Suprathermal ions from coronal holes at 1 au in solar cycles 23 and 24: dependence of ion abundances on solar wind speed

ABSTRACT The relative abundances of thermal and suprathermal C, O, and Fe ions are analysed and compared in solar wind streams from near-equatorial coronal holes (CHs) during quiet periods of nearly two solar cycle minima. Ion fluxes with energies of ∼0.04–2 MeV per nucleon are studied using data from the Ultra Low Energy Isotope Spectrometer (ULEIS) instrument aboard the Advanced Composition Explorer (ACE) spacecraft together with thermal ions in the fast and slow (Maxwellian) solar wind using data from the Solar Wind Ion Composition Spectrometer (SWICS) instrument aboard ACE. The analysis was carried out for quiescent periods in 2006–2012 and 2015–2017 when solar wind flows from near-equatorial CHs were detected at 1 au. Near the minimum of SC23, although they displayed large variability, the C/O and Fe/O ratios of suprathermal ions were, on average, near the corresponding relative abundances of the solar wind. During the decreasing solar activity phase of SC24, suprathermal Fe/O ratios matched those of solar wind ions from CHs. In both cycles, the thermal and suprathermal Fe/O ratios exhibited a similar character of dependence on maximum solar wind speed. Our results suggest that the sources of suprathermal ions from CHs in low-solar activity periods are accelerated solar wind thermal ions. The thermal and suprathermal Fe/O ratios were found to be higher in 2015–2017 in comparison to the ratios measured in 2006–2010. This difference can be attributed to that the second quiet period was selected at times when solar activity has not reached its minimum of SC24 yet.