The First Empirical Determination of the Fe10+ and Fe13+ Freeze-in Distances in the Solar Corona
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 (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 {Fe}}{10+} and {Fe}}{13+} derived from multi-wavelength imaging observations of the corresponding Fe XI ({Fe}}{10+}) 789.2 nm and Fe XIV ({Fe}}{13+}) 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 {Fe}}{10+} and below 1.25 R ⊙ for {Fe}}{13+}. Along open field lines in streamer regions, R f ranges from 1.4 to 2 R ⊙ for {Fe}}{10+} and from 1.5 to 2.2 R ⊙ for {Fe}}{13+}. 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 ⊙.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2018
- DOI:
- 10.3847/1538-4357/aabfb7
- arXiv:
- arXiv:1805.03211
- Bibcode:
- 2018ApJ...859..155B
- Keywords:
-
- eclipses;
- solar wind;
- Sun: corona;
- Sun: coronal mass ejections: CMEs;
- Sun: particle emission;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 22 pages, 7 figures, Video about the paper: https://youtu.be/7j3sQeIcnP8