The action of ion-neutral charge exchange collisions and the large E¿B drifts that can exist in the auroral ionospheric F region lead to an ion velocity distribution which is anisotropic and can be peaked at finite v⊥. Such distributions are unstable to the Post-Rosenbluth instability if the relative ion-neutral drift velocity exceeds 1.8 times the neutral thermal velocity. Such a drift velocity requires a large electric field, of the order of 50 mV/m or more, but such fields do exist at times in the auroral zone. By balancing the ion collision term in Boltzmann's equation with turbulent diffusion caused by the instability a lower bound of a few percent for the fluctuation level is obtained. In situ observations of this instability should be possible. Furthermore, the instability operates at short wavelengths (~10--20 cm), and so the irregularities should be detectable by a UHF radar directed normal or nearly normal to the magnetic field in the F region. Such geometry is difficult to achieve, however, and as yet there appears to be no clear radar evidence of the irregularities.