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Christos Haldoupis – PhD Thesis Title : VHF Doppler Investigations of the Auroral Ionosphere College of Graduate Studies and Research, Department of Physics and Engineering Physics - Supervisor Prof. George Sofko University of Saskatchewan, Saskatoon, Canada, April, 1978.
Thesis AbstractA bistatic continuous wave (CW) radio Doppler system, operating at 42.1 MHz, was developed and built in order to study the spectral characteristics of the radio backscatter from the auroral ionosphere. The recorded backscatter echoes were analyzed to obtain high resolution Doppler spectra, which in turn provided information about the line-of-sight movements and the nature of the scattering irregularities. The spectral properties suggested two general categories of spectra, named discrete and diffuse, that correspond to electrostatic plasma irregularities generated by different mechanisms. The discrete spectra are of two types, narrow and broad, probably caused by plasma density waves. The diffuse spectra are the most frequent and appear to be due to a random assembly of drifting scatterers, whose motion changes from southward to northward as time proceeds from midnight to the down sector. The narrow discrete or ion-acoustic type, spectra and their corresponding echoes were investigated in detail. Experimental results, based on both radio scatter and ground magnetograms, were used to test the validity of the two-stream instability as a causal mechanism of the ion-acoustic echoes. These results yielded considerable evidence that the two-stream instability mechanism may operate to a limited extent in the auroral plasma. Digital demodulation techniques and spectral analysis were used to study the short term characteristics of the ion-acoustic echoes. It was found that the signal amplitude undergoes deep and quasi-periodic fading with marked periodicities in the 2 to 10 Hz range. Evidence showed that the fading is not due to interference but to the appearance and disappearance of independent spatially coherent structures of ionization. New information was obtained about the lifetime, growth (decay) rate, and velocity variations of the irregularities associated with the ion-acoustic echoes. Although the results were consistent with the concept of plasma wave scatterers, the evidence suggested that the linearized two-stream instability is inadequate in explaining the behavior of the unstable plasma waves. Fading spectral analysis showed that there exist two general types of auroral fading, slow quasi-periodic and rapid random fading. Average amplitude distributions showed that strong specular components occasionally dominate the scatter but that, for the majority of cases, the amplitude distribution obeys approximately the Rayleigh law. The fading analysis supported the existence of at least two scattering processes in radio aurora, in agreement with the Doppler results. By using two linear demodulation units, simultaneous long term variations of the mean frequency and signal strength have been investigated. Also, the average overnight variation of the north-south mean movements of the auroral ionization was established on a statistical basis. It is argued that both variations of the east-west electric field, and the Harang discontinuity may play a key role in the observed north-south movements of the radio aurora. Finally, a new spectral type was observed. This has a double-peaked structure and is associated with strong and short-lived echoes which occur infrequently during the morning time sector. Further study showed that the spectral power alternates in time between the two distinct spectral bands where the peaks are located, suggesting that this spectral type results from two competing plasma processes which cannot operate simultaneously.
Thesis Publications
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