Sixty Years of Element Abundance Measurements in Solar Energetic Particles

Sixty years ago the first observation was published showing solar energetic particles (SEPs) with a sampling of chemical elements with atomic numbers $6 \leq Z \leq 18$ 6 ≤ Z ≤ 18 above 40 MeV amu−1. Thus began study of the direct products of dynamic physics in the solar corona. As we have progressed from 4-min sounding-rocket samples to continuous satellite coverage of SEP events, we have extended the observations to the unusual distribution of element abundances throughout the periodic table. Small “impulsive” SEP events from islands of magnetic reconnection on open magnetic-field lines in solar jets generate huge enhancements in abundances of 3He and of the heaviest elements, enhancements increasing as a power of the ion mass-to-charge ratio as ($A$ A /$Q$ Q )3.6, on average. Solar flares involve the same physics but there the SEPs are trapped on closed loops, expending their energy as heat and light. The larger, energetic “gradual” SEP events are accelerated at shock waves driven by fast, wide coronal mass ejections (CMEs). However, these shocks can also reaccelerate ions from pools of residual suprathermal impulsive ions, and CMEs from jets can also drive fast shocks, complicating the picture. The underlying element abundances in SEP events represent the solar corona, which differs from corresponding abundances in the photosphere as a function of the first ionization potential (FIP) of the elements, distinguishing low-FIP (<10 eV) ions from high-FIP neutral atoms as they expand through the chromosphere. Differences in FIP patterns of SEPs and the solar wind may distinguish closed- and open-field regions of formation at the base of the corona. Dependence of SEP acceleration upon $A$ A /$Q$ Q allows best-fit estimation of ion $Q$ Q -values and hence of the source plasma temperature of ∼1 – 3 MK, derived from abundances, which correlates with recent measures of temperatures using extreme ultraviolet emission from jets. Thus, element abundances in SEPs have become a powerful tool to study the underlying solar corona and to probe physical processes of broad astrophysical significance, from the “FIP effect” to magnetic reconnection and shock acceleration. New questions arise, however, about the theoretical basis of correlations of energy-spectral indices with power-laws of abundances, about the coexistence of separate resonant and non-resonant mechanisms for enhancements of 3He and of heavy elements, about occasional events with unusual suppression of He and about the overall paucity of C in FIP comparisons.