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Published February 2020 | Published
Journal Article Open

Properties of Suprathermal-through-energetic He Ions Associated with Stream Interaction Regions Observed over the Parker Solar Probe's First Two Orbits


The Integrated Science Investigation of the Sun (IS⊙IS) suite on board NASA's Parker Solar Probe (PSP) observed six distinct enhancements in the intensities of suprathermal-through-energetic (~0.03–3 MeV nucleon⁻¹) He ions associated with corotating or stream interaction regions (CIR or SIR) during its first two orbits. Our results from a survey of the time histories of the He intensities, spectral slopes, and anisotropies and the event-averaged energy spectra during these events show the following: (1) In the two strongest enhancements, seen at 0.35 and 0.85 au, the higher-energy ions arrive and maximize later than those at lower energies. In the event seen at 0.35 au, the He ions arrive when PSP was away from the SIR trailing edge and entered the rarefaction region in the high-speed stream. (2) The He intensities either are isotropic or show sunward anisotropies in the spacecraft frame. (3) In all events, the energy spectra between ~0.2 and 1 MeV nucleon⁻¹ are power laws of the form ∝E⁻². In the two strongest events, the energy spectra are well represented by flat power laws between ~0.03 and 0.4 MeV nucleon⁻¹ modulated by exponential rollovers between ~0.4 and 3 MeV nucleon⁻¹. We conclude that the SIR-associated He ions originate from sources or shocks beyond PSP's location rather than from acceleration processes occurring at nearby portions of local compression regions. Our results also suggest that rarefaction regions that typically follow the SIRs facilitate easier particle transport throughout the inner heliosphere such that low-energy ions do not undergo significant energy loss due to adiabatic deceleration, contrary to predictions of existing models.

Additional Information

© 2020. The Author(s). Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2019 December 6; revised 2019 December 27; accepted 2019 December 27; published 2020 February 3. Early Results from Parker Solar Probe: Ushering a New Frontier in Space Exploration This work was supported by NASA's Parker Solar Probe Mission, contract NNN06AA01C. We thank all the scientists and engineers who have worked hard to make PSP a successful mission, in particular the engineers, scientists, and administrators who designed and built IS⊙IS/EPI-Lo, IS⊙IS/EPI-Hi, FIELDS, and SWEAP instrument suites and support their operations and the scientific analysis of its data. For their contributions to the scientific configuration and instrumental analysis, we owe special thanks to P. Kollmann, J. Peachy, and J. Vandegriff at JHU/APL for EPI-Lo and B. Kecman and W. R. Cook at Caltech for EPI-Hi. The IS⊙IS data are available at http://spp-isois.sr.unh.edu/data_public/, as well as at the NASA Space Physics Data Facility. Work at SwRI is also supported in part under NASA grants 80NSSC17K0009, NNX15AG09G, 80NSSC18K0520, NNX17AI17G, 80NSSC19K0079, and NNX13AI75G. S.D.B. acknowledges support of the Leverhulme Trust Visiting Professorship program.

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