Solar particle propagation from 1 to 5 AU

Abstract

A statistical analysis of solar particle events, observed by the GSFC-UNH charged particle detector on board Pioneer 10 and Pioneer 11 from March 1972 to December 1974 (from 1 to 5 AU for each spacecraft), is carried out with the goal of experimentally determining the statistical average interplanetary propagation conditions from 3 to 30 MeV. A numerical propagation model is developed that includes diffusion with a diffusion coefficient of the form k _r =k _o r ^β, convection, adiabatic deceleration, and a variable coronal injection profile. The statistical analysis is carried out by individually analyzing each of five parameters (t _max, ξ(t_max), Δt ₅, τ) that are uniquely defined in a solar particle event. Each of the five parameter data sets were analyzed in terms of both a spacecraft-solar flare connection longitude ≤50°, and a numerical model that employed a variable exponential decaying coronal injection profile.

The five individual parameter analyses are combined with the results that the statistical average radial interplanetary diffusion coefficient from 1 to 5 AU is given by 〈k _r〉 = (1.2 ± 0.4) × 10²¹ cm² s^-1 with 〈β〉 = 0.0± 0.3 for 3.4 to 5.2 MeV protons and 〈k _r〉 = (2.6 ± 0.6) × 10²¹ cm² s^-1 with (β) = 0.0± 0.3 for 24 to 30 MeV protons. Using the classical relationship for the radial scattering mean free path λ_r, i.e. k _r = υλ_r/3, we obtain 〈λ_r〉 = 0.09 ± 0.03 AU and 0.075 ± 0.020 AU for the low and high energy data, respectively. These results show, from 1 to 5 AU and from 3 to 30 MeV, that 〈λ_r〉 is both independent of radial distance and approximately independent of rigidity (for 〈λ_r〉~P ^α, where P = rigidity, α = -0.15 ± 0.20).

The above diffusion coefficients are inconsistent With both the predictions of the diffusion coefficient from present theoretical transport models and with the diffusion coefficient used in modulation studies at low energies.