The 2023 Australian Total Solar Eclipse: Line Emission of Fe XIV, Fe X and Fe XI out to 6 solar radii

TL;DR

The paper demonstrates that visible and near-infrared coronal emission lines Fe XIV 530.3 nm, Fe X 637.4 nm, and Fe XI 789.2 nm can be measured continuously from the solar disk to at least $6\,R_{\odot}$ during the 2023 TSE, enabling the first resolved line-emission ratios for the middle corona. By combining on-band/off-band narrowband imaging with absolute disk calibration and cross-checks against LASCO-C2, the authors infer $T_e$-sensitive line ratios and map coronal structure from equatorial open-field regions to polar streamers. The results reveal hotter $ ext{≈}(1.6$–$2)$ MK streamers and cooler $ ext{≈}(0.7$–$1.4)$ MK open-field regions, with the Fe XIV/Fe X ratio suggesting near-isothermal conditions beyond about $3\,R_{\odot}$. The study highlights the value of eclipse observations for constraining coronal heating and solar wind formation, and argues for future missions capable of multi-line observations to larger heliocentric distances.

Abstract

We present narrowband observations of the Fe XIV (530.3 nm), Fe X (637.4 nm), and Fe XI (789.2 nm) coronal emission lines from the 2023 April 20 Total Solar Eclipse in Australia. We deployed pairs of telescopes for each emission line that were equipped with narrowband filters centered on, and several nanometers away from, the center wavelengths of the lines. The secondary continuum telescopes were used to measure and remove the combined continuum K- (electron) and F- (dust) corona, which dominate coronal emission at optical and infrared wavelengths. Significant emission was detected from all three lines from 1.03 solar radii (Rs) continuously outward to at least 6 Rs. The brightness of the lines and continuum are absolutely calibrated to the solar disk, and are validated by a comparison with LASCO-C2 observations made at the same time. Using these observations, we inferred the line emission ratios resolved throughout the middle-corona (defined as 1.5 to 6 Rs) for the first time. These line ratios are a probe of the electron temperature, which have important implications for constraining models of coronal heating and the characterization of solar wind formation, yet these emission lines have scarcely been quantified beyond 3 Rs in the corona. This study demonstrates the enduring potential of eclipse observations for coronal physics and suggests that future spacecraft missions could observe these lines farther out than has been attempted previously.

Figures (9)

• Figure 1: White-light and narrowband images of the corona during the TSE from Exmouth, Australia, on 2023 April 20. The top image was taken with a 1000mm lens, while the bottom-left image shows a wider field-of-view from a 200mm lens (composited with the 1000mm image for the low corona). The middle-right panel shows a composite of Fe xiv (green) and Fe x (red) emission (see Section \ref{['sec:Narrow']}). The arrows in each image indicate the direction of solar North. The bottom right panel shows a slit-less flash spectrum of the corona with the strong Hydrogen, Helium, and Iron lines labeled.
• Figure 2: The ionization equilibrium abundances of Fe x, Fe xi, and Fe xiv as a function of $T_e$. Data are shown as spline interpolated values from the CHIANTI database Dere1997Dufresne2024.
• Figure 3: Emission of Fe xiv recorded with narrowband images (see Section \ref{['sec:Narrow']}). The panels on the left show the absolute line emission (in units of mean solar disk brightness) in log space, while the panels on the right show the line emission divided by the continuum (K+F corona) brightness near the same wavelength of the line. The top panels on each size show the corona with a window of 2.5 $R_{\odot}$ (left) and 4.5 $R_{\odot}$ (right) with Solar North upwards, while the bottom pairs of panels are in polar coordinates (Helioprojective distance vs Position Angle) extending out to 3 (top) and 6 $R_{\odot}$ (bottom).
• Figure 4: Same format as Figure \ref{['fig:FeXIV']} for Fe x emission.
• Figure 5: Same format as Figures \ref{['fig:FeXIV']} and \ref{['fig:FeX']} for Fe xi emission.