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 analyzed and compared in solar wind streams from near-equatorial coronal holes during quiet periods of nearly two solar cycle minima. Ion fluxes with energies of ∼0.04–2 MeV/nucleon are studied using data from the ULEIS instrument aboard the ACE spacecraft together with thermal ions in the fast and slow (Maxwellian) solar wind using data from the SWICS instrument aboard ACE. The analysis was carried out for quiescent periods in 2006–2012 and 2015–17 when solar wind flows from near-equatorial coronal holes (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 2015–17 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.

Solar Cycle Variation of Suprathermal Heavy Ion Composition and Spectra during Quiet Times near 1 AU

“…They are variously associated with CIRs (Chotoo et al 2000), ISEPs (Hovestadt et al 1984b,a;Desai et al 2006b), and GSEPs (Desai & Mason 2001;Mason et al 1999), depending on the phase of solar activity. In particular, prior observations suggest that CIRs are the dominant source of ST ions during solar minima (Desai et al 2006c;Dayeh et al 2009Dayeh et al , 2017Zel'dovich et al 2011Zel'dovich et al , 2018Zel'dovich et al , 2021 and both ISEP and GSEP events dominate ST sources during solar maxima (Dayeh et al 2017(Dayeh et al , 2009Desai et al 2006c). To quantify the similarity of suprathermal abundances to the known populations in Table 2, we have generated plots of quiet time X/O normalized to them Quiet Time (X/O) : Known (X/O) as a function of M/Q, averaging over each solar cycle extremum and all years from Figure 3.…”

“…ST ions are likely associated with CIRs in solar minima (Desai et al 2006c;Dayeh et al 2009Dayeh et al , 2017Zel'dovich et al 2011Zel'dovich et al , 2018Zel'dovich et al , 2021. CIRs are formed when coronating fast solar wind streams from coronal holes overtake slow wind streams.…”

“…Observations of quiet times can be identified as the lowest count levels in energetic particle detectors With such methods Dayeh et al (2017) showed that the number of quiet hours observed by ACE/ULEIS (Mason et al 1998) strongly anti-correlates with sunspot number (SSN). Multiple studies have also shown that ST abundances (normalized to oxygen, X/O) resemble solar energetic particle (SEP) events-both impulsive and gradual-in solar maxima and CIRs in solar minima (Desai et al 2006c;Dayeh et al 2009Dayeh et al , 2017Zel'dovich et al 2011Zel'dovich et al , 2018Zel'dovich et al , 2021. That the long term changes in ST ions correlate with SSN likely reflects changes in both the sources from which ST ions are drawn and the acceleration mechanisms associated with these sources (Zel'dovich et al 2018).…”

“…More recently, the name has become associated with the time profile of the energetic particles associated with the event (Reames 1999). 5 Mason et al (2016Mason et al ( , 2002 6 Allen et al (2019); Mason et al (2008); Ebert et al (2012); Fisk & Lee (1980); Richardson (2004); Desai et al (2006c); Dayeh et al (2017Dayeh et al ( , 2009Zel'dovich et al (2011Zel'dovich et al ( , 2018Zel'dovich et al ( , 2021 The reminder of this paper is organized as follows. Section 2.1 describes our quiet time selection criteria and its uncertainty metric.…”

Proton Spectra for the Interplanetary Space Derived From Different Environmental Models

“…Some information is provided by ACE, but as already mentioned, ACE data does not cover the entire solar cycle 23 and is therefore rejected, but will be included in spectra modelling of succeeding cycles. See e.g., Zeldovich et al (2021) for suprathermal ion data.…”