  In this paper, with two-dimensional (2-D) hybrid simulations, we study the
generation mechanism of filamentary structures downstream of a quasi-parallel
shock. The results show that in the downstream both the amplitude of magnetic
field and number density exhibit obvious filamentary structures, and the
magnetic field and number density are anticorrelated. Detailed analysis find
that these downstream compressive waves propagate almost perpendicular to the
magnetic field, and the dominant wave number is around the inverse of ion
kinetic scale. Their parallel and perpendicular components roughly
satisfies(where and represent the parallel and in-plane perpendicular
components of magnetic field, is the wave number in the perpendicular
direction, and in the ion gyroradius), and their Alfven ratio also roughly
agree with the analytical relation (where and indicate the Alfven ratio and
plasma beta), while the corresponding cross helicity and compressibility show
good agreement with previous theoretical calculations. All these properties are
consistent with those of kinetic slow waves (KSWs). Therefore, we conclude that
the filamentary structures in the downstream of a quasi-parallel shock are
produced due to the excitation of KSWs.
