Azimuthal propagation of low-energy solar-flare protons as observed from spacecraft very widely separated in solar azimuth

Abstract

The azimuthal propagation of solar protons with kinetic energies in the range ∼10 to ∼30 Mev is investigated by using observations made in the period December 1967 through August 1968 with University of Chicago instruments on board the earth satellite Imp 4 and the deep‐space probes Pioneer 6 and Pioneer 7, which were very widely separated from the earth during this period. In the period studied, nine solar‐proton events were observed simultaneously at all three spacecraft. In 6 of the 9 events discussed, the proton fluxes approached uniformity around the sun at the orbit of earth, and in 5 of these 6, the decay was divisible into 2 phases. During phase 1 azimuthal gradients were strong and the decay of the flux was fast, with exponential time‐decay constants in the range ∼10 to 20 hours. During phase 2, azimuthal gradients were weak or nonexistent and decay was slow, with time constants generally in excess of 40 hours. The times required for rise to maximum intensity and approach to uniformity, if interpreted in terms of the 1967 model of L. F. Burlaga, are found consistent with the existence of diffusion coefficients κ∥ > 4.3 × 1020 cm²/sec and κ⊥ ∼ 1020 to 1021 cm²/sec at the orbit of earth. From observations of the energy spectra of the protons in these events, it is concluded that the azimuthal propagation process did not depend strongly on particle energy and that consequently there was no simple general dependence of the spectral form on azimuthal separation from the flare site. Though detailed interpretation and discussion of the theoretical implications of these results are deferred to a later paper, alternative models for the understanding of the observations are mentioned briefly.

Intense solar near‐relativistic electron events at 0.3 AU

“…Additional evidence that these structures affect the transport of energetic particles comes from the observation of large spatial regions uniformly filled with energetic particles that often exist behind shocks [e.g., Reames et al, 2012]. These energetic particle reservoirs are large volumes in which there are small longitudinal, latitudinal, and field-aligned intensity gradients [McKibben, 1972;Roelof et al, 1992;Lario et al, 2006;Lario, 2010]. The observation of these reservoirs indicates that large CME-associated shocks must be effective at containing energetic particles behind them.…”

The Two Sources of Solar Energetic Particles

“…Reservoirs were first reported from the Interplanetary Monitoring Platform (IMP) 4 and Pioneer 6 and 7 observations of ~20 MeV protons spanning ~180 o in solar longitude by McKibben (1972). Twenty years later reservoirs were seen extending over 2.5 AU radially between IMP 8 and Ulysses by Roelof et al (1992).…”

Estimation of the Release Time of Solar Energetic Particles Near the Sun

“…However, some SEP events could be simultaneously observed by multiple spacecraft with a very wide spatial distribution that could be much wider than the size of the flare. In order to interpret this phenomenon, two scenarios were proposed: (1) particles can cross magnetic field lines in the interplanetary space with perpendicular diffusion (McKibben 1972;Dresing et al 2012Dresing et al , 2014, and (2) particles can propagate in the solar atmosphere (Wibberenz et al 1989;Dresing et al 2014). However, the SEP community later realized that CMEs are important for particle acceleration, especially in large SEP events (Mason et al 1984;Gosling 1993;Zank et al 2000;Li et al 2003).…”