Suprathermal Ions in the Outer Heliosphere

Peter Kollmann, M. E. Hill, R. L. McNutt, L. E. Brown, R. C. Allen, G. Clark, B. Andrews, N. Salazar, J. Westlake, G. Romeo, J. Vandegriff, M. Kusterer, D. Smith, S. Jaskulek, R. Decker, A. F. Cheng, S. M. Krimigis, C. M. Lisse, D. G. Mitchell, H. A. WeaverP. Delamere, H. A. Elliott, E. Fattig, G. R. Gladstone, P. W. Valek, S. Weidner, F. Bagenal, M. Horányi, J. A. Kammer, D. Kaufmann, C. B. Olkin, M. R. Piquette, J. R. Spencer, A. J. Steffl, S. A. Stern, L. A. Young, K. Ennico, I. R. Linscott, D. F. Strobel, M. E. Summers, J. R. Szalay

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


Suprathermal ions form from interstellar gas that is first ionized into pickup ions and then accelerated to tens and hundreds of keV in energy. The resulting suprathermal ion spectra with hundreds of keV have been previously observed throughout the heliosphere; however, measurements at lower energies, around the pickup ion cutoff energy where they are accelerated from, were limited to <10 au. Here we present a statistical study of suprathermal ions in the keV to hundred keV energy range. We use the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on the New Horizons spacecraft, which recorded observations at a wide range of heliocentric distances, and compare these measurements to charge energy mass spectrometer (CHEMS) observations on Cassini, which cruised to and remained at Saturn. We find that the power-law exponents of suprathermal ion intensity over energy are between -1 and -2, change abruptly close to discontinuities that are likely corotating merged interaction regions, correlate with the solar wind bulk speed, and show a long-term evolution on the timescale of the solar cycle. The independent measurements from New Horizons and Cassini are consistent, confirming the first fully calibrated measurements from the New Horizons/PEPSSI instrument.

Original languageEnglish (US)
Article number46
JournalAstrophysical Journal
Issue number1
StatePublished - May 1 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • acceleration of particles
  • interplanetary medium
  • plasmas
  • shock waves


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