Charge State Evolution in the Solar Wind. Iii. Model Comparison With Observations

Abstract

We test three theoretical models of the fast solar wind with a set of remote sensing observations and in-situ measurements taken during the minimum of solar cycle 23. First, the model electron density and temperature are compared to SOHO/SUMER spectroscopic measurements. Second, the model electron density, temperature, and wind speed are used to predict the charge state evolution of the wind plasma from the source regions to the freeze-in point. Frozen-in charge states are compared with Ulysses/SWICS measurements at 1 AU, while charge states close to the Sun are combined with the CHIANTI spectral code to calculate the intensities of selected spectral lines, to be compared with SOHO/SUMER observations in the north polar coronal hole. We find that none of the theoretical models are able to completely reproduce all observations; namely, all of them underestimate the charge state distribution of the solar wind everywhere, although the levels of disagreement vary from model to model. We discuss possible causes of the disagreement, namely, uncertainties in the calculation of the charge state evolution and of line intensities, in the atomic data, and in the assumptions on the wind plasma conditions. Last, we discuss the scenario where the wind is accelerated from a region located in the solar corona rather than in the chromosphere as assumed in the three theoretical models, and find that a wind originating from the corona is in much closer agreement with observations.

Nonequilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind (Invited Review)

“…It is possible to do this with a variety of models including different Poynting flux values or magnetic field geometries (open or closed) in different regions such as coronal holes, and even eruptions constituting transients in the solar wind (e.g., Lynch et al, 2011;Gruesbeck et al, 2011;Landi et al, 2012aLandi et al, ,b, 2014Lepri et al, 2012;Oran et al, 2015;Rodkin et al, 2016).…”

“…Evolution of charge state along an open magnetic field line was studied in a series of papers by Landi et al (2012aLandi et al ( ,b, 2014. It was found that windinduced departures from ionization equilibrium existed in the solar atmosphere, producing up to a factor of 3 differences in total radiative losses of the upper chromosphere and transition region when compared to equilibrium ion composition (Landi et al, 2012b).…”

Coronal plasma diagnostics from ground‐based observations

“…(left) Cranmer et al [] coronal hole/fast wind model. (right) Empirical model with the wind source region located in the corona (from Landi et al []).…”

“…One of the leading proposed mechanisms for heating the solar corona (and accelerating the solar wind) is dissipation of magnetic waves. This method powers many first-principle, physics-based global coronal models, such as the Alfvén Wave Solar Model [van der Holst et al, 2014]. There are many mechanisms proposed for how to dissipate magnetic waves and release their energy into the solar atmosphere, but all require observational evidence.…”

“…Some models of the solar corona based on Alfvén waves predict that toward the top of large-scale loops the ion and electron temperatures decouple, as electrons are able to conduct the energy deposited by Alfvén waves downward along the loop, while ions are much less efficient at that [van der Holst et al, 2014]. This leads to a strong enhancement of T ion at the loop top while the electron temperatures is almost constant; this increase strongly affects line widths while keeping their intensities unchanged.…”