Photo voltaic sort II radio bursts are slow-drifting emissions in radio dynamic spectra, historically interpreted as signatures of shock waves pushed by quick coronal mass ejections (CMEs). They supply the first diagnostics for energetic electron acceleration and transport in photo voltaic eruptions. Nevertheless, a puzzling class of metric sort II bursts happens with none detectable CME and is linked solely to comparatively weak photo voltaic flares and small-scale jets. The basic query is: what drives the shock wave in these occasions? A latest examine by Cui et al. (2026) introduced clear observational proof {that a} jet-induced disturbance can steepen right into a shock inside particular coronal constructions, providing a brand new rationalization for these non-CME sort II bursts. The work additionally reviews a associated sort N radio burst, a uncommon tracer of electron beams propagating and mirroring inside giant coronal loops.
The occasion occurred on Might 8, 2023, related to a weak C3.1-class photo voltaic flare and a small-scale coronal jet, however no CME seen in SOHO/LASCO or STEREO coronagraphs. The radio spectrum was primarily recorded by the high-performance photo voltaic radio spectrograph at meter wavelengths lately developed on the Chashan photo voltaic radio observatory (CSO, Chang et al. 2024). As proven in Determine 1, it revealed a number of fast-drifting sort III bursts and a definite sort N burst through the flare impulsive section, adopted by a kind II burst with clear band-splitting within the gradual section.
Determine 1. Radio dynamic spectrum noticed by CSO (110-300 MHz), Learmonth (80-110 MHz), and GERMANY-DLR (50-80 MHz), displaying sort III and sort III-like (J, N) bursts adopted by a kind II burst.
The Sort N Burst and In-Loop Electron Transport
The kind N burst, named for its attribute N-shaped spectral profile, is a variant of sort III bursts. As illustrated in Determine 2, its distinctive form arises from an electron beam propagating alongside a big closed coronal loop: first shifting outward (unfavorable frequency drift), then passing the loop apex and touring sunward (optimistic drift), and eventually being mirrored through magnetic mirroring on the distant loop leg (unfavorable drift once more). Radio imaging from the Nançay Radioheliograph (NRH) traced this path, displaying the radio supply shifting from the flare website to a distant location and again. These observations present direct proof of electron beams (~15 keV) being injected into and trapped inside large-scale closed coronal magnetic fields.
Determine 2. (a) Excessive-resolution (0.1 s) CSO dynamic spectrum highlighting the harmonic emission of the sort N burst. (b) Schematic of sort N burst formation: crimson arrows point out electron beam propagation, blue and inexperienced curves signify closed and open magnetic area strains, ellipses denote NRH-observed supply areas as proven in panels (c) and (d).
Origin of the Sort II Burst
The kind II burst displays well-defined fundamental-harmonic bands, band splitting, and herringbone fantastic constructions (Determine 3). By combining NRH radio imaging with EUV pictures from SDO/AIA, it exhibits that the sort II sources coincide spatially with a weak, wave-like perturbation entrance propagating by close by closed loops at ~880 km/s, far quicker than the jet entrance itself. The velocity of this EUV perturbation matches the shock velocity (~800 km/s) inferred from the drift price of the sort II burst utilizing the Newkirk coronal density mannequin. These outcomes reveal that the jet launched a quick disturbance that propagated into dense closed loop methods with low Alfven speeds, the place it steepened right into a fast-mode shock. This shock then accelerated electrons to supply the noticed sort II radio emission.
Determine 3. (a) CSO dynamic spectrum of the sort II burst. (b1-b4) AIA 171Å running-difference pictures and (c1-c4) authentic pictures overlaid with NRH sources at 173 MHz and 228 MHz.
This examine reported a metric sort II burst pushed by a small-scale jet eruption and missing CME affiliation. Whereas metric sort II bursts are broadly attributed to CME-driven shocks, some occasions can as an alternative be linked to flare blast waves, jet eruptions, or localized reconnection processes (Su et al. 2015; Morosan et al. 2023). These occasions spotlight that the coronal setting itself (particularly dense, low Alfvenic closed loop methods) performs a decisive position in enabling shock formation and electron acceleration in weak photo voltaic eruptions. Future observations with new-generation radio imaging spectrographs will probably be essential for figuring out how widespread this jet-driven shock mechanism is and for refining fashions of photo voltaic actions.
Based mostly on a latest paper: Y. Cui, X. Kong, Z. Li, et al., A photo voltaic jet-induced perturbation propagating by coronal loops and in-loop electron beam transport as indicated by sort II and sort 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
