Solar Physics

, 294:32 | Cite as

Helium Suppression in Impulsive Solar Energetic-Particle Events

  • Donald V. ReamesEmail author


We have studied the element abundances and energy spectra of the small “He-poor” impulsive solar energetic-particle (SEP) events, comparing them with other impulsive SEP events with more-normal abundances of He. He-poor events can have abundances as low as He/O ≈ 2, while both impulsive and gradual SEP events usually have source abundances of \(30 \leq \mbox{He/O} \leq 100\) with mean values of ≈ 50 – 60. He/C ratios are not only low, but often decrease with energy in He-poor events. Abundance enhancement patterns of other elements with atomic numbers \(6 \leq Z \leq 56\), and likely values of their mass-to-charge ratios \(A\)/\(Q\), are generally unaltered in He-poor events, as are the probable source-plasma temperatures of 2.5 – 3.2 MK for all impulsive SEP events. One He-poor event is also an example of a rarer C-poor event with \(\mbox{C/O} = 0.08 \pm 0.04\), suppressed by a factor over five from the mean. We discuss suggestions of a possible \(A\)/\(Q\) threshold during acceleration and of the sluggish ionization of He entering the corona, because of its uniquely high first ionization potential (FIP), but the suppression of He and the decline of He/C with energy is difficult to explain if both He and C are fully ionized with \(A\mbox{/}Q = 2\) as expected at 2.5 – 3.2 MK. Although less dramatic, a possible excess enhancement of Ne in some impulsive SEP events is also considered. Possible causes of the large ≈ 30% spectral and abundance variations in impulsive events are also discussed. However, the physics of the He-poor events remains a mystery.


Solar energetic particles Solar system abundances Solar flares 



The author thanks Chee Ng and Steve Kahler for helpful comments on this manuscript.

Disclosure of Potential Conflicts of Interest

The author declares he has no conflicts of interest.


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© Springer Nature B.V. 2019

Authors and Affiliations

  1. 1.Institute for Physical Science and TechnologyUniversity of MarylandCollege ParkUSA

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