Plasma instabilities in auroral ionospheric density gradients from 120-200 km altitude

Recent observations by the Sonderstrom incoherent scatter radar show narrow field aligned filaments of enhanced plasma density extending from ~100 to 400 km in altitude, with widths of order 1 km transverse to B, and with peak density of ~5e11/m3. These features typically appear at the footprint of nightside reconnection where strong, sheared, ionospheric flows exist. We examine the stability of such structures when immersed within plasma convection using a 2-D (plane perpendicular to the magnetic field) hybrid plasma simulation. Initial results indicated the relatively rapid development (order seconds) of gradient drift type instability waves; for parameters near 200 km altitude, and convection velocity of 600 m/s, such plasma columns are found to be unstable to waves with a wavelength of ~10 to 20 m. These results have implications for the interpretation of HF backscatter measurements in such ionospheric regions.