Possible Role of Coronal Streamer as Magnetically-closed Structure in Shock-induced Energetic Electrons and Metric Type II Radio Bursts

Abstract:

Solar type II radio bursts are excited by energetic electrons accelerated at coronal eruption-driven shocks. Streamers are quasi-steady and dense, and magnetically-closed structures in the corona, where the Alfven speed is much lower and plasma outflow is much slower than that of the surroundings, therefore are expected to facilitate the formation/enhancement of shocks. In recent studies, streamers have been suspected to be important on the generation of type II radio bursts and the morphology of radio dynamic spectra. In this study, we first present two type II events in which the type II ends upon the CME front (shock) passing by the streamer tip (cusp). The observations lead us to conjecture that the large-scale closed magnetic field of the streamer may be important to electron acceleration at coronal shocks and excitation of type II bursts. To validate this physical implication, we develop a streamer-shock model consisting of a streamer and an outward-propagating shock, and perform a test-particle simulation. It shows that only those electrons that are injected within the closed field regions can be accelerated efficiently, and the trapping effect via closed field lines allows the trapped electrons to return to the shock front multiple times and be repetitively accelerated. We suggest that the scenario may be potentially important to the generation of more metric type IIs considering the fact that most solar eruptions originate from closed field regions. This scenario also provides an explanation to the ending frequencies of many metric type IIs and the long-standing issue of the disconnection between metric and interplanetary type II bursts.