Persistent Sunward Flow of ~1.6 MeV Protons at 1 AU

Abstract

It has been found that except during the onset of prompt solar particle events the diffusive flow of 1.3 to 2.3 MeV protons at 1 AU is predominantly sunward. The anisotropy of the proton intensity has been measured with the Caltech Electron/Isotope Spectrometer aboard IMP-7 for periods between prompt solar particle events from 72/273 to 74/2. The diffusive anisotropy, which has been determined by subtracting the independently determined convective anisotropy from the observed anisotropy, is predominantly directed toward the sun with a mean radial component of 14%. This sunward diffusion is typical of intensities from 0.012 to 1.2 (cm^² sec sr MeV)^(−1) for 1.3 - 2.3 MeV protons and indicates that a positive radial gradient is characteristic of these modestly enhanced fluxes. The direction of the flow is opposite to that which would be produced by previously proposed models which involve a continuous solar source for the observed particles. A steady-state propagation model which includes adiabatic energy loss with particle injection beyond 1 AU produces the average observed anisotropy for κ_rr∼4 × 10^20 cm^² sec^(−1).

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