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

Desai, M. I. and Mitchell, D. G. and Szalay, J. R. and Roelof, E. C. and Giacalone, J. and Hill, M. E. and McComas, D. J. and Christian, E. R. and Schwadron, N. A. and McNutt, R. L., Jr. and Wiedenbeck, M. E. and Joyce, C. and Cohen, C. M. S. and Ebert, R. W. and Dayeh, M. A. and Allen, R. C. and Davis, A. J. and Krimigis, S. M. and Leske, R. A. and Matthaeus, W. H. and Malandraki, O. and Mewaldt, R. A. and Labrador, A. and Stone, E. C. and Bale, S. D. and Pulupa, M. and MacDowall, R. J. and Kasper, J. C. (2020) Properties of Suprathermal-through-energetic He Ions Associated with Stream Interaction Regions Observed over the Parker Solar Probe’s First Two Orbits. Astrophysical Journal Supplement Series, 246 (2). Art. No. 56. ISSN 1538-4365. doi:10.3847/1538-4365/ab65ef.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription ItemIS⊙IS data
Desai, M. I.0000-0002-7318-6008
Mitchell, D. G.0000-0003-1960-2119
Szalay, J. R.0000-0003-2685-9801
Roelof, E. C.0000-0002-2270-0652
Hill, M. E.0000-0002-5674-4936
McComas, D. J.0000-0001-6160-1158
Christian, E. R.0000-0003-2134-3937
Schwadron, N. A.0000-0002-3737-9283
McNutt, R. L., Jr.0000-0002-4722-9166
Wiedenbeck, M. E.0000-0002-2825-3128
Joyce, C.0000-0002-3841-5020
Cohen, C. M. S.0000-0002-0978-8127
Ebert, R. W.0000-0002-2504-4320
Dayeh, M. A.0000-0001-9323-1200
Allen, R. C.0000-0003-2079-5683
Krimigis, S. M.0000-0003-2781-2386
Leske, R. A.0000-0002-0156-2414
Matthaeus, W. H.0000-0001-7224-6024
Malandraki, O.0000-0002-4751-6835
Mewaldt, R. A.0000-0003-2178-9111
Labrador, A.0000-0001-9178-5349
Stone, E. C.0000-0002-2010-5462
Bale, S. D.0000-0002-1989-3596
Pulupa, M.0000-0002-1573-7457
MacDowall, R. J.0000-0003-3112-4201
Kasper, J. C.0000-0002-7077-930X
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, 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.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Leverhulme TrustUNSPECIFIED
Subject Keywords:Solar wind ; Corotating streams ; Interplanetary particle acceleration
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Solar wind (1534); Corotating streams (314); Interplanetary particle acceleration (826)
Record Number:CaltechAUTHORS:20200203-092721940
Persistent URL:
Official Citation:M. I. Desai et al 2020 ApJS 246 56
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101065
Deposited By: George Porter
Deposited On:03 Feb 2020 22:47
Last Modified:16 Nov 2021 17:58

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