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Structure and Dynamics of the Inner Corona Measured from the DEB Initiative 2024 Eclipse Image Sequence

This paper presents results from the Dynamic Eclipse (DEB) Initiative's 2024 eclipse observations, revealing tightly correlated coronal brightness profiles across North America, measuring a Ludendorff index of 0.0761, and detecting both polar downflows and transient outflows that partially align with and partially challenge existing MHD models of the inner corona.

Original authors: Matthew J Penn, Robert Baer, Chris Mandrell, Corinne Brevik, Castor Fu, Mike Conley, Richard Danley, Harvey Henson, David Iadavida, Jonathan Mangin, Chris Midden, Brodye Miller, Claude Plymate, Teresa

Published 2026-07-15

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CC BY 4.0

Original authors: Matthew J Penn, Robert Baer, Chris Mandrell, Corinne Brevik, Castor Fu, Mike Conley, Richard Danley, Harvey Henson, David Iadavida, Jonathan Mangin, Chris Midden, Brodye Miller, Claude Plymate, Teresa Plymate, Zack Stockbridge, Kevin Rasso, Autumn Awalt, Avery Awalt, Gracie Awalt, Josh Awalt, Lauree Rasso, Mike Chartrand, Noah Lambert, Zoee Rasso, Kevin Cobble, Amelia Menezes, Andrew Yu, Felix Mei, Hanson Du, Ramesh Pattar, Vishrut Kumaran, Robert Auburger, Brian J. Drake, Seth Antozzi, Chloe Rectanus, Grayson Garner, Michael Weiss, Bill Kloepping, Deborah Grubis, Maryanne Angliongto, Ashvik Chilakala, Carl Buz McCullough, Mark Bremer, Christina Adair, Amber Huffman, Charon Adair, Gabbie Graham, Nick Tillerson, Fred Isberner, Candy Isberner, Harry Treece, Lisa Sikorski, Sage Julian-Fralish, Heidi Schran, Addison Greene, Bibodh Baral, Crow Ely, Nolan Hodgson, Olive Blackmar Rice, Olivia Andrews, Olivia Freeman, Quinn Donnelly, Serenity Prince Kirk, Carmen Tran, Brianna Blanchard, Kyan West, Landon Bevier, Murali Saravanan, Jeremy Wright

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Sun wearing a giant, glowing crown made of super-hot gas. This "crown" is called the corona, and for over a century, scientists have tried to take a snapshot of it during a total solar eclipse. But usually, they can only get one quick peek from a single spot on Earth.

Enter the DEB Initiative, a massive team of citizen scientists (including students, teachers, and astronomy clubs) who acted like a giant, moving camera crew. On April 8, 2024, they set up 82 stations across North America, stretching from Texas to Maine. While clouds blocked some views, 11 of these sites had clear skies and managed to capture the corona for a total of 49 minutes as the Moon's shadow rolled across the continent.

Think of it like a relay race where each runner passes the baton of "seeing the Sun" to the next. By stitching together images from these 11 locations, the team created a time-lapse movie of the Sun's atmosphere, revealing how it moves and changes in real-time.

The Sun's Shape: Not Just a Perfect Circle

For decades, scientists have used a specific math trick called the Ludendorff index to describe how "squashed" or flat the Sun's corona looks. It's like measuring if a balloon is perfectly round or slightly oval.

The paper measured this directly from the new images and found the index to be 0.0761 ± 0.0007 at a distance of 2.0 solar radii (twice the Sun's width).

However, the authors argue that many previous studies tried to guess this number by drawing a straight line through measurements taken closer to the Sun. The paper shows that this "guessing game" (extrapolation) is wrong. If you try to predict the shape at 2.0 solar radii by just extending a line from lower heights, you get the wrong answer. Instead, the team suggests using two new, smoother measurements—how oval the light rings are and the angle of that oval—to describe the Sun's shape more accurately.

The Solar Wind: A Rollercoaster of Speeds

The real magic happened when the team looked at how the gas in the corona was moving. They used a clever computer trick called "optical flow" (similar to how video games track moving objects) to measure the speed of the gas.

They found two very different types of traffic:

The Downward Rush: Near the Sun's North Pole, they spotted a dark stream of gas falling inward. It was moving at an average speed of -37 ± 3 km s⁻¹ (that's about 82,000 miles per hour!) and slowing down (decelerating) at 14 ± 3 m s⁻². This matches predictions from a computer model by Y. Li et al. (2026), which suggested that gas should fall in this specific spot.
The Upward Blast: In other areas, gas was shooting outward. The fastest burst they saw was zooming away at 105 km s⁻¹. This happened on the eastern edge of the Sun. Interestingly, the computer model didn't predict this specific fast blast. The authors suggest this was likely a sudden, temporary event (a "transient event") that the model, which is designed for steady patterns, couldn't see coming.

What the Paper Rules Out

The team was very careful about what they didn't find. They noticed that at the very edges of their images (farther than 3.7 solar radii), the brightness didn't match the expected physics. They concluded this wasn't a new discovery about the Sun, but rather "stray light"—glare from the atmosphere or the telescope itself messing up the data. So, they decided to ignore anything beyond 3.7 solar radii in their structural analysis to keep their results clean.

They also ruled out the idea that the Sun's shape changes in a simple, straight-line way as you move away from the surface. The "rule of thumb" that says the flattening increases linearly doesn't hold up when you look at the actual data.

