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Subauroral polarization streams (SAPS) are regions of enhanced westward plasma flow associated with an E × B drift, which is driven by a strong poleward electric field caused by magnetosphere-ionosphere coupling through the closure of Region 2 field-aligned currents (Foster & Burke, 2002). These plasma flows are accompanied by density, or conductivity, troughs (Spiro et al., 1978) that are formed due to increased recombination caused by larger ion frictional heating from the increased plasma velocity (Schunk et al., 1976). This decreased conductivity then further increases the electric field which creates a feedback loop that results in a continued lowering of the conductivity (Anderson et al., 1993). SAPS occur in the dusk-midnight sector and have a latitudinal width of a few degrees. In the literature, subauroral ion drifts and polarization jets are other terms that have been used to describe narrow SAPS. The SAPS velocities can range from Abstract Density irregularities have been observed in subauroral polarization streams (SAPS). One hypothesis of the cause of this ionospheric turbulence, based on the background morphology, is the gradient drift instability (GDI). This work models the GDI using a two-dimensional electrostatic fluid model to determine if it is a viable cause of turbulence generation in SAPS. A statistical study of different velocity profiles, based on SuperDARN radar and Global Positioning System total electron content data, is used to prescribe parameters in the numerical model. The parameter space of different SAPS profiles is explored to study the effect on GDI development. As the velocity shear is initialized closer to the unstable density gradient, the GDI becomes increasingly damped. For these cases, the density and electric potential turbulence cascades obtained from the numerical model follow power laws of about −5/3 or −2, which is in agreement with observational data. If the region of sheared velocity overlaps the density gradient, the GDI becomes stabilized. The latitudinal location of maximum GDI growth depends on the density profile, the velocity profile, and the neutral wind direction. Using velocity profiles with regions of low velocity shear can cause instabilities that grow inside SAPS which have turbulence cascades with different behavior. In all parameter regimes considered, the GDI turbulence is precluded from extending through regions of velocity shear. Turbulence is generated for a variety of SAPS relevant conditions; therefore, the GDI has been shown to be a viable candidate for generating ionospheric irregularities in SAPS.Plain Language Summary Turbulence in the ionosphere is important to understand because it can negatively impact radio communication signals such as Global Positioning System (GPS) signals.For example, when GPS signals travel through a turbulent region in the ionosphere, a device's position estimates become less accurate. This work helps scientists understand how turbulence is generated in the ionosphere by simulating a phenomenon called subauroral pol...

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Studies have shown that in sheared E×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.

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