Solar Wind C, N, and O Abundances and the Solar Metallicity

Abstract:

Solar wind composition provides important constraints to solar composition and to the processes that modify such compositional patterns in the atmospheres of the Sun and of active stars. There are a number of ways that composition can be observed, including spectroscopy, helioseismology, and the collection of solar samples either in the form of solar wind or energetic particles. In either case, models are needed to infer compositional constraints from observations. For example, models are needed to interpret solar spectroscopy results, and the evolution of these has recently led to significant changes to the previously accepted solar composition. The collection of solar samples requires a different type of consideration. Most solar wind and energetic particle samples are fractionated according to first ionization potential (FIP) as first pointed out by Hovestadt et al. in the seventies – elements with FIP below 10 eV are enhanced relative to elements at higher FIP, and He and possibly Ne are further depleted. Besides FIP fractionation there are indications from both isotopic and elemental data that mass fractionation, either through gravitational and/or collisional processes, may also play a role.

Based on comparisons of in situ data with coronal spectroscopy it is evident that most of these processes occur at the interface between the photosphere and the corona. However, the high-latitude corona near solar minimum appears to undergo much less fractionation, if any at all. Thus it provides a heliospheric sample that is – to within our observational constraints – photospheric in nature. The low-latitude heliosphere further provides direct access to plasmas that have the fractionation pattern qualitatively and quantitatively similar to the one observed in the corona.

We present a recent reanalysis of the SWICS observations on both Ulysses and ACE using modern statistical tools. Concentrating on C, N, and O, which together with the recently published Ne (Shearer et al., ApJ, 2014) contribute 96% of the solar metallicity, we find that the solar wind metallicity is significantly higher than the recent compilation of spectroscopic abundances (Asplund et al., ARAA, 2009). It is more in line with earlier spectroscopic results and, more importantly, not incompatible with helioseismology results of the solar interior.