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1. The 2023 Australian Total Solar Eclipse: Line Emission of Fe XIV, Fe X, and Fe XI out to 6 Solar Radii

   https://doi.org/10.3847/1538-4357/ae2658  

   Boe, Benjamin; Habbal, Shadia; Druckmüller, Miloslav; Štarha, Pavel; Štarha, Matěj; Hoderová, Jana; Constantinou, Sage; Ayars, Eric; Casillas, Daniell (January 2026, The Astrophysical Journal)

   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 (R⊙) continuously outward to at least 6 R⊙. 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 1.5–6 R⊙) 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 R⊙ 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. 

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2. Data from "The 2023 Australian Total Solar Eclipse: Line Emission of Fe XIV, Fe X, and Fe XI out to 6 Solar Radii"

   https://doi.org/10.5281/zenodo.18601399  

   Boe, Benjamin; Habbal, Shadia; Druckmüller, Miloslav; Štarha, Pavel; Štarha, Matěj; Hoderová, Jana; Constantinou, Sage; Ayars, Eric; Casillas, Daniell (February 2026, Zenodo)

   Narrowband imaging data of the corona collected during the 2023 Total Solar Eclipse on April 20 in Australia. These data contain observations of the ionic emission lines of Fe X (637.4 nm), Fe XI (789.2 nm), and Fe XIV (530.3 nm) as well as corresponding continuum observations of the K and F corona near each line. The line emission data have already had the continuum subtracted.The data are contained in seperate files for both Cartesian coordinates and polar coordinates. Each file contains two layers: the first being the scientific data product of integrated brightness (in mean solar disk brightness units), and the second is an estimate of the uncertainty of the observation. Additional metadata are included in the FITS headers of each file.These data have already appeared in Boe et al. 2026, ApJ, 997, 171. See that publication for further details.https://iopscience.iop.org/article/10.3847/1538-4357/ae2658  

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3. Identifying the Coronal Source Regions of Solar Wind Streams from Total Solar Eclipse Observations and in situ Measurements Extending over a Solar Cycle

   Habbal, S. R.; Druckmuller, M. (April 2021, The astrophysical journal)

   null (Ed.)

   This Letter capitalizes on a unique set of total solar eclipse observations, acquired between 2006 and 2020, in white light, \ion[Fe xi] 789.2 nm (\Tfexi\ = $$1.2 \pm 0.1$$ MK) and \ion[Fe xiv] 530.3 nm (\Tfexiv\ = $$ 1.8 \pm 0.1$$ MK) emission. They are complemented by \insitu\ Fe charge state and proton speed measurements from ACE/SWEPAM-SWICS, to identify the source regions of different solar wind streams. The eclipse observations reveal the ubiquitous presence of open structures throughout the corona, invariably associated with \ion[Fe xi] emission from $$\rm Fe^{10+}$$, thus revealing a constant electron temperature, \Tc\ = \Tfexi\, in the expanding corona. The \insitu\ Fe charge states are found to cluster around $$\rm Fe^{10+}$$, independently of the 300 to 700 km $$\rm s^{-1}$$ stream speeds, referred to as the continual solar wind. $$\rm Fe^{10+}$$ thus yields the fiducial link between the continual solar wind and its \Tfexi\ sources at the Sun. While the spatial distribution of \ion[Fe xiv] emission, from $$\rm Fe^{13+}$$, associated with streamers, changes throughout the solar cycle, the sporadic appearance of charge states $$> \rm Fe^{11+}$$, \insitu, exhibits no cycle dependence regardless of speed. These latter streams are conjectured to be released from hot coronal plasmas at temperatures $$\ge \rm $$ \Tfexiv\ within the bulge of streamers and from active regions, driven by the dynamic behavior of prominences magnetically linked to them. The discovery of continual streams of slow, intermediate and fast solar wind, characterized by the same \Tfexi\ in the expanding corona, places new constraints on the physical processes shaping the solar wind. 

