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 ISʘIS/EPI-Lo, ISʘIS/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. PSP data are available at the NASA Space Physics Data Facility. Work at SwRI is supported in part under NASA grants 80NSSC20K1815, 80NSSC18K1446, 80NSSC21K0112, 80NSSC20K1255, and 80NSSC21K0971. W.H.M. is partially supported by the PSP/ISʘIS project at the University of Delaware, subcontract SUB0000317 from Princeton University, and by NASA PSP Guest Investigator grant 80NSSC21K1765. A.F. was supported by NASA grant 80NSSC23K1625. J.F.D. and M.S. were supported by the FIELDS team of the Parker Solar Probe (NNN06AA01C), by NASA grants 80NSSC20K1813, 80NSSC22K0352, and 80NSSC25K7754, and NSF grant PHY2109083.
Proton and Heavy Ion Acceleration by Magnetic Reconnection at the Near-Sun Heliospheric Current Sheet
Creators
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1.
Southwest Research Institute
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2.
The University of Texas at San Antonio
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3.
University of Maryland, College Park
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4.
Princeton University
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5.
University of California, Berkeley
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6.
Johns Hopkins University Applied Physics Laboratory
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7.
California Institute of Technology
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8.
Goddard Space Flight Center
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9.
University of New Hampshire
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10.
University of Delaware
Abstract
Magnetic reconnection at the near-Sun heliospheric current sheet (HCS) dissipates the Parker spiral and converts magnetic energy into plasma kinetic energy. During Encounter 14 at a radial distance of ~16.25 R⊙, Parker Solar Probe observed an HCS crossing where reconnection-driven acceleration—likely facilitated by merging large-scale flux tubes—energized protons up to ∼400 keV. This energy gain is ≈1000 times greater than the available magnetic energy per particle. We present here a comprehensive analysis of pitch-angle distributions and differential energy spectra for protons and heavy ions (He, O, and Fe) in conjunction with local wave activity during this crossing. Our results provide the first direct in situ observations of simultaneous proton and heavy-ion energization during HCS reconnection. Crucially, we find that heavy-ion power-law spectral indices differ significantly from those of protons, contradicting previous simulations that predict species-independent slopes. We further demonstrate that ion beams and anisotropies produced during reconnection drive waves in the ion cyclotron range of frequencies. Finally, we show that proton pitch-angle scattering is stronger than that of heavy ions, which may account for the flatter spectra or harder spectral indices observed in the heavy-ion populations. These observations provide definitive evidence for in situ reconnection-driven acceleration at the near-Sun HCS and necessitate the inclusion of species-dependent transport and acceleration efficiencies in contemporary reconnection-based particle energization models.
Copyright and License
© 2026. The Author(s). Published by the American Astronomical Society. 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.
Acknowledgement
Files
Desai_2026_ApJ_1000_300.pdf
Additional details
Funding
- National Aeronautics and Space Administration
- NNN06AA01C
- National Aeronautics and Space Administration
- 80NSSC20K1815
- National Aeronautics and Space Administration
- 80NSSC18K1446
- National Aeronautics and Space Administration
- 80NSSC21K0112
- National Aeronautics and Space Administration
- 80NSSC20K1255
- National Aeronautics and Space Administration
- 80NSSC21K0971
- University of Delaware
- SUB0000317
- National Aeronautics and Space Administration
- 80NSSC21K1765
- National Aeronautics and Space Administration
- 80NSSC23K1625
- National Aeronautics and Space Administration
- 80NSSC20K1813
- National Aeronautics and Space Administration
- 80NSSC22K0352
- National Aeronautics and Space Administration
- 80NSSC25K7754
- National Science Foundation
- PHY2109083
Dates
- Submitted
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2026-01-09
- Accepted
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2026-02-20
- Available
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2026-03-31Published
Caltech Custom Metadata
- Caltech groups
- Physics Department , Space Radiation Laboratory , Division of Physics, Mathematics and Astronomy (PMA)
- Publication Status
- Published