Investigation of the generation source of decameter-scale sub-auroral ionospheric irregularities during geomagnetically quiet periods

The mid-latitude SuperDARN radars have identified quiet-time decameter-scale density irregularities in the night-side sub-auroral ionosphere that have been proposed to be responsible for the observed low-velocity Sub-Auroral Ionospheric Scatter (SAIS). The physical mechanism responsible for such common irregularities is still unknown. Joint collaborative experiments using the SuperDARN HF radar located at Wallops Island, Virginia and the Millstone Hill incoherent scatter radar (ISR) have determined that these irregularities are located at the ionospheric footprint of the plasmapause and in a region of opposed electron density and electron temperature gradients [1]. In this paper, the temperature gradient instability (TGI) has been extended into the kinetic regime appropriate for HF radar frequencies and modeled as the potential source of these irregularities. The TGI growth rate for this event [1] is computed and compared to another common instability mechanism, the gradient drift instability (GDI) [6]. We conclude that the observed decameter-scale ionospheric irregularities are likely produced by the TGI or a cascade product from it. A "Particle In Cell" (PIC) simulation for TGI utilizing the gyro-kinetic approach has been successfully developed to study the nonlinear evolution of the TGI instability. This allows detailed study of saturation amplitude and particle transport. The simulation results show the saturation mechanisms and confirm the possible wave cascading of TGI into the decameter scale regime of the radar observations..