Solar Energetic Particle Event on July 9, 1996

Fig. 1. Intensity profiles of protons as observed by ERNE from 9:04 UT on July 9 to 24:00 UT on July 10, 1996. The three lowest energy channels upto 12 MeV represent ERNE/LED observations, and channels above 12 MeV ERNE/HED observations. The intensity in the lowest energy channel is multiplied by a factor of ten. The bottom panel gives the angle between the ERNE sensor axis and the interplanetary magnetic field observed by WIND/MFI. The highlighted area represents a disturbance in magnetic field during the period 12:50-16:00 UT. MFI data is provided by WIND/MFI team.

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.

Fig. 2. Pitch-angle distributions of the 14-17 MeV protons during four time intervals on July 9-10, 1996: A: July 9, 10:10-13:00 UT, B: July 9, 13:00- 16:00 UT, and C: July 9, 16:00-19:30 UT, and D: July 9, 19:30 UT - July 10, 08:00 UT. The distribution is given both with the respect to the average field measured by WIND/MFI (solid circles) and to the symmetry axis estimated from the directional scatter diagram (histogram). The horizontal dashed line represents the isotropic background of the cosmic radiation.

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.

Interplanetary Particle Transport

The interplanetary transport of energetic particles during most of the solar event on July 9, 1996, was extremely rapid, scatter-free. The protons arrived at the SOHO distance as a beam of particles. The proton beam was narrow with a half width in the range 25-40 degrees with respect to the symmetry axis of the beam.

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.

To be published in Solar Physics under title "Recurrence of energetic particle flux anisotropy as observed by ERNE on July 9, 1996" by Torsti, J., Laitinen, T., Vainio, R., Kocharov, L., Anttila, A., and Valtonen, E.