  Mature neutron stars are expected to have several superfluid components.
Strong evidence for this is provided by the glitches that have been observed in
dozens of pulsars. The underlying idea behind most glitch models is that, as
the neutron star crust spins down due to the emission of electromagnetic
radiation, the superfluid component lags behind until a critical point is
reached and angular momentum is transferred from the superfluid to the crust,
leading to the spin-up associated with the glitch. In this Letter we describe a
superfluid analogue of the two-stream instability that is well known in plasma
physics, and provide evidence that this instability is likely to be relevant
for neutron stars. This is a new physical mechanism which may play a key role
in explaining the glitch mechanism and which could also prove to be relevant in
laboratory experiments on superfluid Helium.
