Shock-accelerated electrons during the fast expansion of a coronal mass ejection

Context. Some of of the most prominent sources for energetic particles in our Solar System are huge eruptions of magnetised plasma from the Sun called coronal mass ejections (CMEs), which usually drive shocks that accelerate charged particles up to relativistic energies. In particular, energetic electron beams can generate radio bursts through the plasma emission mechanism. The main types of bursts associated with CME shocks are type II and herringbone bursts. However, it is currently unknown where early accelerated electrons that produce metric type II bursts and herringbones propagate and when they escape the solar atmosphere. Aims. Here, we investigate the acceleration location, escape, and propagation directions of electron beams during the early evolution of a strongly expanding CME-driven shock wave associated with herrinbgone bursts. Methods. We used ground-based radio observations from the Nancay Radioheliograph combined with space-based extreme-ultraviolet and white-light observations from the Solar Dynamics Observatory and and the Solar Terrestrial Relations Observatory. We produced a three-dimensional (3D) representation of the electron acceleration locations which, combined with results from magnetohydrodynamic (MHD) models of the solar corona, was used to investigate the origin of the herringbone bursts observed. Results. Multiple herringbone bursts are found close to the CME flank in plane-of-sky images. Some of these herringbone bursts have unusual inverted J shapes and opposite drifting herringbones also show opposite senses of circular polarisation. By using a 3D approach combined with the radio properties of the observed bursts, we find evidence that the first radio emission in the CME eruption most likely originates from electrons that initially propagate in regions of low Alfven speeds and along closed magnetic field lines forming a coronal streamer. The radio emission appears to propagate in the same direction as a coronal wave in three dimensions. Conclusions. The CME appears to inevitably expand into a coronal streamer where it meets ideal conditions to generate a fast shock which, in turn, can accelerate electrons. However, at low coronal heights, the streamer consists of exclusively closed field lines indicating that the early accelerated electron beams do not escape. This is in contrast with electrons which, in later stages, escape the corona so that they are detected by spacecraft.