Persistent Sub-MeV Ion Enhancement Downstream of an Interplanetary Shock: Multispacecraft Observations near L1

Abstract On 2026 February 4, an interplanetary shock crossed the L1 region and was followed about 7.7 hr later by a heliospheric current sheet/heliospheric plasma sheet (HCS/HPS), while energetic-ion intensities continued to evolve throughout the intervening downstream region. The shock was observed by IMAP, Wind, ACE, DSCOVR, and SOLAR-1, providing a multispacecraft view of its geometry. Multispacecraft timing and local shock-normal analyses show that the shock is highly quasiperpendicular but spatially nonplanar across L1. Energetic-ion intensities exhibit a pronounced energy-dependent response. Sub-MeV intensities increase well upstream of the shock and remain enhanced throughout the downstream region until approaching the HCS/HPS, whereas intensities above ∼1 MeV show a much weaker downstream enhancement and tend to peak near the shock. The energy spectra also soften downstream, with the fitted spectral break shifting from several MeV near the shock to lower energies farther downstream. Pitch-angle distributions evolve from pronounced field-aligned streaming upstream to enhanced intensities at oblique-to-perpendicular pitch angles downstream, with the anisotropy persisting through much of the downstream sheath and gradually broadening toward the HCS/HPS. A secondary current sheet embedded within the downstream HPS exhibits signatures consistent with a localized reconnection exhaust. Overall, the observations indicate that the sub-MeV ion population is strongly influenced by the shock, while its subsequent downstream evolution is shaped primarily by energy-dependent transport. Localized energization within the disturbed downstream sheath may provide an additional contribution to the sub-MeV population.

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Publication Details

Journal
The Astrophysical Journal Supplement Series
Published
2026-11-04
DOI
https://doi.org/10.3847/1538-4365/aeb300
Primary Topic
Solar and Space Plasma Dynamics
Type
article
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article

Persistent Sub-MeV Ion Enhancement Downstream of an Interplanetary Shock: Multispacecraft Observations near L1

Yiming Jiao, Keiichi Ogasawara, M. J. Starkey, M. I. Desai et al.
The Astrophysical Journal Supplement Series
Solar and Space Plasma Dynamics
article

Persistent Sub-MeV Ion Enhancement Downstream of an Interplanetary Shock: Multispacecraft Observations near L1

Yiming Jiao, Keiichi Ogasawara, M. J. Starkey, M. I. Desai, Leng Ying Khoo, Ashok Silwal, J. Giacalone, T. S. Horbury, John Grant Mitchell, Xingyu Zhu, Jamie S. Rankin, R. B. Torbert, Gary Paul Zank, Lingling Zhao, S. Livi, Drew L. Turner, M. A. Dayeh, Zeping Jin, E. R. Christian, M. E. Cuesta, M. Gkioulidou, Samuel T. Hart, D. Vassiliadis, Savvas Raptis, David J. McComas, Nibuna S.M. Subashchandar
article en

Abstract

Abstract On 2026 February 4, an interplanetary shock crossed the L1 region and was followed about 7.7 hr later by a heliospheric current sheet/heliospheric plasma sheet (HCS/HPS), while energetic-ion intensities continued to evolve throughout the intervening downstream region. The shock was observed by IMAP, Wind, ACE, DSCOVR, and SOLAR-1, providing a multispacecraft view of its geometry. Multispacecraft timing and local shock-normal analyses show that the shock is highly quasiperpendicular but spatially nonplanar across L1. Energetic-ion intensities exhibit a pronounced energy-dependent response. Sub-MeV intensities increase well upstream of the shock and remain enhanced throughout the downstream region until approaching the HCS/HPS, whereas intensities above ∼1 MeV show a much weaker downstream enhancement and tend to peak near the shock. The energy spectra also soften downstream, with the fitted spectral break shifting from several MeV near the shock to lower energies farther downstream. Pitch-angle distributions evolve from pronounced field-aligned streaming upstream to enhanced intensities at oblique-to-perpendicular pitch angles downstream, with the anisotropy persisting through much of the downstream sheath and gradually broadening toward the HCS/HPS. A secondary current sheet embedded within the downstream HPS exhibits signatures consistent with a localized reconnection exhaust. Overall, the observations indicate that the sub-MeV ion population is strongly influenced by the shock, while its subsequent downstream evolution is shaped primarily by energy-dependent transport. Localized energization within the disturbed downstream sheath may provide an additional contribution to the sub-MeV population.

The Astrophysical Journal Supplement SeriesVol. 288(1)
Southwest Research Institute (US), Goddard Space Flight Center (US), National Oceanic and Atmospheric Administration (US), University of Arizona (US), Johns Hopkins University (US), University of New Hampshire (US), Princeton University (US), University of Michigan (US), NOAA National Environmental Satellite Data and Information Service (US), NOAA Office of Space Weather Observations (US), Imperial College London (GB), University of Alabama in Huntsville (US)
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