Composition of low energy solar particles (0.5 – 225 keV/e) measured by the WIND-SC during impulsive and gradual flares Original Research Article

The composition of 6 impulsive or 3He rich and 4 gradual solar particle events was studied at the low energy end of their spectrum using SMS-WIND measurements to access an energy range not accessible to date by other experiments. The investigated events were found to have mostly a cutoff energy for 3He ions at ∼30–225 keV/e, but H+, 4He++, 4He+, CNO and sometimes 56Fe10+ ions could be detected during both types of flares in that energy/charge range. The 4He++H+ ratio of impulsive flares increased between Eq = 6.5 – 225 keV/e and 30 – 225 keV/e in most examples, indicating that 4He++ is preferentially accelerated whereas during gradual flares that ratio showed no big variations. The 3He and Fe-enrichment observed at higher energies and the acceleration of 1H+ and 4He++ can be explained by selective gyroresonance processes as proposed e.g. by Miller and Vinas (1993). The flux decrease of 3He at low energies would be caused by a temperature decrease in the flare region at the end of the active flare phase. The density increase in the flare region removes then 3He by coulomb collision from the resonance process with H+-ion cyclotron waves, whereas H+ and 4He++ ions are further accelerated by X-mode waves.