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The Abundance of Helium in the Source Plasma of Solar Energetic Particles

Abstract

Studies of patterns of abundance enhancements of elements, relative to solar coronal abundances, in large solar energetic-particle (SEP) events, and of their power-law dependence on the mass-to-charge ratio, \(A/Q\), of the ions, have been used to determine the effective source-plasma temperature, \(T\), that defines the \(Q\)-values of the ions. We find that a single assumed value for the coronal reference He/O ratio in all SEP events is often inconsistent with the transport-induced power-law trend of the other elements. In fact, the coronal He/O varies rather widely from one SEP event to another. In the large Fe-rich SEP events with \(T \approx 3~\mbox{MK}\), where shock waves, driven out by coronal mass ejections (CMEs), have reaccelerated residual ions from impulsive suprathermal events that occur earlier in solar active regions, \(\text{He}/\text{O} \approx 90\), a ratio similar to that in the slow solar wind, which may also originate from active regions. Ions in the large SEP events with \(T < 2~\mbox{MK}\) may be accelerated outside active regions, and have values of \(40 \leq \text{He}/\text{O} \leq 60\). Mechanisms that determine coronal abundances, including variations of He/O, are likely to occur near the base of the corona (at \(\approx 1.1~\mathrm{R}_{\mathrm{S}}\)) and thus to affect both SEPs (at 2 – \(3~\mathrm{R}_{\mathrm{S}}\)) and the solar wind. Other than He, reference coronal abundances for heavier elements show little temperature dependence or systematic difference between SEP events; He, the element with the highest first-ionization potential, is unique. The CME-driven shock waves probe the same regions of space, at \(\approx 2~\mathrm{R}_{\mathrm{S}}\) near active regions, which are also likely sources of the slow solar wind, providing complementary information on conditions in those regions.

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Acknowledgements

The author thanks Steve Kahler for many helpful discussions of subjects included in this manuscript.

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Correspondence to Donald V. Reames.

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Reames, D.V. The Abundance of Helium in the Source Plasma of Solar Energetic Particles. Sol Phys 292, 156 (2017). https://doi.org/10.1007/s11207-017-1173-5

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  • DOI: https://doi.org/10.1007/s11207-017-1173-5

Keywords

  • Solar energetic particles
  • Solar flares
  • Coronal mass ejections
  • Solar system abundances
  • Solar wind