🔭 astrophysics

UK White Paper on Space-based total solar eclipse observations: structure and dynamics of the solar atmosphere

This UK White Paper proposes the Moon-Enabled Sun Occultation Mission (MESOM), a space-based initiative utilizing total solar eclipses to investigate the Sun's magnetic fields, atmospheric dynamics, and space weather impacts, thereby addressing key scientific goals outlined in the UK's National Space Strategy and STFC roadmap.

Original authors: Lucie M. Green, Nicola Baresi, Huw Morgan, Matt Gunn, Amir Caspi, Daniel B. Seaton, Giulio Del Zanna, Yeimy J. Rivera, Francisco Javier Bailen, David Orozco Suarez, Anthony R. Yeates, Peter Wyper, Naw

Published 2026-06-05

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CC BY 4.0

Original authors: Lucie M. Green, Nicola Baresi, Huw Morgan, Matt Gunn, Amir Caspi, Daniel B. Seaton, Giulio Del Zanna, Yeimy J. Rivera, Francisco Javier Bailen, David Orozco Suarez, Anthony R. Yeates, Peter Wyper, Nawin Ngampoopun, Nathalia Alzate, Simone Di Matteo, Erika Palmerio, Richard J. Morton, Eleanna Asvestari, Manuela Temmer, Greta Cappello, Teodora Mihailescu, Karl Battams

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

The Big Idea: A Space-Based "Moon Shadow"

Imagine trying to study the Sun's outer atmosphere (the corona). Normally, the Sun's blinding face is like a giant, bright spotlight that makes it impossible to see the faint, wispy smoke (the corona) swirling around it.

Usually, scientists have to wait for a total solar eclipse on Earth, where the Moon naturally blocks the Sun's face, allowing us to see the corona for a few minutes. But these events are rare and only happen in specific spots on Earth.

The MESOM Mission proposes a clever solution: send a spacecraft to a special spot in space where the Moon always blocks the Sun for the spacecraft.

The Analogy: Think of the Moon as a giant, cosmic "fingertip" held up to a flashlight. The spacecraft sits in the shadow of that fingertip.
The Result: Every month (about every 29.6 days), the spacecraft gets a perfect, total eclipse. This gives scientists over 400 minutes of "darkness" every two years to study the Sun without the glare, without needing to build complex mechanical shields (which often cause their own problems).

The Four "Eyes" of the Mission

The spacecraft, called MESOM, carries four special instruments (a "payload") that weigh less than a small car (under 40 kg). Think of them as four different cameras, each with a special job:

MagCHILS (The Magnetic Mapmaker):
- What it does: It takes pictures of invisible magnetic lines using a special filter.
- The Analogy: Imagine trying to see wind. You can't see the wind itself, but if you sprinkle glitter in the air, the glitter shows you where the wind is blowing. MagCHILS looks at a specific color of light (infrared) to see the "glitter" of magnetic fields in the Sun's atmosphere, telling us how strong they are and how they are shaped.
HiBri (The Super-Sharp Zoom Lens):
- What it does: It takes incredibly fast, high-definition videos of a small patch of the Sun.
- The Analogy: This is like a high-speed camera on a race car. It snaps photos so fast (every 4 seconds) and so clearly that it can see tiny ripples and waves moving through the Sun's gas, which helps explain how the Sun gets so hot.
LoBri (The Wide-Angle Context Camera):
- What it does: It takes a wider view of the Sun and measures the density of the gas.
- The Analogy: If HiBri is a microscope, LoBri is a drone flying high above a city. It sees the whole neighborhood. It helps scientists understand how the small ripples seen by HiBri connect to the big, global storms on the Sun. When the Moon isn't blocking the Sun, this camera acts like a "heliospheric imager," watching the solar wind blow out into space.
CHILS (The Chemical Detective):
- What it does: It breaks light down into a rainbow (spectrum) to identify what elements are present.
- The Analogy: This is like a forensic lab. By looking at the specific "fingerprints" of light from different elements (like iron or argon), it tells scientists exactly what the Sun is made of, how hot it is, and how the gas is moving.

The Big Questions They Want to Answer

The paper lists five main mysteries the mission hopes to solve:

The Magnetic Skeleton: How are the Sun's magnetic fields arranged? (Currently, we have to guess based on models; this mission will take the actual "X-ray" to see the truth).
The Heating Mystery: Why is the Sun's outer atmosphere millions of degrees hotter than its surface? The mission will look for "waves" (like sound waves) that might be carrying heat up from the surface.
The Recipe of the Sun: Why does the Sun's atmosphere have different ingredients in different places? The mission will track how heavy and light elements mix and separate as they rise.
The Storm Makers: How do tiny magnetic snaps (reconnection) build up into giant explosions (solar flares and CMEs) that can mess up Earth's technology?
The Wind's Origin: How does the solar wind (a stream of particles) start? The mission will watch the "birth zone" where the wind leaves the Sun to see what makes it speed up.

Bonus: The Comet Hunters

While the Sun is blocked, the spacecraft can also look at the dark sky around it.

The Analogy: It's like looking at a dark stage while the spotlight is off.
The Goal: They hope to catch "sun-grazing" comets (comets that fly very close to the Sun) as they break apart. This will help scientists understand what these comets are made of and what forces tear them apart as they get too close to the heat.

Why the UK is Leading This

The paper highlights that the UK has been the world leader in studying solar eclipses for decades.

The Team: It's a global effort involving the UK, Europe, the US, and Australia.
The Strategy: This isn't just about science; it's about building a "superpower" in space technology, training a new generation of engineers, and improving how we predict "space weather" (storms that can affect satellites and power grids on Earth).

In short: MESOM is a mission to put a camera in the Moon's shadow, giving us a permanent, perfect view of the Sun's hidden atmosphere to solve the mysteries of how our star works and how it affects our planet.

