Pulsars moving through \ac ism produce bow shocks detected in hydrogen H \alpha line emission .
The morphology of the bow shock nebulae allows one to probe the properties of \ac ism on scales \sim 0.01 pc and smaller .
We performed 2D \ac rmhd modeling of the pulsar bow shock and simulated the corresponding H \alpha emission morphology .
We find that even a mild spatial inhomogeneity of \ac ism density , \delta \rho / \rho \sim 1 , leads to significant variations of the shape of the shock seen in H \alpha line emission .
We successfully reproduce the morphology of the Guitar Nebula .
We infer quasi-periodic density variations in the warm component of \ac ism with characteristic length of \sim 0.1 pc .
Structures of this scale might be also responsible for the formation of the fine features seen at the forward shock of Tycho \ac snr in X-rays .
Formation of such short periodic density structures in the warm component of \ac ism is puzzling , and bow-shock nebulae provide unique probes to study this phenomenon .