Robert Burston

Turbulent times in the northern polar ionosphere

A model is presented of the growth rate of turbulently generated irregularities in the electron concentration of northern polar cap plasma patches. The turbulence is generated by the short-term fluctuations in the electric field imposed on the polar cap ionosphere by electric field mapping from the magnetosphere. The model uses an ionospheric imaging algorithm to specify the state of the ionosphere throughout. The growth rates are used to estimate mean amplitudes for the irregularities, and these mean amplitudes are compared with observations of the scintillation indices S-4 and sigma(phi) by calculating the linear correlation coefficients between them. The scintillation data are recorded by GPS L1 band receivers stationed at high northern latitudes. A total of 13 days are analyzed, covering four separate magnetic storm periods. These results are compared with those from a similar model of the gradient drift instability (GDI) growth rate. Overall, the results show better correlation between the GDI process and the scintillation indices than for the turbulence process and the scintillation indices. Two storms, however, show approximately equally good correlations for both processes, indicating that there might be times when the turbulence process of irregularity formation on plasma patches may be the controlling one.

Polar cap plasma patch primary linear instability growth rates compared

Four primary plasma instability processes have been proposed in the literature to explain the generation of phase scintillation associated with polar cap plasma patches. These are the gradient drift, current convective, and Kelvin-Helmholtz instabilities and a small-scale “turbulence” process. In this paper the range of possible values of the linear growth rates for each of these processes is explored using Dynamics Explorer 2 satellite observations. It is found that the inertial turbulence instability is the dominant process, followed by inertial gradient drift, collisional turbulence, and collisional shortwave current convective instabilities. The other processes, such as Kelvin-Helmholtz, collisional gradient drift, and inertial shortwave current convective instabilities, very rarely (<1% of the time) give rise to a growth rate exceeding 1/60, that is deemed to be significant (in publications) to give rise to GPS scintillation.

Analysis of the Regional Ionosphere at Low Latitudes in Support of the Biomass ESA Mission

Biomass is a spaceborn polarimetric P-band (435 MHz) synthetic aperture radar (SAR) in a dawn–dusk low Earth orbit. Its principal objective is to measure biomass content and change in all the Earth’s forests. The ionosphere introduces the Faraday rotation on every pulse emitted by low-frequency SAR and scintillations when the pulse traverses a region of plasma irregularities, consequently impacting the quality of the imaging. Some of these effects are due to total electron content (TEC) and its gradients along the propagation path. Therefore, an accurate assessment of the ionospheric morphology and dynamics is necessary to properly understand the impact on image quality, especially in the equatorial and tropical regions. To this scope, we have conducted an in-depth investigation of the significant noise budget introduced by the two crests of the equatorial ionospheric anomaly (EIA) over Brazil and Southeast Asia. This paper is characterized by a novel approach to conceive a SAR-oriented ionospheric assessment, aimed at detecting and identifying spatial and temporal TEC gradients, including scintillation effects and traveling ionospheric disturbances, by means of Global Navigation Satellite Systems ground-based monitoring stations. The novelty of this approach resides in the customization of the information about the impact of the ionosphere on SAR imaging as derived by local dense networks of ground instruments operating during the passes of Biomass spacecraft. The results identify the EIA crests as the regions hosting the bulk of irregularities potentially causing degradation on SAR imaging. Interesting insights about the local characteristics of low-latitudes ionosphere are also highlighted.

A comparison of the relative effect of the Earth's Quasi-D.C. and A.C. electric field on Gradient Drift Waves in large-scale plasma structures in the polar regions.

Radio signals traversing polar-cap plasma patches and other large-scale plasma structures in polar regions are prone to scintillation. This implies that irregularities in electron concentration often form within such structures. The current standard theory of the formation of such irregularities is that the primary Gradient Drift Instability drives a cascade from larger to smaller wavelengths that manifest as variations in electron concentration. The electric field can be described as the sum of a quasi-D.C. and an A.C. component. Whilst the effect of the quasi-D.C. component has been extensively investigated in theory and by modelling, the contribution of the A.C. component has been largely neglected. This paper investigates the relative contributions of both components, using data from the Dynamics Explorer 2 satellite. It concludes that the contribution of the A.C. electric field to irregularity growth cannot be neglected. This has consequences for our understanding of large-scale plasma structures in polar regions (and any associated radio scintillation) as the A.C. electric field component varies in all directions. Hence, it effect is not limited to the trailing edge of such structures, as it is for the quasi-D.C. component. This raises the need for new experimental and modelling investigations of these phenomena.