Minor element abundances in the corona and solar wind

Abstract

Using a time-dependent numerical model that spans the chromosphere, corona and solar wind, abundance variations resulting from gravitational settling in the chromosphere and corona have been studied.

\noindent Gravitational settling in the chromosphere may lead to a depletion in the abundances of minor elements in the solar wind relative to the photosphere. The observed overabundance (relative to the photosphere) of elements with a first ionization potential (FIP) lower than 10~eV in the solar wind, and the underabundance of high-FIP elements such as He and Ne is a long standing problem in solar physics. In Paper~I and Paper~II, we study the significance of the magnetic flux tube expansion factor on the abundances of high-FIP elements He, Ne and O in the solar wind. With a low expansion factor, of order 15-20, we can reproduce the observed fast solar wind He abundance, but a high expansion factor, of order 40-100, is required to reproduce the observed fast solar wind Ne abundance.

\noindent In the corona, gravitational settling leads to local abundance enhancements. The magnitude and location of a coronal abundance enhancement depends on the minor element heating rate and the background plasma parameters. Observations of minor element abundances in the corona, in combination with modelling, may place constraints on the minor element heating rates, thereby providing new constraints for models and theories on coronal heating.

\noindent In Paper~I it is shown that, for proton-electron plasma parameters in accordance with observations from polar coronal holes, a coronal He abundance enhancement will occur in the region 1.2 - 2~$R_\odot$, where no observations of the He abundance exist. It can only be avoided by strong heating of the He ions, which leads to very high outflow velocities for both He and protons in the corona. In Paper~II we show that such high outflow velocities in the corona causes O and Ne to be pushed far out of ionization equilibrium, and leads to ion fractions for O and Ne that are not in accordance with in-situ observations in the fast solar wind. The results from Paper~I and Paper~II suggest that He abundance enhancements are common in the region above 1.2~$R_\odot$. In Paper~II we also investigate minor element abundance enhancements in the corona in a H-He background, using O as an example. We find that for O temperatures and outflow velocities in accordance with observations from coronal holes, coronal O abundance enhancements will occur at heights above 1.3~$R_\odot$.

\noindent In Paper~III we study Fe elemental abundance enhancements in the slow solar wind. The results are compared to the observations of \cite{Habbaletal2007}, who found evidence of coronal density enhancements of Fe$^{+10}$ and Fe$^{+12}$, particularly along streamer edges. We show that the observations of \cite{Habbaletal2007} are consistent with an Fe elemental abundance enhancement, but owing to existing uncertainties about the plasma conditions in the source regions of the slow solar wind we cannot place any strong constraints on the coronal Fe heating rate. However, by comparing the modeled Fe ion fractions with in-situ observations in the slow solar wind we find that a background model with coronal hole-like densities and a high outflow velocity in the corona provides the best fit to observations.