Relativistic solar protons on 1989 October 22: Injection and transport along both legs of a closed interplanetary magnetic loop

Abstract

Worldwide neutron monitor observations of relativistic solar protons on 1989 October 22 have proven puzzling, with an initial spike at some stations followed by a hump with bidirectional flows and a very slow decay. We analyze data from polar monitors, which measure the directional distribution of solar energetic particles (mainly protons) at rigidities of ~ 1-3 GV. The inferred density and anisotropy are simultaneously fit by simulating the particle transport for various magnetic field configurations and determining the best-fit injection function near the Sun. The data are not well fit for an Archimedean spiral field, a magnetic bottleneck beyond Earth, or particle injection along one leg of a closed magnetic loop. A model with simultaneous injection along both legs of a closed loop provides the best explanation. Refined fits indicate a very low spectral index of turbulence, q < 1, and hence an unusually low correlation length of magnetic fluctuations in the loop, a loop length of 4.7 ± 0.3 AU, and escape from the loop on a time scale of 3 hours.