A solar energetic particle event was observed on July 9, 1996, by the ERNE instrument on board the SOHO spacecraft. The arrival of the first protons at energies > 20 MeV took place at 9:55 UT, 51 minutes after the maximum in the X-ray and H-alpha radiation of a flare located at S10W30, 9:04 UT. The LASCO observations on board SOHO showed that a large coronal mass ejection occurred simultaneously, probably in association with the flare.
The rise phase of the particle intensity at all energies was exceptionally rapid. At 12:50 UT, the intensities dropped in all energy channels. The drop was most impressive at low energies, almost two orders of magnitude at 1.6-3.0 MeV, and extremely rapid, 2-3 minutes. Simultaneously the magnetic field instrument MFI on board the WIND spacecraft, not far from SOHO, detected a sharp and large change in the magnetic field direction.
The analysis of the directional measurements of ERNE in energy range 14-17 MeV shows a presence of a strong flux anisotropy during the whole period 10:10-12:50 UT. The directional distribution is strongly peaked with respect to a flux axis parallel to the magnetic field guiding the transport of charged particles in space.
In the above figure, an isotropic particle flux would be a horisontal line. From 12:50 UT until about 16:00, the directional analysis of the proton fluxes refers to a slight anisotropy or to a fact that the flux anisotropy was below the sensor resolution. After that the flux anisotropy was found to have recurred indicating the long-running injection of particles in the fluxtubes connected to the SOHO spacecraft. These experimental results lead to tight limits on the particle injection and transport models. The first period of the anisotropy and its recurrence phase cover 24 hours.
About three hours after the event onset, the spacecraft passed a strong interplanetary field disturbance, which caused the intensity to drop and changed the strong anisotropy of the directional proton fluxes to a nearly isotropic distribution in the energy range 14-17 MeV .
The observed recurrence of a beam-like anisotropy after six hours of the event onset suggests that the energetic particle injection in the SEP event on July 9-10, 1996, was extended lasting until the end of the analysis period, i.e. 24 hours.