Abstract
Context. The elemental composition of the solar wind differs from the solar photospheric composition. Elements with low first ionization potential (FIP) appear enhanced compared to O in the solar wind relative to the respective photospheric abundances. This so-called FIP effect is different in the slow solar wind and the coronal hole wind. However, under the same plasma conditions, for elements with similar FIPs such as Mg, Si, and Fe, comparable enhancements are expected. Aims. We scrutinize the assumption that the FIP effect is always similar for different low FIP elements, namely Mg, Si, and Fe. Methods. Here we investigate the dependency of the FIP effect of low FIP elements on the O7+/O6+ charge state ratio depending on time, that is the solar activity cycle, and solar wind type. In addition, we order the observed FIP ratios with respect to the O7+/O6+ charge state ratio into bins and analyze separately the respective distributions of the FIP ratio of Mg, Si, and Fe for each O7+/O6+ charge state ratio bin. Results. We observe that the FIP effect shows the same qualitative yearly behavior for Mg and Si, while Fe shows significant differences during the solar activity maximum and its declining phase. In each year, the FIP effect for Mg and Si always increases with increasing O7+/O6+ charge state ratio, but for high O7+/O6+ charge state ratios the FIP effect for Fe shows a qualitatively different behavior. During the years 2001–2006, instead of increasing with the O7+/O6+ charge state ratio, the Fe FIP ratio exhibits a broad peak or plateau. In addition, the FIP distribution per O7+/O6+ charge state bin is significantly broader for Fe than for Mg and Si. Conclusions. These observations support the conclusion that the elemental fractionation is only partly determined by FIP. In particular, the qualitative difference in behavior with increasing O7+/O6+ charge state ratio between Fe on the one hand and Mg and Si on the other hand is not yet well explained by models of fractionation.
Highlights
The solar elemental composition has been under investigation for several decades (Feldman 1992; Schmelz et al 2012; Meyer 1985)
Since elements with similar first ionization potentials (FIPs) show a similar elemental composition in the solar wind relative to the photosphere, this is called the “FIP effect” and the underlying mechanism behind this effect separates ions from neutrals, and is assumed to act preferentially on ions rather than on neutral atoms. It is still unknown whether low FIP elements are enhanced in the solar wind compared to the photosphere or whether instead high FIP elements are depleted
With the help of statistical tests, for each O charge state ratio bin we investigated whether the distributions of FIP ratios of each pair of low FIP elements are likely to follow the same or different distributions
Summary
The solar elemental composition has been under investigation for several decades (Feldman 1992; Schmelz et al 2012; Meyer 1985). Since elements with similar first ionization potentials (FIPs) show a similar elemental composition in the solar wind relative to the photosphere, this is called the “FIP effect” and the underlying mechanism behind this effect separates ions from neutrals, and is assumed to act preferentially on ions rather than on neutral atoms It is still unknown whether low FIP elements are enhanced in the solar wind compared to the photosphere or whether instead high FIP elements are depleted. The Heavy Ion Sensor (HIS) is intended as the main tool for investigating potential slow solar wind source regions Both instruments can determine the strength of the FIP effect for several elements, and coordinated observations of the same plasma packages are planned. We utilize the available observations of the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) to scrutinize this assumption and investigate the differences in the behavior of these low FIP elements
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