Interstellar He+ Ring-Beam Distributions: Observations and Implications

Interstellar pickup ions, which are believed to be injected as a ring-like velocity distributions into the bulk solar wind, are exposed to fluctuations of the Interplanetary Magnetic Field (IMF) and thus undergo pitch-angle scattering. It is generally assumed that this process can be very efficient on short time scales, and thus velocity distributions of pickup ions were believed to be fully isotropic in nature [Vasyliunas and Siscoe, 1976]. However, unambiguous evidence for this assumption has yet to be found. Indeed, observations of reduced one-dimensional w-spectra for quasi-parallel and -perpendicular IMF orientations revealed a significant difference in flux and spectral shape of interstellar He+ pickup ions, which e.g., Saul et al. [2007], Möbius et al. [1998], and Isenberg [1997] attributed to ineffective scattering across 90° pitch-angle. Here we report systematic measurements of the distribution of the incident angle of interstellar pickup He+ ions as observed by STEREO/Plasma and Supra-Thermal Ion Composition (PLASTIC). We have organized these observations according to the angle spanned by the IMF vector, B, and the solar wind velocity vector. Our measurements show clear evidence of a relatively local injection of He+ pickup ions into the solar wind, which are then seen as a ring distribution perpendicular to B. Changes of the spectral shape and a reduced flux of interstellar He+ during radial IMF configuration, as observed by, e.g., Ulysses/Solar Wind Ion Composition Spectrometer (SWICS), SOHO/Charge Time-Of-Flight (CTOF), Active Magnetospheric Particle Tracer Explorers/SUprathermaL Energy Ionic Charge Analyzer (SULEICA), have generally been attributed to inefficient scattering across 90° pitch-angle. Our observations of the pitch-angle distribution of interstellar He+ suggest that these changes are instead a result of locally injected pickup ions that escape detection for IMF configuration in which the Solar Wind Sector of PLASTIC, as well as SWICS, CTOF, and SULEICA, are not sensitive to the measurement of the locally injected pickup ion ring. A 2-D cut of the model He+ phase space density in the (vx, vy) plane at vz = 0 km/s for the situation in which B is perpendicular to the solar wind velocity vector. It shows an isotropic pickup ion shell distribution (inner green ring at 250 km/s) superposed with a freshly injected pickup ion ring perpendicular to B that fits our observations made by STEREO/PLASTIC.