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4. On the use of solar eclipses to study the ionosphere

   Liles, W; Mitchell, C.; Cohen, M.; Earle, G.; Frissell, N.; Kirby-Patel, K.; Lukes, L.; Miller, E.; Moses, M.; Nelson, J.; et al (January 2017, 15th International Ionospheric Effects Symposium)

   Exploring the effects of solar eclipses on radio wave propagation has been an active area of research since the first experiments conducted in 1912. In the first few decades of ionospheric physics, researchers started to explore the natural laboratory of the upper atmosphere. Solar eclipses offered a rare opportunity to undertake an active experiment. The results stimulated much scientific discussion. Early users of radio noticed that propagation was different during night and day. A solar eclipse provided the opportunity to study this day/night effect with much sharper boundaries than at sunrise and sunset, when gradual changes occur along with temperature changes in the atmosphere and variations in the sun angle. Plots of amplitude time series were hypothesized to indicate the recombination rates and reionization rates of the ionosphere during and after the eclipse, though not all time-amplitude plots showed the same curve shapes. A few studies used multiple receivers paired with one transmitter for one eclipse, with a 5:1 ratio as the upper bound. In these cases, the signal amplitude plots generated for data received from the five receive sites for one transmitter varied greatly in shape. Examination of very earliest results shows the difficulty in using a solar eclipse to study propagation; different researchers used different frequencies from different locations at different times. Solar eclipses have been used to study propagation at a range of radio frequencies. For example, the first study in 1912 used a receiver tuned to 5,500 meters, roughly 54.545 kHz. We now have data from solar eclipses at frequencies ranging from VLF through HF, from many different sites with many different eclipse effects. This data has greatly contributed to our understanding of the ionosphere. The solar eclipse over the United States on August 21, 2017 presents an opportunity to have many locations receiving from the same transmitters. Experiments will target VLF, LF, and HF using VLF/LF transmitters, NIST?s WWVB time station at 60 kHz, and hams using their HF frequency allocations. This effort involves Citizen Science, wideband software defined radios, and the use of the Reverse Beacon Network and WSPRnet to collect eclipse-related data. 

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5. On the use of solar eclipses to study the ionosphere

   Liles, W; Mitchell, C.; Cohen, M.; Earle, G.; Frissell, N.; Kirby-Patel, K.; Lukes, L.; Miller, E.; Moses, M.; Nelson, J.; et al (May 2017, 15th International Ionospheric Effects Symposium)

   Exploring the effects of solar eclipses on radio wave propagation has been an active area of research since the first experiments conducted in 1912. In the first few decades of ionospheric physics, researchers started to explore the natural laboratory of the upper atmosphere. Solar eclipses offered a rare opportunity to undertake an active experiment. The results stimulated much scientific discussion. Early users of radio noticed that propagation was different during night and day. A solar eclipse provided the opportunity to study this day/night effect with much sharper boundaries than at sunrise and sunset, when gradual changes occur along with temperature changes in the atmosphere and variations in the sun angle. Plots of amplitude time series were hypothesized to indicate the recombination rates and reionization rates of the ionosphere during and after the eclipse, though not all time-amplitude plots showed the same curve shapes. A few studies used multiple receivers paired with one transmitter for one eclipse, with a 5:1 ratio as the upper bound. In these cases, the signal amplitude plots generated for data received from the five receive sites for one transmitter varied greatly in shape. Examination of very earliest results shows the difficulty in using a solar eclipse to study propagation; different researchers used different frequencies from different locations at different times. Solar eclipses have been used to study propagation at a range of radio frequencies. For example, the first study in 1912 used a receiver tuned to 5,500 meters, roughly 54.545 kHz. We now have data from solar eclipses at frequencies ranging from VLF through HF, from many different sites with many different eclipse effects. This data has greatly contributed to our understanding of the ionosphere. The solar eclipse over the United States on August 21, 2017 presents an opportunity to have many locations receiving from the same transmitters. Experiments will target VLF, LF, and HF using VLF/LF transmitters, NIST’s WWVB time station at 60 kHz, and hams using their HF frequency allocations. This effort involves Citizen Science, wideband software defined radios, and the use of the Reverse Beacon Network and WSPRnet to collect eclipse-related data. 

   more » « less