  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.
