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Spatial and Temporal Variations of 2 – 10 MeV nuc−1 He/H in Gradual Solar Energetic Particle Events

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In a previous work (Kahler and Brown, 2021; henceforth KB21) we found that 4 – 53 MeV nuc−1 He/H ratios measured at peak intensities of 43 gradual solar energetic-particle (SEP) events were significantly correlated with solar-wind (SW) He/H ratios and with SEP event peak intensities. Here, we extend that work by examining He/H ratios from onsets through peaks of 12 large 1.8 – 10.0 MeV nuc−1 SEP events observed with the Low Energy Telescope (LET) on both STEREO A and B (STA/B) spacecraft when their longitudinal separation angle was < 90∘ in 2013 – 2014. We discuss the challenges of choosing matching five 1-hour periods of STA/B SEP profiles to characterize He/H in the LET energy ranges 1.8 – 3.6, 4.0 – 6.0, and 6.0 – 10.0 MeV nuc−1. The roles of rigidity-dependent transport or of anomalous He production in large variations of He/H are assessed with 6.0 – 10.0 MeV nuc−1 O/He. The STA/B SW data do not include He/H, but we confirm the KB21 results that SEP He/H extends over a range of 0.001 to 0.1, decreasing with increasing energy, and increasing with event peak H intensities. Differences of STA/B log He/H are not correlated with STA/B angular separations. The six cases of SEP events occurring within interplanetary coronal mass ejections (ICMEs) show somewhat higher H peak intensities and He/H than those outside ICMEs. We interpret the low He/H events as predominately SEP3 and high He/H as SEP4 events in the Reames (2020) SEP event-classification system.

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Acknowledgments

S.K., D.B, and A.L. were supported by AFOSR grant 18RV-COR122, the AFRL Space Scholar Program, and AFRL contract FA9453-15-C-0050, respectively. We acknowledge use of the LET SEP data and the ICME and SIR lists at the STEREO web site. This work benefits from discussions within the International Space Science Institute (ISSI) Team ID 425 ‘Origins of 3He-rich SEPs’. Comments by the reviewer substantially improved the manuscript.

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Authors and Affiliations

  1. Air Force Research Laboratory, Space Vehicles Directorate, 3550 Aberdeen Ave., Kirtland AFB, NM, 87117, USA

    S. W. Kahler

  2. University of North, Texas, USA

    D. Brown

  3. Atmospheric Environmental Research, Albuquerque, NM, 87110, USA

    A. G. Ling

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  1. S. W. Kahler
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Correspondence to S. W. Kahler.

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Kahler, S.W., Brown, D. & Ling, A.G. Spatial and Temporal Variations of 2 – 10 MeV nuc−1 He/H in Gradual Solar Energetic Particle Events. Sol Phys 297, 51 (2022). https://doi.org/10.1007/s11207-022-01971-0

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