The Big Picture

This study didn't just take pretty pictures; it proved that you can measure the speed and direction of the Sun's atmosphere from the ground using a network of amateur telescopes. The results show a mix of gas falling down and shooting up, which mostly agrees with the latest computer simulations, except for those surprise bursts of speed.

The authors suggest that future eclipse projects should keep using this "relay race" method. By combining pictures of the Sun's brightness with measurements of how fast the gas is moving, we can build much better models of our star's atmosphere. It's a reminder that sometimes, you don't need a giant space telescope to see the universe's secrets; you just need a lot of friends with good cameras and clear skies.

Technical Summary: Structure and Dynamics of the Inner Corona Measured from the DEB Initiative 2024 Eclipse Image Sequence

Problem and Motivation
For over a century, networks of observers have captured images of the inner solar corona during total solar eclipses to study its structure and dynamics. Building on citizen science networks established for the 2017 eclipse, the Dynamic Eclipse Broadcast (DEB) Initiative aimed to expand this capability for the April 8, 2024, total solar eclipse. The primary challenge addressed was the collection of high-quality, high-dynamic-range (HDR) coronal data across a wide geographic baseline despite suboptimal weather conditions (cirrus and lower clouds) that compromised many sites. The goal was to utilize a network of amateur-grade equipment to measure coronal brightness profiles, structural flattening, and radial plasma velocities, providing data to test Magnetohydrodynamic (MHD) models of the corona.

Methodology
The DEB Initiative established a network of 82 sites across North America, utilizing standardized equipment: Askar FMA 180 refractors (40mm aperture, 180mm focal length), Player One Neptune M cameras (Sony IMX178 CMOS, 14-bit ADC), and iOptron SkyHunter mounts. The camera's spectral sensitivity peaks at 520nm with a bandwidth of 390–780nm.

Due to poor weather, usable data were obtained from only 11 sites spanning a 2,700 km baseline, covering 49 minutes of eclipse evolution. The data processing pipeline involved:

HDR Image Generation: Sequences of varied exposures were captured every 5 seconds to create HDR images, which were then co-added to improve signal-to-noise ratios.
Alignment and Calibration: Subframes were aligned using phase correlation (trinarized to remove star contamination). An Affine transform matrix (zero shear) was computed to map all site images to a reference site (Site 1), correcting for translation, rotation, and scale. Solar radius was calibrated to 345.7 pixels based on uneclipsed disk measurements.
Static Analysis: Azimuthally averaged radial intensity profiles were computed. The Ludendorff flattening parameter was measured by comparing isophote diameters near the equator versus the poles. The authors also proposed fitting ellipses to brightness isophotes to derive ellipticity and position angle as alternative structural parameters.
Dynamic Analysis: Radial velocities were measured using two methods: cross-correlation for tracking specific features (like stars and dark downflows) and optical flow (Farneback method) for general plasma motion. These measurements were compared against MHD models, specifically the line-of-sight integrated predictions from Y. Li et al. (2026).

Key Results

Coronal Brightness: Radial brightness profiles from all 11 sites were tightly correlated from 1.2 to 4.0 solar radii ( $R_\odot$ ). The DEB data showed excellent agreement with published photometric intensities (Boe et al. 2025) up to 3.7 $R_\odot$ . Above this height, divergence from theoretical F-corona predictions suggested the presence of stray light, leading the authors to restrict structural analysis to heights below 3.7 $R_\odot$ .
Structural Flattening: The directly measured Ludendorff index at 2.0 $R_\odot$ was 0.0761 ± 0.0007. The authors found that extrapolation techniques used in previous literature (linear or parabolic fits to lower heights) yielded significantly different values (e.g., 0.0829), suggesting that such extrapolation methods may introduce inaccuracies in characterizing the corona. They propose that ellipticity and position angle of the semi-major axis, derived from isophote fitting, provide smoother and more accurate structural characterizations.
Plasma Dynamics:
- Downflows: A "tadpole-like" dark feature near the north pole (PA 16°) was tracked moving inward. It exhibited an average radial velocity of -37 ± 3 km s⁻¹ and a deceleration of 14 ± 3 m s⁻². This speed and deceleration align with previous white-light measurements (Sheeley & Wang) and the Y. Li et al. (2026) MHD model predictions for a pseudo-streamer near the north pole.
- Outflows: Radial outflows were detected in the southwestern and northwestern regions. The fastest observed outflow, located above the eastern limb, reached 105 km s⁻¹. The authors note this event was likely a transient phenomenon not captured by the data-constrained MHD model, which predicted a different location for maximum outflow (southwestern limb).
- Model Comparison: Qualitative comparison with the Y. Li et al. (2026) model showed agreement regarding the mixture of outflows and downflows at low heights and the specific polar downflow. However, discrepancies in the location of the fastest outflow highlight the model's inability to capture transient events and the challenges of comparing line-of-sight integrated observations with meridional plane model slices.

Significance and Claims
The paper claims that the DEB Initiative successfully demonstrated the viability of using a distributed network of amateur-grade equipment to generate scientifically robust, high-dynamic-range coronal data. The study provides a direct measurement of the Ludendorff index that challenges the validity of common extrapolation techniques used in the field. Furthermore, it presents the first radial velocity measurements from an eclipse image sequence that can be directly compared to MHD model predictions. The authors conclude that future eclipse experiments combining coronal intensity and radial velocity measurements can more tightly constrain models of the inner corona, particularly by distinguishing between steady-state flows and transient events.

The Sun's Shape: Not Just a Perfect Circle

The Solar Wind: A Rollercoaster of Speeds

What the Paper Rules Out

The Big Picture

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