• The Height of F-Layer Irregularities in the Arctic Ionosphere

      Bates, Howard F. (Geophysical Institute at the University of Alaska, 1959-03)
      Results and interpretations of oblique incidence soundings of the arctic ionosphere are presented. Anomalous echoes are found to be prevalent in high latitudes in contrast to lower latitudes where 2F groundscatter predominates. One of the echoes seen regularly at College, Alaska has been identified as direct F-layer (IF) backscatter. The observations of the IF echo provide direct evidence of the presence of irregularities in the F-layer between heights of 350 and 600 km. The IF echoes are recorded regularly at night and occasionally during the day in disturbed periods. They appear to be associated with auroral ionization. Simultaneous reception of 2F echoes from the north and the south indicates that at times the reflecting layer is tilted. Tilt-angles in the vicinity of 2 to 3 degrees are found. The 2F echoes from the north usually connect to the extraordinary branch of the vertical incidence trace while the 2F echoes from the south appear to connect to the ordinary branch. The analysis of groundscattered (2F) echoes is extended from a plane to a spherical geometry, and it is shown that a geometrical extension of the plane earth theory is adequate. The observed range-frequency dependence differs only slightly from that predicted by the latter theory.
    • An HF sweep frequency study of the arctic ionosphere

      Bates, Howard F. (Geophysical Institute at the University of Alaska, 1961-04)
      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 overcast. The cloud size and distribution depend primarily upon the zenith angle of the sun, the smaller the zenith angle, the smaller and fewer the clouds. Geomagnetic effects do not appear to influence the behavior or the F region clouds, although this point is somewhat uncertain. The E region clouds which produced the slant Es echo were definitely correlated with geomagnetic activity but not to solar radiation, there appeared to be no connection between the causes of the E and the F region direct scatter echoes even though they were propagated by the same general mechanisms. This study raised several new problems whose explanations might contribute greatly to ionospheric research. The sweep frequency sounding technique is a good method, but the system sensitivity must be raised greatly over that of the present equipment if meaningful results are to be obtained.