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 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 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 R .

CME-induced Thermodynamic Changes in the Corona as Inferred from Fe xi and Fe xiv Emission Observations during the 2017 August 21 Total Solar Eclipse

“…It is also important to remember that once the plasma reaches the freeze-in distance ≈ 1.2 − 1.4 R for coronal holes and up to 1.6 − 1.8 R for streamers, as shown by Boe et al (2018), the ionic abundance (and so the inferred T e ) will remain constant as the plasma flows outward in the solar wind. Beyond the freeze-in distance, our inferred T e will be representative of what the T e was in the low corona when the plasma was below the freeze-in distance.…”

Solar Wind and Heavy Ion Properties of Interplanetary Coronal Mass Ejections

“…The interpretation of charge-state data is complex, as coronal electron density, temperature and plasma velocities all play a role in determining the ionisation and recombination rates (Landi et al, 2012;Zhao et al, 2014). Coronal mass ejections may significantly disrupt ambient coronal structure, further complicating interpretation of charge-state data (Boe et al, 2018;Ding and Habbal, 2017). Thus, ideally, self-consistent dynamical modelling (Shen et al, 2017) should be used to enable interpretation of in situ heavy ion observations in terms of coronal processes.…”

Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)

“…These will facilitate the quantitative evaluation of suggested heating processes in varying magnetic environments. Measurement of the line-continuum and line-line intensity ratios of ionic species, such as Fe IX to XV, that are also found in in-situ measurements of the fast and slow solar wind, will allow determination of the electron-density, temperature, and chargestate evolution of the solar-wind plasma, informing our understanding of the acceleration and heating processes (Figure 10, Landi et al, 2012;Landi, Habbal, and Tomczyk, 2016;Boe et al, 2018). Comparison between observations and theoretical studies of fast and slow magneto-acoustic wave-mode conversion, shock formation, and dissipation (e.g.…”