Disparity among low first ionization potential elements

Christian Albrechts University at Kiel, Germany
e-mail: heidrich@physik.uni-kiel.de

Received: 18 May 2018
Accepted: 25 August 2018

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.