Multi-Scale Structuring of the Polar Ionosphere

Understanding a radically new sensing capability for polar ionospheric science introduced by observational evidence recently provided by the electronically steerable Resolute Bay Incoherent Scatter Radar (RISR).

April 1, 2023

Air Force Office of Scientific Research, Arlington, VA

A three-dimensional view of an F region plasma density structure. The slices at 350 km and 250 km as well as the vertical slice show the electron density as derived from RISR-N data. The location of the radar beams are marked as black circles on the horizontal slices. (Image: Air Force Office of Scientific Research)

Ionospheric variability is a critical consideration for communication systems, GNSS, and space asset management. At high magnetic latitudes, the convergent magnetic field acts as a lens, focusing electromagnetic power originating from solar wind-magnetosphere interactions into a limited latitudinal range. The geometry and ensuing complex coupling processes result in extreme multi-scale time-dependent variations in the structure and composition of the ionized gases in Earth’s outer atmosphere. Understanding the mechanisms and technological consequences of these interactions benefits from distributed heterogeneous time-dependent measurements of the ionosphere-thermosphere-magnetosphere system, and their application as constraints on predictive space weather models.

This research used collaborative observations by UHF incoherent scatter radar (ISR), the HF SuperDARN radar network, and wide-angle optical imagers, supported by first-principles numerical modeling, to clarify the driving mechanisms and physical consequences of these interactions. Critical new observational evidence has been provided by the electronically steerable Resolute Bay Incoherent Scatter Radar (RISR), which has introduced a radically new sensing capability to polar ionospheric science. The results of this research include both technical contributions related to the application of phased array ISR in the polar cap, and scientific contributions arising from the application of these techniques. The major published results may be summarized as follows:

Numerical simulation of densities, temperatures, and cross-field plasma flows within density cavities along auroral boundaries has revealed extreme plasma parameters creating sites of instabilities and turbulence.
These extreme frictional heating events lead to anomalous spectral characteristics in Incoherent Scatter Radar (ISR) measurements. We have developed a Zakharov simulation framework that enables extraction of useful plasma information from such distorted spectra.
Application of tomographic analysis to fine-scale auroral forms appearing at the boundaries of these turbulent flow channels enabled us to quantify the width, temporal scales, and particle energies using model based inversion techniques.
The work on this project was synergistic with AF-sponsored efforts to develop a space-based sensor network comprised of cubesats for analyzing small-scale field-aligned current systems in the aurora. This project partially supported a feasibility study carried out in collaboration with Planet Labs using their 278-element cubesat constellation.

The software toolset developed under this grant enables routine analysis of common volume measurements of the ionosphere-thermosphere-magnetosphere system in response to changing solar wind conditions. This toolset is currently being extended to include data from the SuperDARN HF radar network, GPS scintillation sensors in the polar cap, and Fabry-Perot Interferometer (FPI) measurements of the neutral wind field. The high-level data products produced through this data fusion approach can be directly applied as constraints for regional and global models of the geospace system, improving space weather predictive capabilities.

The relative contributions of transport, precipitation, recombination, field-aligned currents, and thermal diffusion in controlling plasma structures in the polar cap remains poorly understood. A major impediment has been the lack of diagnostic measurements able to provide the requisite three-dimensional view of the evolving ionospheric state. The electronically steerable advanced modular ISR (AMISR) sensors fill this need by allowing acquisition of information in multiple directions simultaneously. Using common volume observations by RISR, PolarDARN, and an all-sky spectral imaging system, we have carried out the first quantitative and experimentally verified calculation of three-dimensional plasma continuity at the geomagnetic pole.

This work was performed by Joshua Semeter for the Air Force Office of Scientific Research. For more information, download the Technical Support Package (free white paper) at mobilityengineeringtech.com/tsp under the Sensors category. AFOSR-0211