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On the Origin of Close-Range E Region Echoes Observed by SuperDARN HF Radars in the Mid- and High Latitudes



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The Super Dual Auroral Radar Network (SuperDARN) is a global network of coherent high frequency (HF) radars located in the polar, high- and mid-latitudes of both the Northern and Southern hemispheres. This thesis deals with close-range SuperDARN echoes (oblique HF backscatter from the lower part of the ionosphere). The aim of this thesis is to shed light on the origin of these echoes. Previous studies have been content to propose explanations for the origin of these echoes without thorough checking of the proposed mechanisms against constraints available from various radars and other important information. For the purpose of clarifying the situation, a chain of SuperDARN radars in the Northern and Southern hemispheres and several years of daily statistics have been used. This has allowed for several findings. Notably, the close-range SuperDARN echoes show diurnal and seasonal variations and their properties with respect to signal-to-noise-ratio, Doppler velocity and Doppler width vary. Three distinct populations of close-range HF backscatter have been established: (1) a morning population (0400-0700 LT), (2) a midday summer population (0800-1300 LT) and (3) a pre-midnight (2100-2300 LT) population. The morning population is associated with meteor trails which are observed to be peaking near local dawn as expected, and already suggested by previous research. High latitude SuperDARN radars also had echoes (pre-midnight population) with higher Doppler velocities than the others yet the Doppler velocities are smaller than that expected from auroral E region echoes. Given the time and location of this population of echoes, it has been concluded that they are a special class of high latitude E region echoes at high aspect angle which have been termed ``high aspect irregularity region" echoes in the past. Lastly, the midday summer population was found to be too high for polar mesosphere summer echoes and too early for plasma instabilities. It is proposed that these SuperDARN echoes are produced either from contribution from meteors trails or by neutral turbulence which is suspected (from other work) to be present near 100 km. The properties of the midday summer population resembles those of meteor trails as they have the same power, and the same altitude and have high summer occurrence as expected for meteors. Their late morning occurrence could be due to particular look direction of individual radars which may change the occurrence statistics in the presence of meteor showers. With respect to neutral turbulence, the drift of the midday summer population is similar to that of neutral wind.



Near-range echoes, HF backscatter, SuperDARN



Doctor of Philosophy (Ph.D.)


Physics and Engineering Physics




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