In this study we evaluate the effect of power-law spectra with rigidity dependent exponential cut-off, as often observed, on the mean ionic charge of Fe. We show that power law spectra with an exponential cut-off, with e-folding energy E0 (M/Q) ˜ (Q/M)alpha , alpha ≈ 1, and E0
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[Show abstract][Hide abstract] ABSTRACT: The spectral and compositional characteristics of solar energetic particles (SEPs) reflect a number of factors, including the nature of the acceleration process and the accessible seed population. We review recent discoveries about these issues and discuss current hypotheses about the relationship between flares and CME-driven shocks in producing SEPs above a few tens of MeV/nucleon in large, gradual events. We also briefly review recent results on spectra and composition in impulsive events. We particularly emphasize how significant correlations among spectral and compositional characteristics serve to constrain models of SEP production.
[Show abstract][Hide abstract] ABSTRACT: We have surveyed the energy spectra of ~0.1–100 MeV nucleon^(-1) C, O, and Fe nuclei associated with the
passage of 72 interplanetary (IP) shocks observed on board the ACE spacecraft during the period 1997 October–2002 October. Our main results are as follows: (1) The spectral fit parameters are independent of the local shock
properties. (2) About 7% of the events exhibit increasing Fe/O ratios with energy; the remaining events have
Fe/O ratios that either remain constant or decrease with energy. (3) The Fe/O ratio in the shock-associated
particles is typically ~30% lower than in the ambient population. (4) The fractionation pattern of the elemental
abundances, the O spectra, and the energy-dependence of Fe/O at the IP shocks are remarkably similar to those of
the ambient interplanetary suprathermal ion population. We suggest that the IP shocks studied here reaccelerate
energetic particle seed spectra composed of ions from impulsive and gradual solar energetic particle events by
systematic rigidity-dependent mechanisms in which higher rigidity ions are accelerated less efficiently than lower
Full-text · Article · Dec 2008 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: Shocks driven by fast coronal mass ejections (CMEs) are generally believed to be the dominant accelerators in large, gradual solar energetic particle (SEP) events. A key challenge for this notion has been the highly variable spectral and compositional characteristics of these events above a few tens of MeV per nucleon. We have recently proposed that this variability results from the interplay of two factors: evolution in the shock-normal angle as the shock moves outward from the Sun; and a compound seed population, typically comprising at least suprathermals from the corona (or solar wind) and suprathermals from flares. We present here a simple analytical implementation of these ideas. Our calculations semiquantitatively reproduce key features of the observed variability, including spectral morphologies and energy dependence in Fe/O, 3He/4He, and mean ionic charges, in ways that are consistent with correlations in the data. The model makes a prediction for the average high-energy Fe/O enhancement that is borne out by 30 years of observations; the model also provides a quantitative explanation for the Breneman & Stone fractionation effect, a fundamental but previously unexplained aspect of SEP phenomenology. Our calculations must be bolstered by future efforts incorporating realistic CME-shock simulations and a rigorous treatment of particle transport. Suprathermal densities in the corona, as well as details of the injection process at shocks of arbitrary obliquity, require further investigation. Nevertheless, these first results suggest a comprehensive framework for understanding the complexity of high-energy variability in terms of shock physics for most, if not all, large SEP events.
Preview · Article · Dec 2008 · The Astrophysical Journal