Interpretation of the statistics of occurrence of Alouette I Earth echoes

Abstract

The strength of Earth echoes on Alouette I ionograms, tabulated in Alouette I booklets (ALOSYN), is used to produce contours of the percentage occurrence of Earth echoes on polar projection maps of the Earth. These maps can be used to show the relative noise level over much of the ocean area of the Earth. In general, a very low percentage occurrence of Earth echoes corresponds to regions of high radio noise level due to thunderstorm activity and to h.f. radio wave interference. An unusually low percentage occurrence of Earth echoes is observed over Greenland suggesting that the radio waves are absorbed in the thick ice sheet. An extremely high percentage occurrence of Earth echoes near the north magnetic dip pole can be explained by the collimating or focussing effect of field-aligned ionization irregularities in the ionosphere. There is some indication that sea is a better reflector of radio waves than land.