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.