  In some solar energetic particle (SEP) events, a counter-streaming particle
beam with a deep depression of flux near 90 degrees pitch angle during the
beginning phase is observed. Two different interpretations exist in the
community to explain this interesting phenomenon. One explanation invokes the
hypothesis of an outer reflecting boundary or a magnetic mirror beyond the
observer. The other one considers the effect of the perpendicular diffusion on
the transport process of SEPs in the interplanetary space. In this work, we
revisit the problem of the counter-streaming particle beams observed in SEP
events and discuss the possible mechanisms responsible for the formation of
this phenomenon. We clarify some results in previous works.
