Echoes at D‐heights with special reference to the Pacific Islands

Radio echoes obtained from an equivalent height of 50 km above Pitcairn Island by the usual vertical incidence pulse technique are attributed to D-layer reflections. Previous literature has been collected, and many conflicting statements on the occurrence and behaviour of D-layer echoes are revealed. A total of 1302 observations of the layer, taken at Pitcairn Island between October 1, 1944, and November 16, 1945, have been analysed, showing: (i) A frequency-range largely contained between 2.5 and 5.0 Mc; (ii) Occurrence during 65 per cent of all daytime observations, although night occurrence is not excluded; (iii) Both a summer and a winter minimum of activity; (iv) A strength of approximately one-third the E-layer-echo strength, and all the typical characteristics of a sporadic region. Similar echoes have been reported from Raoul Island (Kermadec Group) during approximately 75 per cent of the daytime in the summer of 1944–45, and from Christmas Island during 87 per cent of the daytime in the period June 11–30, 1945. It is considered that the echoes may be characteristic of a tropical region.