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Solar particle events with helium-over-hydrogen enhancement in the energy range up to 100 MeV nucl-1

  • Authors: Torsti, J.; Kocharov, L.; Laivola, J.; Pohjolainen, S.; Plunkett, S. P.; Thompson, B. J.; Kaiser, M. L.; Reiner, M. J.
  • Publication: Solar Physics, v. 205, Issue 1, p. 123-147, 2002
  • Solar events of 1998 May 27 and 1999 December 28
  • Statistical study
  • ADS link
  • Abstract: Flux measurements of solar energetic particles (SEPs) in the ERNE instrument onboard SOHO indicate that the abundance of 4He-nuclei compared to protons in the energy range up to 100 MeV nucl-1 was exceptionally high during the particle events on 27 May 1998 and 28 December 1999. The 4He/p ratio stayed between 0.15-0.50 for more than ten hours. There was also a prolonged enhancement in helium-3, 3He/4H ~1%. Observations of EIT and LASCO on board SOHO confirm that the originators of both SEP events were western eruptions, flares and coronal mass ejections (CMEs). The onset of the SEP release took place close to the maximum of flares which were probably triggered by the rising CMEs. The observations suggest that the SEP events were started with the flare-(pre)accelerated particles, but impact of the CME-associated shocks might explain the continuation and modification of the helium and proton fluxes well after the flare production. These observations support the idea that the helium enhancements in the CME-associated events reflect the availability of seed particles that originate previously in flares.


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