Recurrence of Energetic Particle Flux Anisotropy as Observed by Erne on 9 July 1996 |
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Authors: | J Torsti T Laitinen R Vainio L Kocharov A Anttila E Valtonen |
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Institution: | (1) Space Research Laboratory, Department of Physics, Turku University, FIN-20014, Finland |
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Abstract: | A solar energetic particle event was observed on 9 July 1996, by the ERNE sensors LED and HED on board the SOHO spacecraft.
The arrival of the first protons in the energy range >20 MeV took place at 09:55 UT, 43 min after the maximum in the X-ray
and Hα radiation of a flare located at S10 W30. The rise phase of the particle intensities at all energies was exceptionally
rapid. At 12:50 UT, the intensities dropped in all energy channels. Simultaneously, the magnetic field instrument MFI on board
WIND, not far from SOHO, detected a sharp and large change in the magnetic field direction. The analysis of the directional
measurements of ERNE in the energy range 14–17 MeV shows the presence of a strong flux anisotropy during the whole period
10:10–12:50 UT. From 12:50 UT until about 16:00 UT, the directional analysis of the proton fluxes gives only a weak anisotropy
at the limit of the sensor resolution. Later on, the flux anisotropy was found to recur, indicating a continuous injection
of particles into the flux tubes connected to the SOHO spacecraft. These experimental results lead to strict limits on particle
injection and transport models. The first period of the anisotropy and its recurrent phase cover 24 hours. This suggests an
extended injection of particles. The strength and stability of the anisotropy indicate that, during these periods, SOHO was
in an interplanetary sector where the particle transport was almost scatter-free. On the other hand, during the intermediate
3-hr period, we observed particles which traveled in a sector of diffusive transport or which were retarded by magnetic field
disturbances not far from the observation site. |
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