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Microwave Quasi-periodic Pulsation with Millisecond Bursts in A Solar Flare on 2011 August 9

An peculiar microwave quasi-periodic pulsation (QPP) accompanying with a hard X-ray (HXR) QPP of about 20 s duration occurred just before the maximum of an X6.9 solar flare on 2011 August 9. The most interesting is that the microwave QPP is consisting of millisecond timescale superfine structures. Each microwave QPP pulse is made up of clusters of millisecond spike bursts or narrow band type III bursts. There are three different frequency drift rates: global frequency drift rate of microwave QPP pulse group, frequency drift rate of microwave QPP pulse, and frequency drift rate of individual millisecond spikes or type III bursts. The physical analysis indicates that the energetic electrons accelerating from a large-scale highly dynamic magnetic reconnecting current sheet above the flaring loop propagate downwards, impact on the flaring plasma loop, and produce HXR bursts. The tearing-mode (TM) oscillations in the current sheet modulate HXR emission and generate HXR QPP; the energetic electrons propagating downwards produce Langmuir turbulence and plasma waves, result in plasma emission. The modulation of TM oscillation on the plasma emission in the current-carrying plasma loop may generate microwave QPP. The TM instability produces magnetic islands in the loop. Each X-point will be a small reconnection site and accelerate the ambient electrons. These accelerated electrons impact on the ambient plasma and trigger the millisecond spike clusters or the group of type III bursts. Possibly each millisecond spike burst or type III burst is one of the elementary burst (EB). Large numbers of such EB clusters form an intense flaring microwave burst.