  Pulsars moving through 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 ISM on scales $\sim 0.01$ pc and smaller. We performed 2D RMHD
modeling of the pulsar bow shock and simulated the corresponding H$\alpha$
emission morphology. We find that even a mild spatial inhomogeneity of 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 ISM with a characteristic length of $\sim0.1$~pc.
Structures of this scale might be also responsible for the formation of the
fine features seen at the forward shock of Tycho SNR in X-rays. Formation of
such short periodic density structures in the warm component of ISM is
puzzling, and bow-shock nebulae provide unique probes to study this phenomenon.
