How the Sun’s atmosphere blurs its own radio signals

A radio image of the Sun records more than where radiation began. Before reaching a telescope, radio waves cross the corona, the Sun’s turbulent, magnetised outer atmosphere. That journey can enlarge an apparent source, shift its position and change its polarization—the orientation and rotation of its electric field.

A chapter by Soham Dey and colleagues examines this problem, called coronal “seeing.” Available as an arXiv preprint submitted on 9 November 2026, and listed as published in Advancing Astrophysics with the SKA–II, it reviews earlier observations and theory rather than presenting a new observing campaign. Its central argument is that unexpectedly compact radio sources make high-resolution observations worth pursuing, despite the corona’s blurring effects.

What changes on the journey?

The bursts discussed arise through plasma emission: accelerated electrons travelling along magnetic fields excite disturbances in the surrounding electrically charged gas, which produce radio waves. These signals carry information about electron acceleration, density and magnetic structure.

Three propagation processes complicate that information. Large-scale density changes bend the waves through refraction, potentially shifting a source’s apparent location. Smaller density fluctuations scatter radiation along different paths, broadening images and spreading arrival times. In the magnetised plasma, coupling between different propagation modes can also change polarization.

Scattering can make a compact, intense source appear larger and less bright per unit area. The resulting reduction in “brightness temperature,” a measure of radio intensity, need not mean the emitting material is physically cooler. Rapid variations can likewise become smeared before any telescope records them.

Compact sources inside broader emission

The review’s strongest examples include type I noise storms: persistent radio emission with rapid, narrowband bursts, usually associated with active solar regions.

A 2024 study used the upgraded Giant Metrewave Radio Telescope, or uGMRT, to make snapshot images at 220–250 megahertz. Its angular resolution was approximately 20 by 9 arcseconds. The images revealed compact components within diffuse emission, with structures changing on timescales shorter than a minute. The review identifies approximately 9 arcseconds as an upper limit for some of the smallest structures—not a precisely resolved diameter.

Follow-up observations at 250–500 megahertz found multiple sources smaller than 20 arcseconds in active regions near the centre of the solar disc. Together, these results suggest that strong broadening is not inevitable under every coronal condition.

Other observations broaden the picture. LOFAR imaging of shock-associated type II bursts has found compact sources and, in particular events, spatially separated regions producing split frequency bands. Type III bursts, generated by electron beams travelling along open magnetic fields, show apparent trajectories and structures strongly affected by propagation. The chapter also highlights reported linear polarization in type I and type III emission, challenging expectations that coronal propagation would erase it completely.

How to separate source from distortion

Radio interferometers combine measurements from antennas separated by different distances, called baselines. Long baselines provide access to finer angular detail, but they must also be sensitive enough to detect compact emission. A missing signal on a long baseline therefore does not automatically prove that the corona has erased every small structure.

The proposed approach combines images across time and frequency with polarization measurements. Researchers can track source sizes, shapes, positions and burst timing, then compare those changes with propagation models.

Another useful comparison involves fundamental plasma emission and its harmonic, emitted at approximately twice the local plasma frequency. Fundamental emission is especially susceptible to propagation near its source. Harmonic emission is generally less affected there, but is not immune: some observations show comparable apparent sizes despite different time evolution.

What the SKA could clarify

The authors argue that the Square Kilometre Array Observatory’s sensitivity on long baselines and broad frequency coverage could constrain scattering more directly. SKA-Low’s 50–350 megahertz band would probe strongly affected emission, while higher-frequency SKA-Mid observations would explore conditions where scattering is weaker.

Spacecraft provide complementary measurements closer to the Sun. However, the current spacecraft receivers discussed operate below about 20 megahertz, so they do not overlap SKA-Low’s band. Coordinated observations would follow the same events at different frequencies and heights, rather than make identical-frequency comparisons.

Important uncertainties remain. Published source sizes use different definitions and corrections, making comparisons indicative rather than exact. Turbulence geometry can itself generate multiple apparent components, so compactness alone does not prove an undistorted view. Better instruments cannot simply undo propagation that has already occurred.

The opportunity is therefore twofold: recover more of the emitting source’s behaviour, and use the distortions themselves to investigate the corona through which the radiation travelled.