DECREASES IN THE ANTISUNWARD FLUX OF INTERSTELLAR PICKUP HE-FIELD( ASSOCIATED WITH RADIAL INTERPLANETARY MAGNETIC)

Observations of interstellar pickup He+ by the Suprathermal Energy Ion ic Charge Analyzer (SULEICA) instrument on the AMPTE/IRM spacecraft sh ow significant reductions in the antisunward flux of these particles i n the solar wind frame when the interplanetary magnetic field is direc ted radially. During such times, which can be as short as the 20-min t ime resolution of our instrument, the typically sharp cutoff in the pi ckup ion distribution at spacecraft-frame energy E-c = 2 M V-sw(2), wh ere 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 ste ps. We interpret these reductions as the result of inhibited pitch ang le scattering into the observable antisunward region of phase space, i mplying a strong sunward anisotropy during these times. Thus we unambi guously identify the appearance of such anisotropies with the radial o rientation of the magnetic field. We analyze the observations in terms of a steady two-stream model of the ion distribution and use this mod el to estimate an effective lambda, the scattering mean free path of t he 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.