  The Guitar Nebula is an H-alpha nebula produced by the interaction of the
relativistic wind of a very fast pulsar, PSR B2224+65, with the interstellar
medium. It consists of a ram-pressure confined bow shock near its head and a
series of semi-circular bubbles further behind, the two largest of which form
the body of the Guitar. We present a scenario in which this peculiar morphology
is due to instabilities in the back flow from the pulsar bow shock. From
simulations, these back flows appear similar to jets and their kinetic energy
is a large fraction of the total energy in the pulsar's relativistic wind. We
suggest that, like jets, these flows become unstable some distance down-stream,
leading to rapid dissipation of the kinetic energy into heat, and the formation
of an expanding bubble. We show that in this scenario the sizes, velocities,
and surface brightnesses of the bubbles depend mostly on observables, and that
they match roughly what is seen for the Guitar. Similar instabilities may
account for features seen in other bow shocks.