Technical Summary: UK White Paper on Space-based Total Solar Eclipse Observations (MESOM)

Problem Statement
The paper identifies critical gaps in understanding the Sun's atmosphere, specifically regarding the structure and dynamics of the solar atmosphere from the chromosphere to the solar wind. Key unresolved scientific questions include:

The strength and global structure of the Sun's magnetic field, which is currently constrained by models with limited observational validation.
The role of magnetic waves (specifically Alfvénic waves) in heating and accelerating plasma, including the difficulty in estimating total wave energy flux and its partition.
The mechanisms driving variations in plasma composition, density, and temperature across coronal structures.
The role of magnetic reconnection in restructuring the corona from small to large scales and driving escaping plasma.
The fine-scale structure of the extended solar wind and its link to small-scale events near the Sun.
The physical processes leading to the destruction of Kreutz-group comets and non-gravitational forces acting on them.

Current ground-based observations are limited by atmospheric degradation, and existing space-based coronagraphs suffer from diffraction and stray light issues due to internal or external occulters.

Methodology: The MESOM Concept
The proposed solution is the Moon-Enabled Sun Occultation Mission (MESOM). This mission concept utilizes the Moon as a natural occulter to recreate total solar eclipse conditions in space once every synodic month (~29.6 days).

Orbit: The spacecraft will fly a 2:1 synodic resonant orbit around the Earth-Moon barycenter, completing two revolutions per synodic month. This trajectory places the spacecraft near the apex of the Moon's umbral region.
Advantages: This approach eliminates the need for internal/external occulters (simplifying design and avoiding artifacts) and avoids Earth's atmospheric degradation. The large distance between the spacecraft and the Moon minimizes diffraction and stray light.
Observation Strategy: The mission provides over 400 minutes of totality over a two-year nominal operation. Outside of totality, the instruments operate in "heliospheric imager" mode to observe the extended corona and solar wind.

Instrumentation Suite
The payload consists of four coordinated instruments with a total mass of less than 40 kg, designed to provide quantitative measurements of coronal magnetic fields and plasma properties:

MagCHILS (Magnetic-Coronal High-Resolution Imaging Line Spectrometer): An imaging spectropolarimeter targeting the Fe XIII 1074 nm emission line. It offers a field of view (FoV) up to 6 x 6 solar radii ( $R_s$ ) with 10" resolution, enabling vector magnetic field measurements from the upper chromosphere to ~2 $R_s$ .
HiBri (High-resolution Broadband Imager): A visible light imager (580 nm, 100 nm FWHM) with a small FoV (0.5 x 0.5 $R_s$ ). It utilizes a 0.6m aperture three-mirror anastigma telescope to achieve unprecedented spatial resolution (0.24", or 170 km at the Sun) and high temporal cadence (4 seconds).
LoBri (Low-resolution Broadband Imager): A larger FoV (4.2 x 4.2 $R_s$ ) visible light telescope. It provides spatiotemporal resolution of 2.2" (1400 km) and 15s cadence. It includes a filter wheel and polarizers to calculate polarized brightness for density diagnostics. Outside totality, it uses a baffle system to function as a heliospheric imager.
CHILS (Coronal High-resolution Imaging Line Spectrometer): A spectral imager mapping four emission lines (Fe X 789 nm, Fe XI 637 nm, Fe XIV 530 nm, Ar X 554 nm) across a 3.2 x 3.2 $R_s$ FoV. It operates with a 12s cadence in time-series mode to capture small-scale motions and waves.

Key Contributions and Expected Results
The paper outlines how MESOM data will address specific scientific questions:

Magnetic Field Structure: MagCHILS will provide the coronal magnetic field vector required to validate and constrain global magnetic field models, essential for space weather forecasting.
Wave Dynamics: The combination of HiBri (imaging) and CHILS (Doppler/linewidth) will allow for the quantitative study of Alfvénic wave amplitudes, periodicities, and propagation, helping to estimate total wave energy flux.
Plasma Properties: Spectral imaging across multiple emission lines will map elemental abundances (FIP effect), plasma density, and temperature, revealing energy and mass transport mechanisms.
Reconnection and CMEs: High-resolution broadband imaging will link small-scale dynamic events (blobs, inflows) to global coronal restructuring and Coronal Mass Ejections (CMEs), probing how CMEs redistribute open magnetic flux.
Solar Wind and Comets: High-resolution heliospheric imager data will map density features in the extended corona to understand solar wind origins. Additionally, the mission will monitor the fragmentation of Kreutz-group comets to understand sublimation-driven forces and non-gravitational effects.

Significance and Strategic Context
The paper positions MESOM as a strategic initiative for the UK space sector, aligning with the "National Space Strategy" and the "UK Space Frontiers 2035" thematic area on the Sun and heliosphere.

Scientific Leadership: The mission leverages the UK's world-leading expertise in eclipse experimentation, coronal physics, and numerical modeling. It aims to deliver "Pillars" of the National Space Strategy, including unlocking sector growth, fostering international collaboration (UK-Europe-US-Australia consortium), and establishing the UK as a science and technology superpower.
Operational Impact: By providing accurate models of the solar coronal magnetic field, MESOM aims to improve the accuracy and reliability of space weather forecasts.
Broader Engagement: The mission plans to utilize the inherent public interest in solar eclipses for diverse community engagement, including citizen science projects (e.g., comet tracking), school activities, and artist collaborations.
Collaboration: The concept is developed by an international consortium, with specific instrument leadership anticipated from Spain (MagCHILS), the US (LoBri/HiBri), and the UK (CHILS), building on heritage from missions like Solar Orbiter, Proba-3, and Parker Solar Probe.