Interstellar pickup H⁺ Ions at 8.3 AU: Pioneer 10 plasma and magnetic field analyses

Abstract

Analysis of Pioneer 10 plasma and magnetic field observations at 8.3 AU in 1975 provides new evidence for the presence of interstellar pickup hydrogen (H⁺) ions. Use of plasma sensors that look far from the solar wind direction confirms the spherical shell distribution of the pickup ions in velocity space. Phase space density and flux estimates are closely consistent with those from Ulysses measured under similar conditions. Power spectral analyses of the magnetic field data show a distinct signal a little above the proton gyrofrequency, consistent with the presence of Doppler-shifted ion-cyclotron waves generated by H⁺ pickup ions. These results show that the Pioneer data set has the potential for systematic studies of the global properties of interstellar pickup ions.