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

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 second peak, which is difficult to understand in terms of transport along a standard Archimedean spiral magnetic field or a second injection near the Sun. Here we analyze data from polar monitors, which measure the directional distribution of solar energetic particles (mainly protons) at rigidities of ∼1-3 GV. This event has the unusual properties that the particle density dips after the initial spike, followed by a hump with bidirectional flows and then a very slow decay. The spectral index, determined using bare neutron counters, varies dramatically, with energy dispersion features. The density and anisotropy data 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 a better explanation: particles moving along the near leg make up the spike, those coming from the far leg make up the hump, both legs contribute to the bidirectional streaming, and trapping in the loop accounts for the slow decay of the particle density. Refined fits indicate a very low spectral index of turbulence, q < 1, a parallel mean free path of 1.2-2.0 AU, a loop length of 4.7 ± 0.3 AU, and escape of relativistic protons from the loop on a timescale of 3 hr. The weak scattering is consistent with reports of weak fluctuations in magnetic loops, while the low q-value may indicate a smaller correlation length as well.