  Context. While the shapes of many observed bow shocks can be reproduced by
simple astrosphere models, more elaborate approaches have recently been used to
explain differing observable structures. Aims. By placing perturbations of an
otherwise homogeneous interstellar medium in front of the astrospheric bow
shock of the runaway blue supergiant $\lambda$ Cephei, the observable structure
of the model astrosphere is significantly altered, providing insight into the
origin of perturbed bow shock images. Methods. Three-dimensional single-fluid
magnetohydrodynamic (MHD) models of stationary astrospheres were subjected to
various types of perturbations and simulated until stationarity was reached
again. As examples, simple perturbations of the available MHD parameters
(number density, bulk velocity, temperature, and magnetic field) as well as a
more complex perturbation were chosen. Synthetic observations were generated by
line-of-sight integration of the model data, producing H$\alpha$, $70\,\mu$m
dust emission, and bremsstrahlung maps of the perturbed astrosphere's
evolution. Results. The resulting shock structures and observational images
differ strongly depending on the type of the injected perturbation and the
viewing angles, forming arc-like protrusions or bifurcations of the bow shock
structure, as well as rings, arcs, and irregular structures detached from the
bow shock.
