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Concurrent solar and corotating interaction region particle events in August 1996

  • Authors: Torsti, J.; Anttila, A.; Sahla, T.
  • Publication: Journal of Geophysical Research, Volume 104, Issue A5, p. 9891-9902, 1999
  • Solar events of 1996 August 7-18
  • Statistical study
  • ADS link
  • Abstract: The energetic particle telescope ERNE onboard SOHO registered a continuous particle flux enhancement during August 7-18, 1996. The particle event was limited in the low energy range from 1.6 to 8 MeV during the first week of the event. It originated from a corotating interaction region (CIR). At 1630 UT on August 13, an additional new flux of energetic particles emerged with maximum energies above 50 MeV. The southwest directed coronal mass ejection (CME) is the most probable source of the high energy particles. Our spectral analysis revealed a two-component structure with a hard Bessel-type spectrum (αT~0.02) of the CME-injected particles and a soft power law spectrum (index s~4.0) of CIR particles from 1630 UT till 1000 UT on August 14. In the following 3 hours the intensities above 3 MeV decreased more than 10-fold. During the plateau-type intensity maximum phase the intensities remained almost isotropic in the wide ERNE/HED view cone. We claim that the reason for the drop was that the direct connection to the CME was lost but the connection to the CIR was still maintained. The flux of >12 MeV protons still prevailing in interplanetary space had probably been injected earlier by the CME-associated event in its very early phase in the solar atmosphere, and trapped there between the CIR and the Sun. The spectrum of these protons was of power form and very hard, s~1.9. During August 15, the high-energy part hardened further, s~1.1. At the same time, the intensities increased, leading to a second intensity maximum during the first half of August 16. Finally, on August 18, a disconnection from the CIR took place in all energy channels.

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