Possible role of coronal streamer as magnetically-closed structure in   shock-induced energetic electrons and metric type II radio bursts
  Two solar type II radio bursts, separated by ~24 hours in time, are examined together. Both events are associated with coronal mass ejections (CMEs) erupting from the same active region (NOAA 11176) beneath a well-observed helmet streamer. We find that the type II emissions in both events ended once the CME/shock fronts passed the white-light streamer tip, which is presumably the magnetic cusp of the streamer. This leads us to conjecture that the closed magnetic arcades of the streamer may play a role in electron acceleration and type II excitation at coronal shocks. To examine such a conjecture, we conduct a test-particle simulation for electron dynamics within a large-scale partially-closed streamer magnetic configuration swept by a coronal shock. We find that the closed field lines play the role of an electron trap, via which the electrons are sent back to the shock front for multiple times, and therefore accelerated to high energies by the shock. Electrons with an initial energy of 300eV can be accelerated to tens of keV concentrating at the loop apex close to the shock front with a counter-streaming distribution at most locations. These electrons are energetic enough to excite Langmuir waves and radio bursts. Considering the fact that most solar eruptions originate from closed field regions, we suggest that the scenario may be important to the generation of more metric type IIs. This study also provides an explanation to the general ending frequencies of metric type IIs at or above 20-30 MHz and the disconnection issue between metric and interplanetary type IIs. 
Xiangliang Kong, Yao Chen, Fan Guo, Shiwei Feng, Bing Wang, Guohui Du,   and Gang Li
astro-ph.SR