  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.
