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Heavy ions are markers of the physical processes responsible for the densityand temperature distribution throughout the fine scale magnetic structures thatdefine the shape of the solar corona. One of their properties, whose empiricaldetermination has remained elusive, is the 'freeze-in' distance (Rf) where theyreach fixed ionization states that are adhered to during their expansion withthe solar wind. We present the first empirical inference of Rf for Fe10+ andFe13+ derived from multi-wavelength imaging observations of the correspondingFeXI (Fe10+) 789.2 nm and FeXIV (Fe13+) 530.3 nm emission acquired during the2015 March 20 total solar eclipse. We find that the two ions freeze-in atdifferent heliocentric distances. In polar coronal holes Rf is around 1.45 Rsfor Fe10+ and below 1.25 Rs for Fe13+. Along open field lines in streamerregions Rf ranges from 1.4 to 2 Rs for Fe10+ and from 1.5 to 2.2 Rs for Fe13+.These first empirical Rf values: (1) reflect the differing plasma parametersbetween coronal holes and streamers and structures within them, includingprominences and Coronal Mass Ejections (CMEs); (2) are well below the currentlyquoted values derived from empirical model studies; and (3) place doubt on thereliability of plasma diagnostics based on the assumption of ionizationequilibrium beyond 1.2 Rs.

First Empirical Determination of the Fe 10+ and Fe 13+ Freeze-in Distances in the Solar Corona