Fine structure of the lower ionosphere

Pulse soundings of the ionosphere at vertical incidence taken at 100 and 325 kc show pronounced splitting of the echoes received at night. The ranges of most of the echo components are found to lie between about 90 and 130 km. Four possible explanations of splitting are suggested, as follows: (1) Off-vertical reflections, (2) magneto-ionic splitting, (3) partial reflection in a layer of monotonically increasing ionization-density, and (4) division of the region into a series of partially-reflecting partially-transmitting strata. Experimental data in the form of records of virtual height as a function of time indicate that the first three explanations are not acceptable. The fourth explanation, however, is consistent with all available data, including the characteristics of multiple reflections and certain systematic differences in the polarizations of echoes at different heights. It is concluded, therefore, that the lower ionosphere contains at night a system of reflecting strata at discrete heights, capable of both reflecting and transmitting incident electromagnetic energy. A new mechanism of ionospheric layer formation may be required to explain these results.