Observations of Sporadic-E utilising high frequency oblique sounding techniques
thesisposted on 15.12.2014, 10:40 by Paul J. Rylah
Sporadic-E is an 'abnormal', altitude-confined intensification of the E-region plasma density. Its presence may both enable and disable an oblique propagation circuit, depending on the prevalent propagation path geometry, the ambient ionospheric conditions, and the plasma structure of the layer itself. This thesis studies the plasma characteristics of sub-auroral sporadic-E layers, utilising oblique HF 'Chirp' sounders, deployed during times of both quiet and disturbed geomagnetic activity.;During largely quiet geomagnetic conditions, a relationship is established between the sporadic-E top frequency, ftEs, and the layer's signal strength (SS), at a fixed sounder 'Probing Frequency' (PF) such that SS (ftEs/PF)n, with values of 'n' varying between ~ zero and ~ 20. The resulting 'curve' is derived primarily for 'overdense' sporadic-E reflections and is interpreted in terms of the sporadic-E plasma 'cloud' model. A gaussian distribution of differing cloud plasma densities is demonstrated to best model the observed sporadic-E signal strength characteristics. However, this signal strength relationship is increasingly inapplicable to the minority of sporadic-E layers arising from equivalent vertical top frequency (vftEs) growth rates, greater than ~ 0.6 MHz per five minute time interval. Indeed, for vftEs growth rates greater than ~ 1.2 MHz per five minutes, the signal strength is independent of the top frequency. These observations are again consistent with the plasma cloud model, though field-aligned irregularities and off great circle path propagation may account for some of the more temporary vftEs growths.;The thesis concludes with a study of correlated observations of sporadic-E layers and ~ 1 m scale plasma irregularities, as detected by the Wick receiver of the SABRE (Sweden And Britain Radar-aurora Experiment) VHF backscatter radar. The correlated occurrence of the two plasma features, coincided, primarily, with substorm activity. On occasions it was also noted that the backscatter S/N ratio correlated closely with the sporadic-E top frequency, the observation being interpreted in terms of the enhanced auroral electric fields.