Monday

Radio U-bursts and J-bursts are signatures of electron beams propagating along magnetic loops confined to the corona. Given the prevalence of solar magnetic flux to be closed in the corona, it is not clear why type III bursts are more frequently observed than U-bursts or J-bursts. We use LOFAR imaging spectroscopy between 30–80 MHz of low-frequency U-bursts and J-bursts to understand why electron beams travelling along closed coronal loops produce radio emission less often. The different radio source positions were used to model the spatial structure of the guiding magnetic flux tube and then deduce the energy range of the exciting electron beams without the assumption of a standard density model. The radio sources infer a magnetic loop 1 solar radius in altitude, with the highest frequency sources starting around 0.6 solar radii. Electron velocities were found between 0.13 c and 0.24 c, with the front of the electron beam travelling faster than the back of the electron beam. The density along the loop is higher than typical coronal density models and the density gradient is smaller. The large instability distances required before Langmuir waves are produced by some electron beams, and the small magnitude of the background density gradients make closed loops less facilitating for radio emission than loops that extend into interplanetary space. This can result in type III bursts to be more frequently observed that U-bursts or J-bursts.