Energy Spectra vs. Element Abundances in Solar Energetic Particles and the Roles of Magnetic Reconnection and Shock Acceleration

Abstract

We reexamine the relationship between energy spectral indices and element abundance enhancements in solar energetic particle (SEP) events at energies of a few MeV amu−1. We find a correlated behavior only in the largest gradual SEP4 events when all ions are accelerated from the ambient coronal plasma by shock waves driven by fast, wide coronal mass ejections (CMEs). This correlated abundance behavior can track complex time variations in the spectral indices during an event. In other (SEP3) events, CME-driven shock waves, days apart, sample seed particles from a single pool of suprathermal impulsive ions contributed earlier. Of the smaller, Fe-rich, impulsive SEP events, previously related to magnetic reconnection in solar jets, over half are subsequently reaccelerated by CME-driven shock waves (SEP2), causing typical ion intensities to have a 64% correlation with shock speeds. In these SEP2 events, the onset of shock acceleration is signaled by a new component in the abundances, large proton excesses. The remaining SEP1 events lack evidence of shock acceleration. However, for all these events (SEP1–SEP3) with abundances determined by magnetic reconnection, energy spectra and abundances are decoupled.

Element abundance and the physics of solar energetic particles

“…The main reason for the systematic decline with A/Q may be that higher-rigidity elements leak away faster. The energy spectra of ions are correlated with variations in A/Q in these events (Reames, 2021d;Reames, 2022a) and are also relatively steep.…”

“…seed particles, as indicated by the lower temperature, the inclusion of protons in a single seed population, and the return to heavyelement suppression behind the shock. Where abundances show an increase in A/Q, the energy spectra become quite flat in the plateau region (Reames and Ng, 2010;Ng et al, 2012;Reames and Ng, 2014;Reames, 2021a) because of the underlying correlation between abundances and spectra (Reames, 2021d;Reames, 2022a).…”

“…However, large gradual SEP events, even GLEs, can pick up pre-accelerated residual impulsive seed particles from multijet collections, often observed to accumulate near active regions (Desai et al, 2003;Wiedenbeck et al, 2008;Bučík et al, 2014;Bučík et al, 2015;Chen et al, 2015;Reames, 2022a). Recently, Kouloumvakos et al (2023) found an average connection time to 3 He-rich active regions of 4.1 ± 1.8 days, suggesting a width of ∼52⁰ in longitude.…”

“…CME speeds for the three events are 830, 1,199, and 2,465 km s -1 , and the last two events are both GLEs. These "double-dipping" SEP3 events are not uncommon (Reames, 2022a) and often include GLEs.…”

Solar Energetic Particle Charge States and Abundances with Nonthermal Electrons

“…Reames and collaborators have explored these diagnostics in a series of papers (Reames et al 1999;Reames 2000;Reames & Tylka 2002;Reames et al 2014a;Reames 2014Reames , 2016Reames , 2017Reames , 2018aReames , 2020Reames , 2022a. In some events, the average charge state 〈Q〉 of Fe increases with energy, indicating stripping of the Fe ions during acceleration in high-density regions (Barghouty & Mewaldt 1999;Kovaltsov et al 2001;DiFabio et al 2008), but Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.…”