PHASE-SPACE DIFFUSION AND ANISOTROPIC PICK-UP ION DISTRIBUTIONS IN THE SOLAR-WIND - AN INJECTION STUDY

Pick-up ions are produced all over the interplanetary space by ionizat ion of neutral interstellar atoms. Over the last decade it was general ly expected that after pick-up these ions quickly are isotropized in t he velocity space comoving with the solar wind by strong pitch-angle s cattering, though not assimilating to the thermodynamic state of solar wind ions. Recent studies of pick-up ion data obtained with plasma an alyzers on AMPTE and ULYSSES have, however, revealed that during exten ded time periods substantially anisotropic distributions prevail. In t his paper we want to improve the understanding of the evolutionary beh aviour of pick-up ions in interplanetary phase space by an pick-up ion injection study taking into account all relevant diffusion terms desc ribing pitch angle scattering, adiabatic cooling, drifts and energy di ffusion. For particles injected at 1 AU the resulting distribution fun ction stays substantially anisotropic up to distances of the order of 6 AU, unless increased isotropic turbulence levels and non-dissipative wave spectra are considered. The average bulk velocity of pick-up ion s within this distance range is smaller than the solar wind velocity b y factors of between 0.6 and 1.0. Pick-ups are shown to substantially become spread out from that solar wind parcel into which they were ori ginally injected. Derivations of interstellar gas parameters using pic k-up ion flux data can only be dared with additional care in the inter pretation. As a consequence the location of the LISM helium cone axis and the LISM helium temperature are faked in the associated He+ pick-u p ion flux pattern.