F‐region ionosphere‐investigations at low latitudes

Further studies of the F ‐region, ordinarily composed of the F ‐layer at night which separates into the F 1 ‐ and F 2 ‐layers at small zenith‐angles of the Sun, have been continued at the Huancayo Magnetic Observatory of the Department of Terrestrial Magnetism of the Carnegie Institution of Washington during the past year. The separation of the F 1 ‐ and F 2 ‐layers appears to occur because of a decrease of virtual height of the F 1 ‐layer and a marked rise of virtual height of the F 2 ‐layer. To adequately represent this effect, iso‐ionic charts showing the change of virtual height and ionization with time have been constructed. This construction is made possible by a convenient simplification of the virtual height versus frequency graphs obtained from the observed data. An examination of the characteristics of the F 1 ‐layer shows little change of maximum ionization or lowest virtual height for any particular time from day to day. Such changes rarely exceed ±5 per cent. On July 30, 1934, a major magnetic disturbance coincided with a reduction of 15 per cent in noon maximum F 1 ‐layer ionization, indicating a relation of this layer to terrestrial‐magnetic effects. The character of the F 1 critical‐frequency is found to change from day to day, indicating differences in the “degree of separation” at a particular point. Further information is necessary to the interpretation of this effect. The iso‐ionic charts for various zenith‐angles during the day are compared with the results for corresponding zenith‐angles at Washington. Good agreement is found and makes possible an estimate of geographical distribution. This is represented by a three‐dimensional iso‐ionic chart. The single F ‐layer existing at large zenith‐angles and at night is found to decrease in maximum ion‐content on an average during the night, and to begin a rapid increase in maximum ionization before sunrise on the ground but after sunrise at the height of maximum ionization. It is shown that the solar rays active in ionizing the F ‐layer must be therefore filtered out in the lower atmosphere, and the lower limit of this effect is estimated from the data. The character of the variations of maximum ionization and minimum virtual‐heights of the F 2 ‐layer varies greatly from day to day at any particular time as contrasted to the comparatively regular characteristics of the lower layers. No typical curves can be shown, but averages of the F 2 critical‐frequency for periods including small changes in noon zenith‐angle indicate that on an average a minor maximum occurs in the morning, a minimum occurs near noon, and a major maximum occurs in the afternoon, followed by a decrease at night. Individual variations from the average are described. It is found that the F 2 critical‐frequencies are on an average greatest during the southern solstice. The effects are compared to the published data for Washington where the noon values of F 2 critical‐frequency are found to vary in the same manner. This fact introduces a new complication into an already complex problem. The “scattered reflections” reported by other observers above 600 km are apparent at Huancayo. There is also frequently apparent a hitherto undescribed reflection from layers in the F ‐region having a small magnitude decreasing with frequency and a virtual height nearly unchanging with frequency and approximating the lowest virtual height of the F ‐layer involved. Additional stratification of the F ‐region is occasionally observed. It is apparent that extensive data are required to effect a more complete understanding of these phenomena, and an automatic multi‐frequency technique is desirable. It is suggested that a systematic simplification of data, such as described, and the construction of iso‐ionic graphs would lead to an effective and simple representation of such data.