Interstellar helium pickup ion flux variations observed with SOHO/CELIAS

Interstellar pickup ions are formed when neutral atoms freely enter the heliosphere and are subsequently ionized. Once these particles are charged they become a component of the interplanetary plasma and are transported outward with the solar wind (SW). Measurements of these particles give information about the local interstellar gas in which our sun resides. However, fluxes of pickup ions in the solar wind are observed to vary substantially in both magnitude and energy spectrum. Hypothesized is that the main causes of these variations are transport phenomenon in the solar wind, determined by the solar wind and the interplanetary magnetic field. If so, solar wind conditions can be used to predict pickup ion fluxes, and pickup ion fluxes can be used to indicate solar wind transport conditions. To prove the feasibility of this hypothesis, in situ measurements of helium pickup ions from the CTOF instrument on the SOHO spacecraft are used to characterize transport parameters. Results are presented which show the effect of solar wind structures on pickup ions. Compression regions are shown to strongly affect measured fluxes of pickup ions. Observations show that compression regions can also cause an adiabatic heating or a shift in the pickup ion velocity distribution. Also investigated is the effect of interplanetary magnetic field fluctuations and waves on pickup ions. These fluctuations can scatter pickup ions, in both pitch angle and energy. Comparison with WIND/MFI magnetic field measurements show that the pitch angle scattering rate is correlated with the wave-power. A hemispheric model for the pickup ion distribution is used to determine pitch angle diffusion coefficients from observed spectra. We find that the observed scattering rate differs from the predictions of simple diffusion models, especially in low wave-powers. Energy diffusion or particle acceleration is also observed in the pickup ion velocity distribution, most notably in the passage of solar wind disturbances such as shocks but also during quiet times. However, suprathermal pickup ions are not found to be well correlated with resonant wave-power. Finally, measurements of SW parameters are used to de-trend the pickup ion data, reducing the variability of the dataset