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 Fe10+ and Fe13+ derived from multi-wavelength imaging observations of the corresponding Fe XI (Fe10+) 789.2 nm and Fe XIV (Fe13+) 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 corona! holes (CHs)R-f is around 1.45 R-circle dot for Fe10+ and below 1.25 R-circle dot for Fe13+. Along open field lines in streamer regions, R-f ranges from 1.4 to 2 R-circle dot for Fe10+ and from 1.5 to 2.2 R-circle dot for Fe13+. 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-circle dot.