Modeling Variation of Elemental Abundances in ``Gradual'' Solar Energetic Particle Events

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2118 Energetic Particles, Solar, 2139 Interplanetary Shocks, 2149 Mhd Waves And Turbulence, 7513 Coronal Mass Ejections, 7514 Energetic Particles (2114)

Scientific paper

Shock acceleration and transport of energetic ions in ``gradual'' solar energetic particle (SEP) events depend sensitively on the interplanetary (IP) waves which scatter them. Since ions with greater mass-to-charge ratios (A/Q) resonate with waves of longer wavelengths, simultaneous observations of a multitude of SEP ion species constitute a probe of the wave spectrum between the source and observer. Our modeling has shown that the key to understanding time variation of SEP elemental abundances is transport through self-amplified IP waves. Our idealized time-dependent model couples the evolution of SEPs and IP Alfvén waves. It includes, for SEPs: particle motion, magnetic focusing, pitch-angle diffusion due to scattering by waves, solar-wind convection, adiabatic deceleration, and time-dependent energetic ion sources at a CME-driven shock; and for waves: SEP-driven wave growth, wave propagation, and other processes. The model accepts various inputs: shock speed, Alfvén speed, initial wave distribution, ion source spectra, etc., allowing us to explore the causes of event to event variability. The model reveals an important consequence of quasilinear wave-particle interaction: SEPs in large events amplify IP Alfvén waves by orders of magnitude in the inner heliosphere. It makes consistent predictions in general agreement with recent observations, which are not explained by conventional models, e.g. (a) (A/Q)-ordered complex time variation of Fe/O and other abundance ratios at equal energy/amu, (b) early He/H ratio descending from >>1 for small events and events with soft proton spectra, but ascending from <1 for large events with hard proton spectra, (c) smaller ion anisotropy for larger events with harder proton spectra, and (d) early plateau of ~~MeV proton intensities at a few hundred particles /(cm2 s sr MeV). The current model provides a starting point for more realistic studies of SEP transport and shock acceleration coupled to wave evolution. Such studies and others on SEP charge states and coronal abundances promise to reveal where and how SEPs are accelerated, as well as how they impact on the IP medium.

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