  We use linear stability analysis to show that an isotropic phase of elongated
particles with dipolar flow fields can develop nematic order as a result of
their activity. We argue that ordering is favoured if the particles are
flow-aligning and is strongest if the wavevector of the order perturbation is
neither parallel nor perpendicular to the nematic director. Numerical solutions
of the hydrodynamic equations of motion of an active nematic confirm the
results. The instability is contrasted to the well-known hydrodynamic
instability of an ordered active nematic.
