  Studies have shown that in sheared $\mathbf{E}\times\mathbf{B}$ flows in an
inhomogeneous ionospheric plasma, the gradient drift (GDI) or the
Kelvin-Helmholtz (KHI) instability may grow. This work examines the conditions
that cause one of these instabilities to dominate over the other using a novel
model to study localized ionospheric instabilities. The effect of collisions
with neutral particles plays an important role in the instability development.
It is found that the KHI is dominant in low collisionality regimes, the GDI is
dominant in high collisionality regimes, and there exists an intermediate
region in which both instabilities exist in tandem. For low collisionality
cases in which the velocity shear is sufficiently far from the density
gradient, the GDI is found to grow as a secondary instability extending from
the KHI vortices. The inclusion of a neutral wind driven electric field in the
direction of the velocity shear does not impact the dominance of either
instability. Using data from empirical ionospheric models, two altitude limits
are found. For altitudes above the higher limit, the KHI is dominant. For
altitudes below the lower limit, the GDI is dominant. In the intermediate
region, both instabilities grow together. Increasing the velocity shear causes
both limits to be lower in altitude. This implies that for ionospheric
phenomena whose density and velocity gradients span large altitude ranges, such
as subauroral polarization streams, the instabilities observed by space-based
and ground-based observation instruments could be significantly different.
