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Author Li, Gen ♦ Lee, Martin A.
Source United States Department of Energy Office of Scientific and Technical Information
Content type Text
Language English
Subject Keyword ASTROPHYSICS, COSMOLOGY AND ASTRONOMY ♦ ACCELERATION ♦ ALFVEN WAVES ♦ COMPRESSION RATIO ♦ DIFFUSION ♦ INDEXES ♦ MAGNETIC FIELDS ♦ MEAN FREE PATH ♦ QUASILINEAR PROBLEMS ♦ SCATTERING ♦ SHOCK WAVES ♦ SOLAR CYCLE ♦ SOLAR PROTONS ♦ SOLAR WIND ♦ SUN ♦ WAVE PROPAGATION
Abstract The effects of scatter-dominated interplanetary transport on the spectral properties of the differential fluence of large gradual solar energetic particle (SEP) events are investigated analytically. The model assumes for simplicity radial constant solar wind and radial magnetic field. The radial diffusion coefficient is calculated with quasilinear theory by assuming a spectrum of Alfvén waves propagating parallel to the magnetic field. Cross-field transport is neglected. The model takes into consideration several essential features of gradual event transport: nearly isotropic ion distributions, adiabatic deceleration in a divergent solar wind, and particle radial scattering mean free paths increasing with energy. Assuming an impulsive and spherically symmetric injection of SEPs with a power-law spectrum near the Sun, the predicted differential fluence spectrum exhibits at 1 AU three distinctive power laws for different energy domains. The model naturally reproduces the spectral features of the double power-law proton differential fluence spectra that tend to be observed in extremely large SEP events. We select nine western ground-level events (GLEs) out of the 16 GLEs during Solar Cycle 23 and fit the observed double power-law spectra to the analytical predictions. The compression ratio of the accelerating shock wave, the power-law index of the ambient wave intensity, and the proton radial scattering mean free path are determined for the nine GLEs. The derived parameters are generally in agreement with the characteristic values expected for large gradual SEP events.
ISSN 0004637X
Educational Use Research
Learning Resource Type Article
Publisher Date 2015-09-01
Publisher Place United States
Journal Astrophysical Journal
Volume Number 810
Issue Number 1


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