| Summary: | Observations made during 1958 and 1959 using a sweep frequency, HF, oblique sounder located at College, Alaska, are discussed, and selected groups of echoes are illustrated. Groundscatter is the predominant echo type observed on mid-latitude backscatter records, but this is not true in the high latitudes. The majority of the scatter echoes from soundings toward geomagnetic north were direct F region scatter of two main types - IF and constant range echoes. These same echo types were observed from the E region during magnetic disturbances (the slant Es echo corresponds to the IF echo). These echoes were centered about geomagnetic north on swept azimuth soundings and were produced by scatter near the oblique reflection point in the ionosphere. Thus, we conclude that the echoes were caused by scatter from irregularities aligned along the geomagnetic field. The IF echo branches off the first order vertical incidence F region trace and increases linearly in range with frequency. It is produced by scattered energy which is least-time focused. The scatterers are essentially randomly distributed within large regions called clouds in the F region. The height of the irregularities producing the IF echo can be computed if the vertical incidence traces are clear; heights between 200 and 350 km have been found by this technique, indicating that electron density irregularities are not confined only to the lower ionosphere. From the regularity of occurrence of the IF echo, it is highly probable that the irregularities extend throughout much of the F region. The constant range echo is produced by energy scattered from the field-aligned surface of a cloud of irregularities. The soundings were made to the north so the clouds producing the observed constant range echo extended east and west. The IF and the constant range echoes are useful research tools because they indicate the amount of cloudiness present in the F region. During the summer day, the F region is relatively free of clouds, while during the winter night, it is quite ...
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