High-resolution Simulations of Primary Gradient Drift Instability in Polar Cap Patches

Patches of high-density plasma (100-1000 km scales) are commonplace in the polar cap F-region ionosphere. For northward IMF conditions they typically separate from the dayside cusp ionosphere and propagate across the polar cap; often destabilizing along the way and producing radio scintillation. While the general theoretical construct for these instabilities is well developed, gaps still exist in the understanding and characterization of ionospheric plasma instabilities (such as instability seeding/generation, the role of precipitation, influence of MI coupling, etc.) across a range of time and length scales. Observational data from sources such as ISRs have filled some gaps, within the constraints of their temporal and spatial resolutions. Additional insights regarding instabilities in polar cap patches can be realized through computer simulations of these phenomenon.

This work presents new high-resolution simulations of primary gradient-drift instability (GDI) in polar cap patches with the focus on understanding how these mesoscale features cascade to Fresnel scales responsible for scintillation. The GEMINI ionospheric model (https://github.com/gemini3d) is used to better understand the fundamental plasma instability physics occurring on small scales, on the order of approximately 50m, as well as the geophysical importance of such structures (e.g. their influence on radio signals). We focus on an investigation of the mid- to late-stage evolution of the instability, characterizing the effect of diffusion and mixing within the patch, particularly under the influence of ion pressure and ion inertia. Additionally, the simulations also explore the mesoscale redistribution of the plasma components, especially once structures break through the leading edge of the plasma patch. Finally, analysis is conducted on how the spectra of plasma irregularities evolve over time and the extent to which use of the equipotential field line assumption may influence irregularity development.