Closed-field electron acceleration in the solar corona: insights from J- and U-type solar radio bursts

Solar radio bursts (SRBs) provide key diagnostics of energetic electron propagation and coronal magnetic field structures. Among them, J- and U-type bursts are particularly valuable for tracing electron motion along closed magnetic loops, yet they remain less explored than type III bursts in open field lines. Using high-resolution dynamic spectra from the e-CALLISTO network, we analyzed a total of 38 SRB events (J-type 18 events and U-type 20 events) observed during the ascending phase of Solar Cycle 25, from June 2020 to December 2024, to characterize their spectral morphology, frequency drift, and magnetic connectivity. Complementary, Solar Dynamic Observatory/Helioseismic Magnetic Imager (SDO/HMI), Potential-Field Source-Surface Models (PFSS) extrapolations, and SDO/Atmospheric Imaging Assembly (AIA) extreme ultraviolet (EUV) imaging have been used to identify associated active regions and loop geometries. Approximately 30% of the events occurred without associated flares, indicating that small-scale reconnections can also produce SRBs. Application of the Newkirk density model yielded that the drift-rate analysis reveals broadly similar descending branches for J- and U-type SRBs but subtle asymmetries in ascending branches; steeper drift in J-type and smoother drift in U-type, indicating differences in electron propagation and density structuring between low-lying and extended closed coronal loops. For 19 of 38 events, constrained by the PFSS upper boundary (2.5 Rs), the loop heights inferred from the Newkirk model agree with PFSS within ±0.5 Rs, although PFSS tends to underestimate more extended structures. These results demonstrate the diagnostic potential of J- and U-type SRBs for probing electron propagation and closed magnetic structures in the solar corona.