Transport Modeling and Injection Time Profile of Relativistic Solar Protons on 2005 January 20

From an analysis of observations by the Spaceship Earth network of polar neutron monitors, we determined directional distributions of relativistic solar protons as a function of time during the giant ground level enhancement (GLE) of 2005 Jan 20. The distribution was extremely anisotropic at times of peak intensity, with a beaming direction that varied rapidly and was quite different from simultaneous ACE measurements of the magnetic field direction. We infer a differential spectral index of 5.0 at a median proton energy of 1.4 GeV, and the spectrum matches well with spacecraft observations at lower energy. We also numerically modeled the particle transport to study the interplanetary magnetic configuration, interplanetary scattering conditions, and injection profile. In particular, the start time of relativistic particle injection is determined with one-minute accuracy, and is compared with other timing information concerning the flare and CME. This event had two injections of relativistic protons, corresponding closely to the solar radio emission profile at 5 MHz. In the Spaceship Earth observations, the second peak was much more isotropic. The time-intensity and time-anisotropy profiles are well fit when including a magnetic bottleneck beyond Earth. The bottleneck is justified in terms of preceding CMEs and the predicted disturbances in the interplanetary magnetic field. Partially supported by the Thailand Research Fund and NASA LWS grant NNX08AQ18G.