THE SPATIAL COHERENCE OF TRAVELING IONOSPHERIC DISTURBANCES AT MIDLATITUDES

A multifrequency HF Doppler sounder and four spaced receivers were operated near Alma-Ata to form a three-dimensional array of reflection points of HF radio waves. The spacings of reflection points ranged from 5 to 80 km in the vertical and from 30 to 65 km in the horizontal. The purpose of the experiment was to estimate the spatial coherence of travelling ionospheric disturbances (TIDs). Estimation of the coherence length (the distance at which the coherence falls to e-1) in both vertical and horizontal planes is carried out. The coherence often shows peaks at frequencies exceeding the Brunt-Vaisala frequency. Measurements of the slant coherencies have given the opportunity to study the coherence as a function of orientation.