  Many particles are accelerated during solar flares. To understand the
acceleration and propagation processes of electrons, we require the pitch-angle
distributions of the particles. The pitch angle of accelerated electrons has
been estimated from the propagation velocity of a nonthermal microwave source
archived in Nobeyama Radioheliograph data. We analyzed a flare event (an
M-class flare on 2014 October 22) showing cyclical microwave brightenings at
the two footpoint regions. Assuming that the brightenings were caused by the
accelerated electrons, we approximated the velocity parallel to the magnetic
field of the accelerated electrons as 77,000 and 90,000 km/s. The estimated
pitch angle of the accelerated electrons is 69-80 degrees and the size of the
loss cone at the footpoint (estimated from the magnetic field strength in the
nonlinear force-free field model) is approximately 43 degrees. Most of the
accelerated electrons could be reflected at the footpoint region. This feature
can be interpreted as brightenings produced by bouncing motion of the
accelerated electrons.
