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

TL;DR

This study empirically determines the 'freeze-in' distances for Fe10+ and Fe13+ ions in the solar corona, revealing their different formation regions and challenging existing plasma diagnostic assumptions.

Contribution

First empirical measurement of Fe10+ and Fe13+ freeze-in distances using eclipse observations, providing new insights into coronal plasma conditions.

Findings

Fe10+ freezes in at ~1.45 Rs in polar coronal holes.

Fe13+ freezes in below 1.25 Rs in polar coronal holes.

Freeze-in distances vary between coronal holes and streamers.

Abstract

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 (Rf) where they reach fixed ionization states that are adhered to during their expansion with the solar wind. We present the first empirical inference of Rf for Fe10+ and Fe13+ derived from multi-wavelength imaging observations of the corresponding FeXI (Fe10+) 789.2 nm and FeXIV (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 coronal holes Rf is around 1.45 Rs for Fe10+ and below 1.25 Rs for Fe13+. Along open field lines in streamer regions 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 parameters between coronal holes and streamers and structures within them, including prominences and Coronal Mass Ejections (CMEs); (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 Rs.