Abstract
Rotating-spaced loops at Brisbane recorded the azimuths-of-arrival of pulsed 3.84 Mc/s signals from Armidale (202°, 355 km from Brisbane); also pulsed signals at the same frequency, from a transmitter on the site of the loops. These and other normal-incidence records indicate that the irregularities of the F2-layer responsible for “Spread- F” records at Brisbane, are ripples of considerable lateral extent with a “wavelength” varying from 20 km to over 100 km. The position of the ripple in each ionization contour changes with height in such a manner as to suggest a simple relationship with the geomagnetic field orientation. Seasonal variations of the phenomenon show a winter maximum and a summer sub-maximum, and there is a distinct inverse sunspot-cycle relationship. The type of spreading defined as range-spreading generally has a peak of occurrence around midnight, and that defined as frequency-spreading peaks during the sunset and sunrise periods, after an allowance is made for a diurnal variation associated with the changing f 0> F2 value.
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