Trends in Occurrence of Polar Cap SuperDARN Echoes and Electron Density Variations in the Ionosphere

Abstract

This thesis presents three completed objectives related to the operation of the high-frequency (HF) Super Dual Auroral Radar Network (SuperDARN) radars. The first objective was to gain a general understanding of how radio waves propagate through the Earth's ionosphere to result in SuperDARN echoes. To accomplish this, a raytracing program was developed that models propagation through a non-uniform ionospheric electron density distribution. Consistent with previous publications, it was found that ionospheric echoes from the heights of ~200-300 km (F region echoes) occur at distances of ~600-1600 km from the radar. Such echoes are critical for monitoring ionospheric plasma flows, the main scientific objective of the SuperDARN experiment. The second objective in this thesis was assessing the quality of the ionospheric electron density estimates from elevation angle measurements by the Rankin Inlet (RKN) SuperDARN radar. RKN measurements of electron density are compared with concurrent measurements from an ionosonde and an incoherent scatter radar within RKNs field of view. Reasonable agreement between the instruments is shown for daytime conditions. When the density is very low, such as during nighttime, RKN is prone to overestimation. This is related to ambiguities in the phase of the interferometer signal used to determine elevation. Finally, the third and main objective in this thesis was an investigation of factors affecting SuperDARN F region echo occurrence rates. We examine diurnal, seasonal, and solar cycle variations in the occurrence of SuperDARN echoes in the polar cap of both hemispheres. We also examine possible connections between echo occurrence and background electron density, and with electric field/plasma flow velocity. Data from three radars in each hemisphere was considered over a time scale of up to a decade, from 2007-2017. Echo occurrence is shown to increase toward the solar cycle maximum, more distinctly on the nightside, consistent with a general trend of increased background electron density. The pattern of seasonal and diurnal variations in echo occurrence is consistent for all the radars, with more prominent features in the southern hemisphere. The echo occurrence rate is shown to increase with the electron density but only up to a certain threshold value after which the dependence saturates. The echo occurrence rate is also sensitive to the plasma flow velocity magnitude (electric field), which controls the intensity of ionospheric irregularities. This effect is more prominent in the daytime of summer.