Helium Suppression in Impulsive Solar Energetic-Particle Events

Abstract

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 < He/O < 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 < Z < 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 C/O = 0.08 +- 0.04, sup-pressed by a factor over 5 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/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.

28 pages, 12 figures, accepted by Solar Physics

References in corpus (10)

• Abundance Enhancements in Impulsive Solar Energetic-Particle Events with Associated Coronal Mass Ejections
• Multi-Spacecraft Observations of Recurrent 3He-Rich Solar Energetic Particles
• Abundances, Ionization States, Temperatures, and FIP in Solar Energetic Particles
• 3He-Rich Solar Energetic Particles in Helical Jets on the Sun
• Variations in Abundance Enhancements in Impulsive Solar Energetic-Particle Events and Related CMEs and Flares
• The "FIP Effect" and the Origins of Solar Energetic Particles and of the Solar Wind
• The Abundance of Helium in the Source Plasma of Solar Energetic Particles
• Hydrogen and the Abundances of Elements in Impulsive Solar Energetic-Particle Events
• Hydrogen and the Abundances of Elements in Gradual Solar Energetic-Particle Events
• Corotating Shock Waves and the Solar-Wind Source of Energetic Ion Abundances: Power Laws in A/Q

Cited by in corpus (12)

• Solar Energetic Particles (Second Edition)
• Four Distinct Pathways to the Element Abundances in Solar Energetic Particles
• Sixty Years of Element Abundance Measurements in Solar Energetic Particles
• Hydrogen and the Abundances of Elements in Impulsive Solar Energetic-Particle Events
• Hydrogen and the Abundances of Elements in Gradual Solar Energetic-Particle Events
• Fifty Years of 3He-rich Events
• Solar Energetic Particles: Spatial Extent and Implications of the H and He Abundances
• Excess H, Suppressed He, and the Abundances of Elements in Solar Energetic Particles
• Element Abundances of Solar Energetic Particles and the Photosphere, the Corona, and the Solar Wind
• Seeds and Sequences of Element Abundances in Solar Energetic Particle Events
• Variable Ion Compositions of Solar Energetic Particle Events in the Inner Heliosphere: A Field-line Braiding Model with Compound Injections
• A Perspective on Solar Energetic Particles