wind spacecraft enable comparison to the coincident magnetic field wave power. The cross-hemispheric scattering rate is found to be monotonic and exponentially dependent on wave power over the dynamic range considered.