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Two solar type II radio bursts, separated by ~24 hours in time, are examinedtogether. Both events are associated with coronal mass ejections (CMEs)erupting from the same active region (NOAA 11176) beneath a well-observedhelmet streamer. We find that the type II emissions in both events ended oncethe CME/shock fronts passed the white-light streamer tip, which is presumablythe magnetic cusp of the streamer. This leads us to conjecture that the closedmagnetic arcades of the streamer may play a role in electron acceleration andtype II excitation at coronal shocks. To examine such a conjecture, we conducta test-particle simulation for electron dynamics within a large-scalepartially-closed streamer magnetic configuration swept by a coronal shock. Wefind that the closed field lines play the role of an electron trap, via whichthe electrons are sent back to the shock front for multiple times, andtherefore accelerated to high energies by the shock. Electrons with an initialenergy of 300eV can be accelerated to tens of keV concentrating at the loopapex close to the shock front with a counter-streaming distribution at mostlocations. These electrons are energetic enough to excite Langmuir waves andradio bursts. Considering the fact that most solar eruptions originate fromclosed field regions, we suggest that the scenario may be important to thegeneration of more metric type IIs. This study also provides an explanation tothe general ending frequencies of metric type IIs at or above 20-30 MHz and thedisconnection issue between metric and interplanetary type IIs.

Possible role of coronal streamer as magnetically-closed structure in shock-induced energetic electrons and metric type II radio bursts