SummaryNight-time radio reflections from irregularities in the F. region of the ionosphere have been studied at Brisbane (geomagnetic latitude -35.8°), at a fr-equency of 16 Mc/s. The spatial limits and behaviour of the echoes were consistent with the hypothesis of specular reflections from field-aligned columns of ionization, with observational limitations imposed by echo surface configuration, aerial sensitivity, range attenuation, and absorption. The echoes, recorded with range-azimuth display, were of many shapes, but only a U-shape tended to recur. These U-echoes were preferentially observed under conditions of magnetic disturbance. Irregularity occurrence possibly increased with latitude from 45° to 55° geomagnetic latitude. No definite relationship was found between the echoes and v.l.f. noise, radio-star scintillations, spread-F, or auroras.

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