The ionized portion of the Earth’s atmosphere, the ionosphere, forms several layers extending from 80-1000 kilometers in altitude. In the polar regions, during favorable solar wind conditions, portions of the dayside ionosphere detach and propagate across the Earth’s polar cap regions. These “polar cap patches” often undergo instability during passage across the polar cap resulting in a growth of unstable wavelike structures on the trailing edges of the patch. The details of these processes are not understood fully, yet they can result in number of important eects of radio propagation as plasma density structures can refract, diract, and scatter signals. Further, the patches themselves provide a unique opportunity to study coupling from large-scale (100s of km) to small-scale (100s of m) plasma turbulence fundamental to collisional, partly magnetized environments.

This project studied instability and turbulence in polar cap patches via development of improved modeling capabilities to test the eects of various physical processes on patch evolution and their eect on radio scintillation at various frequencies (VHF, UHF, and L-band). We focused primarily on the modeling of primary gradient-drift instability (GDI) and attendant physical connections to secondary processes contributing to ionospheric irregularity formation at Fresnel scales for frequencies of interest. Studies involved combined use of existing and new data from the Resolute Bay Incoherent Scatter radar (RISR) — a high-power, large aperture, phase-array system, a UHF and VHF receiver, and CHAIN GNSS receiver array.

Intellectual Merit: Work conducted during this project substantially claried the extent to which various physical processes contribute to formation of irregularities. Ion inertia, included in our modeling through rst-order corrections to current density calculations, primarily acts to delay (suppress) the formation of primary GDI structures and to result in the formation of overturning, vortex-like structures in the nonlinear stages of GDI. Even a modest amounts of inertia can modify fully static models of GDI development and associated scintillation. Ion and electron pressure processes contribute to cross-eld diusion near L-band Fresnel scales — though the eects in our models are not as pronounced as that of inertia.

Plasma density enhancements in the E-region (e.g., through impact ionization) have been shown to substantially modify GDI growth through electrical shorting of charges accumulated in the Fregion. Lastly, the eects of equipotential eld line (EFL) assumptions commonly made in models like ours seem to be relatively modest; however, basic scaling arguments indicate that this will not be the case at the smallest scales of interest. Plasma and radio propagation modeling case studies have been used to demonstrate VHF scintillation occurrence alongside larger scale patch structures observed in RISR in cases where L-band scintillation is less prevalent, indicating in-progress development of cascading structures that have reached the Fresnel scale for VHF frequencies but not for L-band. Statistical surveys of total VHF and L-band phase scintillation show more variation in the average VHF phase values than in the L-band phase; additionally a positive correlation is shown to exist between the satellite elevation angle and phase variations.

Background and initial conditions are of prime importance to the development of irregularities; however, most modeling eorts use relatively simple planar patches and periodic boundary conditions. Our project has explored the instability of nite (i.e. non-ideal, non-planar) polar cap patches, both in a simple form (e.g. elliptical) and initial directly through the use of interpolated data from RISR. Such patches display are shown to exhibit overall distortion due to charge accumulation along their boundaries, alongside the familiar GDI processes occurring on the trailing edge. Additionally, the instability does not process uniformly across the entire patch structure. As part of this project, we have developed a number of tools potentially useful for initialization of observed patches in the model for case study comparisons beyond those already performed, alongside abilities to include inertia and pressure processes important at small scale — thus improving existing capabilities to model and study irregularity formation processes in other contexts.

Broader Impacts: The project has supported two graduate students: a PhD student and a Master’s student. The project also supported a summer undergraduate student. The GEMINI software and related simulation tools used in this project are open source and available to the public for general and scientic use (https://github.com/gemini3d) under a permissive license. This project has specically added capabilities to do relatively routine simulations of plasma turbulence — relevant to broader operational goals in our research community that seek to develop better prediction and mitigation strategies for scintillation and other trans-ionospheric radio propagation phenomena. This project also resulted in the creation of a number of scintillation data-related open source codes (https://github.com/Polar-Cap-Scintillation)

Last Modified: 02/05/2025
Modified by: Kshitija B Deshpande