The First Empirical Determination of the Fe10+ and Fe13+ Freeze-in Distances in the Solar Corona

Heavy ions are markers of the physical processes responsible for the density and temperature distribution throughout the fine-scale magnetic structures that define the shape of the solar corona. One of their properties, whose empirical determination has remained elusive, is the "freeze-in" distance (R f ) where they reach fixed ionization states that are adhered to during their expansion with the solar wind. We present the first empirical inference of R f for and derived from multi-wavelength imaging observations of the corresponding Fe xi () 789.2 nm and Fe xiv () 530.3 nm emission acquired during the 2015 March 20 total solar eclipse. We find that the two ions freeze-in at different heliocentric distances. In polar coronal holes (CHs) R f is around 1.45 R ⊙ for and below 1.25 R ⊙ for . Along open field lines in streamer regions, R f ranges from 1.4 to 2 R ⊙ for and from 1.5 to 2.2 R ⊙ for . These first empirical R f values: (1) reflect the differing plasma parameters between CHs and streamers and structures within them, including prominences and coronal mass ejections; (2) are well below the currently quoted values derived from empirical model studies; and (3) place doubt on the reliability of plasma diagnostics based on the assumption of ionization equilibrium beyond 1.2 R ⊙.