Relative abundances of quiet-time suprathermal ions at 1 AU

摘要

Possible sources of suprathermal ion fluxes are investigated using ACE, WIND and SOHO observations at 1 AU in 1998-2011. The energy spectra of C, CNO, O, Ne-S, Fe ions and their relative abundances are determined in the energy range of 0.03-3 MeV/n during quiet-time periods of solar activity. 51 quiet intervals were selected under the criteria of minimum contribution of particles arising from previous solar energetic particle events. The behaviour of the abundance ratios Ne-S/O and Fe/CNO confirms our previous finding that the background fluxes of suprathermal ions in the energy range of ~40-100 keV/n can be divided into three types. During the ascending, maximum and descending phases of the solar cycle the abundance ratios Fe/O for almost half of all quiet time periods coincide with the average element abundances of the solar corona whereas the rest were lie within the ratio intervals characteristic for impulsive solar energetic particle events. At solar cycle minimum the ratios are near their solar wind values. The results obtained indicate various origin of suprathermal particles - different seed populations accelerated and different acceleration processes in three quiet-time flux types. Except for quiet solar activity, suprathermal fluxes consist of accelerated coronal ions in the half of quiet periods. In the other half of the periods the fluxes arise from particle acceleration processes similar to those in small impulsive solar flares rich in Fe. The latter population is assumed to be the seed population accelerated in solar energetic particle events. During solar minimum the bulk solar wind particles can serve as seed population.