Interstellar He+ ring‐beam distributions: Observations and implications

Abstract

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 Interplanetary Magnetic Field (IMF), , and the bulk solar wind velocity, . 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 . 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.