Modeling the gradient drift and Kelvin-Helmholtz instabilities as turbulence generation mechanisms in sub-auroral polarization streams.

Abstract:

Sub-auroral polarization streams (SAPS) are regions in the sub-auroral ionosphere of lower plasma density and large westward flow (hundreds to thousands of m/s) driven by a poleward electric field. Observations have shown that this region is highly structured with density irregularities ranging in spatial scales of tens to hundreds of meters. These irregularities are important to understand due to their adverse impact on radio signals. Two instabilities are investigated based on the geometry of the density and velocity gradients, the gradient drift instability (GDI) and the Kelvin-Helmholtz instability (KHI). A newly developed 2D electrostatic fluid model solving the continuity, temperature, and current closure equations is used to study the turbulence generation. Since the turbulence in SAPS is generated on faster time scales than those of the background SAPS dynamics, a perturbed model is used such that the background remains constant in time. The model is spatially discretized using a pseudo-spectral method and evolved in time using a four stage fourth order Runge Kutta method. The physics of the interaction between the GDI and the KHI is examined using different parameter regimes relevant to SAPS. Initial results indicate that the parameters relating to the shape of the SAPS velocity profile play a large role in the two instabilities' interaction. Active versus quiet geomagnetic conditions are also considered through changes in the background SAPS structure. This work will present mechanisms that could produce the turbulence cascade from kilometer scales down to scales observed in SAPS irregularities, tens to hundreds of meters corresponding to HF space weather radar and GNSS signals respectively.