Matthew E. Hill
Johns Hopkins University Applied Physics Laboratory
New Measurements of Suprathermal Ions, Energetic Particles, and Cosmic Rays in the Outer Heliosphere from the New Horizons PEPSSI Instrument
During the period from January 2012 to December 2017 the New Horizons spacecraft traveled from 22 to 41 AU from the Sun, making nearly continuous interplanetary plasma and particle measurements utilizing the SWAP and PEPSSI instruments. We report on newly extended measurements from PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation) that now bring together suprathermal particles above ~2 keV/nuc (including interstellar pickup ions), energetic particles with H, He, and O composition from ~30 keV to ~1 MeV, and cosmic rays above ~65 MeV (with effective count-rate-limited upper energy of ~1 GeV). Such a wide energy range allows us to look at the solar wind structures passing over the spacecraft, the energetic particles that are often accelerated by these structures, and the suppression of cosmic rays resulting from the increased turbulence inhibiting cosmic ray transport to the spacecraft position (i.e., Forbush decreases). This broad perspective provides simultaneous, previously unattainable diagnostics of outer heliospheric particle dynamics and acceleration. Besides the benefit of being recent, in-ecliptic measurements, unlike the historic Voyager 1 and 2 spacecraft, these PEPSSI observations are also totally unique in the suprathermal range; in this region only PEPSSI can span the suprathermal range, detecting a population that is a linchpin to understanding the outer heliosphere.