A study published in Nature Astronomy introduced "Cona," a new coronal adaptive optics (AO) system designed to improve the resolution of solar observations. This system allows astronomers to examine fine structures in the Sun’s corona and better understand phenomena such as coronal heating and solar eruptions.
By correcting atmospheric distortion in real time, Cona produces clear images of solar features like prominences, loops, and coronal rain. These observations reveal detailed plasma behavior and magnetic activity. The findings show that applying AO beyond the solar disk can help resolve features as small as 70 kilometers, offering new opportunities for solar research.
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The Challenge of Observing the Solar Corona
The solar corona, the Sun's outermost atmosphere, is characterized by complex plasma structures with temperatures reaching millions of kelvin. This is much hotter than the Sun’s surface, which is around 6,000 kelvins. Understanding the cause of this extreme heating and what triggers solar eruptions remains a key challenge.
To view fine coronal structures, telescopes with apertures larger than one meter are required to overcome diffraction limits. However, ground-based observations are limited by atmospheric turbulence, which reduces image clarity.
While AO has improved on-disk solar imaging, its application to off-limb observations is more difficult. This is because standard wavefront sensors rely on features found on the solar surface, which are not present in the corona.
Implementing a Novel Coronal Adaptive Optics
To address these limitations, the researchers developed and implemented the Cona system to correct wavefront distortions using coronal plasma features observed in the hydrogen-alpha (Hα) spectral line at 656.3 nm. It was installed on the 1.6-meter Goode Solar Telescope (GST) at Big Bear Solar Observatory, which features an off-axis design that reduces scattered light and enhances sensitivity to faint coronal structures.
Cona uses a modified Shack-Hartmann wavefront sensor designed for coronal imaging. A custom narrowband filter, centered on Hα with a 0.05 nm band-pass, isolates coronal emissions and blocks brighter surface light. Half of the Hα light is used for wavefront sensing, while the other half is directed to scientific imaging through the Visible Imaging Spectrometer (VIS), a Fabry-Pérot interferometer.
The AO software, KAOS Evo 2, enables real-time adjustments by tracking changes in plasma features. It corrects 241 Karhunen-Loève modes at over 2,000 frames per second. This results in Strehl ratios between 20 % and 40 %, which is sufficient for near-diffraction-limited performance. At Hα, the GST's resolution limit is about 85 milliarcseconds, which corresponds to features as small as 60–70 km on the Sun.
New Observations of Coronal Features
Using Cona, researchers captured high-resolution images of off-limb coronal structures such as quiet prominences, loops, and coronal rain. These images showed detailed motions like twisting, braiding, and fine plasma flows that had not been observed with previous ground-based systems.
One notable observation was a twisted coronal plasmoid during the decay of a failed prominence eruption linked to a C6.3-class solar flare. This plasmoid showed multi-threaded structures and moved at speeds up to 90 km/s, indicating ongoing magnetic reconnection. Some strands were narrower than 100 km, close to the telescope’s resolution limit, suggesting structured plasma activity at small scales.
The researchers also analyzed coronal rain, which consists of cool plasma descending from coronal loops. Semi-automated image analysis demonstrated that about half of the rain strands were narrower than 100 km, with some as thin as 64 km. These results supported theoretical predictions of finer magnetic structures, highlighting the need for advanced three-dimensional (3D) radiative magnetohydrodynamic (MHD) simulations.
Applications for Solar Physics and Optical Instrumentation
The implementation of high-order AO for off-limb solar observations represents a significant advancement in solar astronomy and optical engineering. By enabling stable, diffraction-limited imaging of faint coronal structures, it opens new possibilities for studying plasma motion, magnetic reconnection, and coronal heating processes.
Routine observation of features smaller than 100 km will help enhance understanding of rapid, small-scale events such as flares, jets, and spicules. The improved signal-to-noise ratio also supports high-resolution spectroscopic and polarimetric measurements, which are crucial for probing magnetic fields and plasma conditions in the solar atmosphere.
Cona’s wavefront sensor is designed to be low-cost and modular, making it suitable for integration into other solar telescopes, such as the 4-meter Daniel K. Inouye Solar Telescope. Its adaptability may help broaden the use of coronal AO systems in the future.
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Conclusion and Future Directions
The use of high-order AO in off-limb solar imaging has enabled researchers to observe fine structures in the corona with a resolution of 60–70 kilometers. This capability has revealed dynamic processes such as twisted magnetic strands and fast-moving plasmoids, which were previously obscured by atmospheric effects.
The system's image stability also supports advanced spectroscopic and polarimetric studies, offering new insights into magnetic reconnection, coronal heating, and solar eruptions. Future work may involve the development of multi-conjugate AO systems with laser guide stars to expand correction over larger coronal areas. As laser technology for daytime use advances, it may help overcome current limitations in viewing the lower and outer corona.
These developments have the potential to improve our understanding of the Sun’s atmosphere, refine models of solar magnetic activity, and contribute to more accurate space weather forecasting.
Journal Reference
Schmidt, D., Schad, T.A., Yurchyshyn, V. et al. (2025). Observations of fine coronal structures with high-order solar adaptive optics. Nat Astron. DOI: 10.1038/s41550-025-02564-0, https://www.nature.com/articles/s41550-025-02564-0
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