Atmospheric physics : a study of F-region travelling ionospheric disturbances (1980)
AuthorsCooper, Justinshow all
A Fast-Fourier-Transform method has been used to calculate a series of one-dimensional ground diffraction patterns which arise from point-source illumination of a moving sinusoidal reflector. Correlation analysis was performed on the set of ground patterns to investigate the relationship between the derived pattern drift velocity and random velocity, and the velocity of the moving reflector. Application of the correlation analysis of Briggs, Phillips and Shinn (1950) to the complex temporal correlations of the ground patterns between spaced receivers gave a true velocity value which agreed with that of the reflector. The derived random velocity was negligible, as desired for a correct description of unchanging reflector shape. The true velocity obtained from the spatial amplitude correlations using the spatial correlation method of Briggs (1968a) was found to agree closely with the ground velocity of the single specular reflection point on the sinusoidal surface. An undesirable feature was a high value of estimated random velocity. The amplitude temporal correlations and complex spatial correlations gave no useful information. Synoptic ionograms taken at 15-minute intervals at Christchurch, New Zealand, indicated the presence of a medium-scale travelling ionospheric disturbance on 19th February, 1978. The phase velocity vфh = 117 (±10) m. sec-¹, horizontal wavelength λh = 560 (±90) km, and period T = 80-90 minutes. Phase-path data for an altitude of 200 km on the same day show a series of harmonics of a 90-minute fundamental in their maximum entropy power spectra. The harmonics of the phase range arose from movement of a specular reflecting point on a quasi-sinusoidal surface. The existence of several wave cycles of similar frequency in the data and the presence of moderately high magnetic activity allows the auroral electrojet as a possible source for the disturbance observed. No Doppler shifting of gravity-wave frequency arising from the presence of neutral winds was observed.