Implications of Proton Anisotropy Development Observed by the Erne Instrument During the 9 July 1996 Solar Particle Event

We analysed the solar particle event following the 9 July 1996 solar flare. High-energy protons were detected by the ERNE instrument on board SOHO. Anisotropy of arriving protons revealed very peculiar non-monotonic development. A short period of almost isotropic distribution was imbedded into the prolonged period of beam-like distribution of 14–17 MeV protons. This implies the existence of a narrow magnetic channel with a much smaller mean free path than in the surrounding quiet solar wind plasma. We used Monte Carlo simulations of interplanetary transport to fit the observed anisotropies and intensity–time profiles. Proton injection and transport parameters are estimated. The injection scenario is found to be very close to the scenario of the 24 May 1990 event, but the intensity and the interplanetary transport parameters are different. The extreme anisotropy observed implies prolonged injection of high-energy protons at the Sun and at the interplanetary shock front, and either a very large mean free path (≥ 5 AU) outside the slow transport channel, or alternatively, a somewhat smaller mean free path (≈2 AU) and enhanced focusing between the Sun and the Earth.