Modeling Iron Abundance Enhancements in the Slow Solar Wind

Abstract

We have studied the behavior of Fe ions in the slow solar wind, using a fluid model extending from the chromosphere to 1 AU. Emphasis is on elemental "pileup" in the corona, i.e., a region where the Fe density increases and has a local maximum. We study the behavior of individual Fe ions relative to each other in the pileup region, where Fe +10 and Fe +12 have been used as examples. We find that elemental pileups can occur for a variety of densities and temperatures in the corona. We also calculate the ion fractions and obtain estimates for the freezing-in distance of Fe in the slow solar wind. We find that the freezing-in distance for iron is high, between 3 and 11 R , and that a high outflow velocity, of order 50-100 km s-1 , in the region above the temperature maximum is needed to obtain ion fractions for Fe +10 and Fe +12 that are consistent with observations.

Ion Charge States in a Time-Dependent Wave-Turbulence-Driven Model of the Solar Wind

“…In particular, as Fig. 3b of Byhring et al (2011) shows, a single outflow speed for all ions is only a crude approximation. Introducing a more realistic evolution of the plasma, starting from evolving the temperature of electrons and protons separately, is a first step into this direction that will be implemented next.…”

“…Although our model was shown to provide results compatible with observations (Lionello et al, 2014a), we cannot categorically exclude that a different heating model could yield not only the same plasma parameters at 1 AU, but also conditions in the lower corona causing higher ionization. Needless to say, a more accurate calculation of charge states would also require multi-fluid simulations (e.g., Ofman, Abbo, and Giordano, 2013) or even multi-ions simulations (e.g., Byhring et al, 2011). In particular, as Fig.…”

“…In those instances charge states must be calculated with a time-dependent scheme, which is then relaxed to a steady state for the steady solar wind solutions computed here (Shen et al, 2015). Although the evolution of the charge state distribution in the solar wind has been studied with sophisticated multi-fluid models (Buergi and Geiss, 1986;Esser, Edgar, and Brickhouse, 1998;Ko et al, 1999;Chen, Esser, and Hu, 2003;Byhring et al, 2011), connecting in situ measurements with coronal spectroscopic data still remains problematic (Landi et al, 2014). This very difficulty makes the reproduction of charge-states in MHD computational models of the solar corona a robust constraint for the validation of the models themselves.…”

New Solar Wind Diagnostic Using Both in Situ and Spectroscopic Measurements

“…Another limitation of this technique lies in the implicit assumption made by the ionization code that all elements have the same dynamic evolution. For example, Kohl et al (2006) indicate several instances where UVCS observations indicate that the proton and oxygen speeds are significantly different in the early stages of solar wind acceleration; Byhring et al (2011) showed through numerical models that decrease in Coulomb coupling between iron ions and hydrogen caused the former to have a slower speed than the latter; they also showed that each ion decoupled its velocity from hydrogen at a different height due to the different values of Coulomb cross sections. The presence of different velocity profiles for each element in the inner corona will lead to, if all elements are considered together, significant confusion in the interpretation of the predictedto-observed comparison.…”

Carbon Ionization Stages as a Diagnostic of the Solar Wind

“…For example, in situ measurements of ion abundances have been used to measure the temperature of coronal source regions of the wind (Bochsler et al 1986;Geiss et al 1995;Hefti et al 2000;von Steiger et al 2000;Gloeckler & Geiss 2007), as well as to discriminate between theoretical models of solar wind acceleration (Bürgi & Geiss 1986). More recently, Ko et al (1997) combined Ulysses measurements of fast solar wind ion abundances with a theoretical model of ionization and recombination of solar wind plasma to infer the temperature profile of the lower solar corona; Chen et al (2003) and Byhring et al (2011) combined in situ measurements and solar wind models to study the presence of differential flows or abundance enhancements of heavy ions in the solar wind close to the Sun.…”