THE POSSIBLE ROLE OF CORONAL STREAMERS AS MAGNETICALLY CLOSED STRUCTURES IN SHOCK-INDUCED ENERGETIC ELECTRONS AND METRIC TYPE II RADIO BURSTS

Abstract

Two solar type II radio bursts, separated by ~24 hours in time, are examined\ntogether. Both events are associated with coronal mass ejections (CMEs)\nerupting from the same active region (NOAA 11176) beneath a well-observed\nhelmet streamer. We find that the type II emissions in both events ended once\nthe CME/shock fronts passed the white-light streamer tip, which is presumably\nthe magnetic cusp of the streamer. This leads us to conjecture that the closed\nmagnetic arcades of the streamer may play a role in electron acceleration and\ntype II excitation at coronal shocks. To examine such a conjecture, we conduct\na test-particle simulation for electron dynamics within a large-scale\npartially-closed streamer magnetic configuration swept by a coronal shock. We\nfind that the closed field lines play the role of an electron trap, via which\nthe electrons are sent back to the shock front for multiple times, and\ntherefore accelerated to high energies by the shock. Electrons with an initial\nenergy of 300eV can be accelerated to tens of keV concentrating at the loop\napex close to the shock front with a counter-streaming distribution at most\nlocations. These electrons are energetic enough to excite Langmuir waves and\nradio bursts. Considering the fact that most solar eruptions originate from\nclosed field regions, we suggest that the scenario may be important to the\ngeneration of more metric type IIs. This study also provides an explanation to\nthe general ending frequencies of metric type IIs at or above 20-30 MHz and the\ndisconnection issue between metric and interplanetary type IIs.\n