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Paper: Effect of Solar-Wind Velocity, Magnetic Field and Density on Solar Energetic Particle Transport
Volume: 484, Outstanding Problems in Heliophysics: From Coronal Heating to the Edge of the Heliosphere
Page: 156
Authors: Ng, C. K.
Abstract: In large gradual solar energetic particle (SEP) events, energetic protons greatly amplify ambient upstream Alfvén waves near coronal-mass-ejection (CME) driven shocks. The waves grow until they are swept downstream of the shock. The amplified waves scatter the particles and “flatten” the SEP intensity energy spectrum at low energy at 1 AU, causing the streaming limit phenomenon. Both the wave and SEP intensities maximize near the shock and fall steeply with distance upstream. The SEPs are focused by the longitudinal gradient of the magnetic field B. The wave growth rate increases with energetic proton streaming and varies as f/√(np), with f the energetic proton phase-space density and np the plasma proton number density. Thus, in addition to the SEP release rate at the shock, the environmental quantities: np(r), B(r), the solar-wind velocity Vsw(r), and the Alfvén speed VA(r) also influence SEP transport. At heliocentric distance r≲ 8r, np as well as B deviate significantly from ∼ r–2, Vsw rises slowly from near zero on the photosphere, and VA peaks near 4 r. We have generalized our SEP transport model to take account of realistic radial dependences of the above solar-wind properties down to ∼2 r in addition to the usual processes of wave and particle transport and Alfvén wave growth. The model has been applied to STEREO A observation of the 2011 March 21 SEP event with the preliminary conclusion that wave-damping processes rather than the environmental quantities are more likely to raise the predicted proton intensity at < 5 MeV to the higher observed values.
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