Decreases in the antisunward flux of interstellar pickup He+ associated with radial interplanetary magnetic field

Observations of interstellar pickup He+ by the Suprathermal Energy Ionic Charge Analyzer (SULEICA) instrument on the AMPTE/IRM spacecraft show significant reductions in the antisunward flux of these particles in the solar wind frame when the interplanetary magnetic field is directed radially. During such times, which can be as short as the 20-min time resolution of our instrument, the typically sharp cutoff in the pickup ion distribution at spacecraft-frame energy E-c = 2 M V-sw(2), where M is the helium mass and V-sw is the solar wind speed, is replaced by a smooth decline to background flux levels over several energy steps. We interpret these reductions as the result of inhibited pitch angle scattering into the observable antisunward region of phase space, implying a strong sunward anisotropy during these times. Thus we unambiguously identify the appearance of such anisotropies with the radial orientation of the magnetic field. We analyze the observations in terms of a steady two-stream model of the ion distribution and use this model to estimate an effective lambda, the scattering mean free path of the ions along the field. We obtain values of lambda which range from 0.16 tb 0.76 AU and discuss the significance of these results.