Late solar radio bursts point to particle acceleration after a flare’s peak

A radio climax after the flare peak

Suli Ma and colleagues examined an M1.1 solar flare whose soft X-ray emission peaked at 01:50 UT on 17 December 2014. The strongest decimetric radio activity occurred around 02:12 UT, well into the flare’s decline, as a larger twisted coronal structure erupted.

The team interprets these late bursts as evidence consistent with breakout magnetic reconnection: a rearrangement of magnetic connections between the erupting structure and surrounding fields. Such reconnection can release magnetic energy and accelerate electrons even while the main flare’s X-ray brightness is fading.

The analysis is available as an arXiv preprint submitted on 6 November 2026. The supplied manuscript is marked as submitted to The Astrophysical Journal; the landing metadata also lists a related journal DOI.

Combining radio images with ultraviolet views

The central observations came from the Mingantu Spectral Radioheliograph’s MUSER-I array, which observed between 0.4 and 2 gigahertz. Its measurements have a time resolution of 25 milliseconds and a frequency resolution of 25 megahertz, allowing the researchers to examine rapidly changing radio features.

The team calibrated the antenna measurements and reconstructed images at selected frequencies. They then compared radio-source positions with extreme-ultraviolet images from NASA’s Solar Dynamics Observatory, which revealed the movement of hot and cooler coronal structures. Microwave observations from the Nobeyama Radioheliograph and X-ray measurements from RHESSI provided additional context.

Two active regions were contributing emission during the event. Because GOES soft X-ray measurements combine light from the whole Sun, the researchers used region-by-region ultraviolet brightness to identify AR 12241 as the predominant source of the M1.1 flare.

Three stages of a complicated eruption

The observations suggest three stages. First came an internal eruption involving sheared and twisted magnetic structures. Some inner loops disappeared, flare ribbons appeared, and the main flare reached its peak. MUSER-I detected relatively weak decimetric emission during this stage.

Next, a larger, hotter twisted structure rose and expanded against surrounding loops. Two narrowband radio bursts appeared near 0.67 gigahertz. Their concentrated source positions suggest that electron acceleration was occurring in a localized region.

The final stage brought the strongest successive bursts, accompanying the main structure’s breakout eruption. Five notable radio bursts were identified during this phase. Most emission occurred between 0.4 and 0.8 gigahertz, although the strongest cluster extended to about 1.4 gigahertz.

The first major cluster coincided with a prominent ejection. Four later clusters likely corresponded to further ejections, according to the authors’ comparison of radio brightness and ultraviolet motions. That relationship was not exact: after 02:20 UT, radio-source distances continued increasing even as peak radio brightness declined.

A magnetic boundary offers an explanation

To investigate where acceleration might occur, the researchers extrapolated the coronal magnetic field from surface magnetic measurements. They calculated a quantity called the squashing factor, Q, which identifies regions where neighboring magnetic field lines connect to substantially different locations.

High-Q regions can mark quasi-separatrix layers, boundaries favorable for magnetic reconnection. The modeled high-Q region broadly corresponded to the late radio sources, supporting the proposed breakout interpretation.

The bursts’ rapid frequency changes, narrowband structures and polarization were consistent with plasma emission driven by energetic electron beams. In this process, electrons excite disturbances in the surrounding plasma that can generate radio waves. The radio signatures therefore offer an indirect way to investigate acceleration and transport.

The authors propose that the later reconnection involved a more extended region at higher altitude, with magnetic geometry favorable for accelerating and channeling electrons. Repeated smaller eruptions could supply fresh electron beams to the same boundary. These are physical interpretations of the observations, rather than direct measurements of the accelerating electric fields.

What the observations cannot settle

Important limitations remain. Interference removed the 0.8–1.0 gigahertz interval, preventing continuous tracking of some spectral features. Absolute radio positions were limited by angular resolutions of roughly 130 arcseconds at 0.4 gigahertz and 50 arcseconds at 1.4 gigahertz. The authors also describe their magnetic reconstruction as a topological guide, not a complete recovery of the erupting structure.

No clear additional thermal enhancement accompanied the strongest late bursts, and no clear hard X-ray signature was identified. The team suggests that many accelerated electrons escaped upward or lost energy before reaching denser lower layers; the missing signals do not establish their trajectories uniquely.

Taken together, the findings support a role for late breakout reconnection in this event. They also suggest that a coronal pseudostreamer—a configuration combining underlying magnetic arcades with outward-extending fields—may channel accelerated particles into a narrow region. A single eruption, however, does not establish how generally that picture applies.