The Reception of Radio Echoes From Distant Ionospheric Irregularities

Abstract

For many years it has been generally known that strong, sharply defined radio echoes occasionally return to the sending station, or to points within the normal skip-zone, after having traversed a path which greatly exceeds the round trip distance to the $F$ layer of the ionosphere. Previous investigators tentatively ascribed such effects to reflection from distant mountains or from concentrations of ions in the polar regions. Studies of numerous reflection patterns strongly indicate that the delayed echoes are returned from regions where there is marked curvature of the $F$ layer. A region of this sort normally occurs at the edge of the sunlit zone and can turn back a ray which may have traveled many thousands of kilometers around the dark side of the earth. Small nighttime variations in the curvature of the $F$ layer are of very common occurrence and are believed to explain several phenomena, including much long period, long distance fading. These variations are frequently of cyclic character and may tentatively be attributed to wave motion in the upper atmosphere.