Suprathermal Ions in the Outer Heliosphere

Abstract

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.

Pluto's Interaction With Energetic Heliospheric Ions

“…We use here the PSD f in velocity space (particles per volume in real and in velocity space, not momentum space) that is related to the differential intensity j (particles per time, area, energy interval, and solid angle) through f = (m∕v 2 )j with the particle mass m. f 0 is a constant. Above the pickup ion cutoff, at |⃗ v − ⃗ v b | > v b in the solar wind frame, we assume = −1.5, consistent with observations (Fisk & Gloeckler, 2012;Hill & Hamilton, 2010;Kollmann et al, 2019) and = +1 below the pickup cutoff.…”

Influence of Solar Disturbances on Galactic Cosmic Rays in the Solar Wind, Heliosheath, and Local Interstellar Medium: Advanced Composition Explorer, New Horizons, and Voyager Observations

“…The pickup and suprathermal ion and energetic particle measurements are described in more detail elsewhere (McNutt et al 2008;Hill et al 2009;Kollmann et al 2019a) and in Appendix A.2.…”

“…These predominantly GCR protons show the characteristic hallmarks of solar cycle activity, with the local solar cycle #24 maximum (when cosmic rays intensities reach their minimum) occurring in 2015 (see Figure 3) and the recovery period clearly underway by the beginning of 2016. Bottom two grouped panels show lower-energy, local populations, with (d) dominated by helium, mostly pickup ions and suprathermal particles (Kollmann et al 2019a), down to 2 keV nuc −1 , and (e) showing energetic protons (from 20 to 800 keV). It can be seen that the intensities of these lower-energy populations show no obvious solar cycle dependence but more short-term changes.…”

A living catalog of stream interaction regions in the Parker Solar Probe era

“…As the SIR propagates away from the Sun, the compression can lead to the formation of an SIR-associated forward and reverse shock pair at and often beyond 1 au, which efficiently accelerates particles (see Jian et al 2006Jian et al , 2008Richardson 2018, and references therein). As such, SIRs have been found to be a major source of energetic particles in interplanetary space (e.g., Van Hollebeke et al 1981;Tsurutani et al 1982;Richardson 2018;Kollmann et al 2019). As the main source of the high-speed stream, the coronal hole rotates around the solar rotation axis, and the SIR structure rotates with it.…”