Solar type II radio bursts are slow-drifting emissions in radio dynamic spectra, traditionally interpreted as signatures of shock waves driven by fast coronal mass ejections (CMEs). They provide the primary diagnostics for energetic electron acceleration and transport in solar eruptions. However, a puzzling class of metric type II bursts occurs without any detectable CME and is linked only to relatively weak solar flares and small-scale jets. The fundamental question is: what drives the shock wave in these events? A recent study by Cui et al. (2026) presented clear observational evidence that a jet-induced disturbance can steepen into a shock within specific coronal structures, offering a new explanation for these non-CME type II bursts. The work also reports a related type N radio burst, a rare tracer of electron beams propagating and mirroring within large coronal loops.
The event occurred on May 8, 2023, associated with a weak C3.1-class solar flare and a small-scale coronal jet, but no CME seen in SOHO/LASCO or STEREO coronagraphs. The radio spectrum was mainly recorded by the high-performance solar radio spectrograph at meter wavelengths recently developed at the Chashan solar radio observatory (CSO, Chang et al. 2024). As shown in Figure 1, it revealed multiple fast-drifting type III bursts and a distinct type N burst during the flare impulsive phase, followed by a type II burst with clear band-splitting in the gradual phase.
Figure 1. Radio dynamic spectrum observed by CSO (110-300 MHz), Learmonth (80-110 MHz), and GERMANY-DLR (50-80 MHz), displaying type III and type III-like (J, N) bursts followed by a type II burst.
The Type N Burst and In-Loop Electron Transport
The type N burst, named for its characteristic N-shaped spectral profile, is a variant of type III bursts. As illustrated in Figure 2, its unique shape arises from an electron beam propagating along a large closed coronal loop: first moving outward (negative frequency drift), then passing the loop apex and traveling sunward (positive drift), and finally being reflected via magnetic mirroring at the remote loop leg (negative drift again). Radio imaging from the Nançay Radioheliograph (NRH) traced this path, showing the radio source shifting from the flare site to a distant location and back. These observations provide direct evidence of electron beams (~15 keV) being injected into and trapped within large-scale closed coronal magnetic fields.
Figure 2. (a) High-resolution (0.1 s) CSO dynamic spectrum highlighting the harmonic emission of the type N burst. (b) Schematic of type N burst formation: red arrows indicate electron beam propagation, blue and green curves represent closed and open magnetic field lines, ellipses denote NRH-observed source locations as shown in panels (c) and (d).
Origin of the Type II Burst
The type II burst exhibits well-defined fundamental-harmonic bands, band splitting, and herringbone fine structures (Figure 3). By combining NRH radio imaging with EUV images from SDO/AIA, it shows that the type II sources coincide spatially with a weak, wave-like perturbation front propagating through nearby closed loops at ~880 km/s, far faster than the jet front itself. The speed of this EUV perturbation matches the shock speed (~800 km/s) inferred from the drift rate of the type II burst using the Newkirk coronal density model. These results demonstrate that the jet launched a fast disturbance that propagated into dense closed loop systems with low Alfven speeds, where it steepened into a fast-mode shock. This shock then accelerated electrons to produce the observed type II radio emission.
Figure 3. (a) CSO dynamic spectrum of the type II burst. (b1-b4) AIA 171Å running-difference images and (c1-c4) original images overlaid with NRH sources at 173 MHz and 228 MHz.
This study reported a metric type II burst driven by a small-scale jet eruption and lacking CME association. While metric type II bursts are widely attributed to CME-driven shocks, some events can instead be linked to flare blast waves, jet eruptions, or localized reconnection processes (Su et al. 2015; Morosan et al. 2023). These events highlight that the coronal environment itself (especially dense, low Alfvenic closed loop systems) plays a decisive role in enabling shock formation and electron acceleration in weak solar eruptions. Future observations with new-generation radio imaging spectrographs will be critical for determining how common this jet-driven shock mechanism is and for refining models of solar activities.
Based on a recent paper: Y. Cui, X. Kong, Z. Li, et al., A solar jet-induced perturbation propagating through coronal loops and in-loop electron beam transport as indicated by type II and type N radio bursts, A&A, 706, A197, 2026. DOI: 10.1051/0004-6361/202555230
References
Cui, Y., Kong, X., Li, Z., et al. 2026, A&A, 706, A197
Chang, S., Wang, B., Lu, G., et al. 2024, ApJS, 272, 21
Su, W., Cheng, X., Ding, M., et al. 2015, ApJ, 804, 88
Morosan, D., Pomoell, J., Kumari, A., et al. 2023, A&A, 675, A98