On the propagation of electromagnetic waves through the atmosphere
A general method of tackling the problem of the propagation of electromagnetic waves in the ionosphere has been developed and the current methods of Appleton, Hartree, Saha, Rai and Mathur, etc., have been deduced as special cases from the general results. The different assumptions by Appleton, Hartree, Bose, Booker and Rai, as regards the condition of reflexion of the waves from the ionosphere, have been shown to be identical. A symbol-correspondence chart for the different symbols used by the different workers has been given to facilitate the understanding of the parallelism between the different methods. Polarization of the radio waves have been discussed fully.
A reciprocity theorem on the propagation of radio waves via the ionosphere
ABSTRACT The reciprocity theorem for electrical systems which include a radiation link was believed to be true only when the media within the system have dielectric constants which are symmetric tensors. This condition is not fulfilled by the ionosphere, so that the reciprocity theorem is not generally applicable when the radiation link includes one or more reflexions from the ionosphere. It is here proved that, when the ionosphere is horizontally stratified, and when the path from transmitter to receiver is in the magnetic meridian (north-south and south-north transmission), the reciprocity theorem applies (a) when the transmitting and receiving aerials both radiate or receive waves whose electric vector is in the plane of incidence, and (b) when both aerials radiate or receive waves whose electric vector is horizontal. Further, (c) if the electric vector radiated or received is horizontal for one aerial and in the plane of incidence for the other, then there is reciprocity in signal amplitude, but the phase changes for transmission in the two directions differ by 180°. These results are valid for any law of variation of electron density and collision frequency with height. They are based on a ‘full-wave’ theory, and therefore apply to all frequencies. They are unaffected if the path includes multiple reflexions, and if allowance is made for the curvature of the earth.
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 F1- and F2-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 F1- and F2-layers appears to occur because of a decrease of virtual height of the F1-layer and a marked rise of virtual height of the F2-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 F1-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 F1-layer ionization, indicating a relation of this layer to terrestrial-magnetic effects. The character of the F1 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 F2-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 F2 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 F2 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 F2 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.