This thesis is a study of the structure of that part of the ionosphere lying between 60 and 120 km. In the usual terminology the ionized parts of the atmosphere in this altitude range are called the ionospheric D and E regions, the boundary between them occurring at a height of 90 km. Above this height the E region extends upwards to 140 km, the base of the F region. Since the ionization below 50 km is not enough to effect the propagation or radio waves this height effectively marks the bottom of the D region. There is also another system of nomenclature based on the neutral gas temperatures of the atmosphere. The mesosphere lies in the altitude range 50 to 85 km, which is a region of decreasing temperature with height. Because they refer to the same height range the terms mesosphere and D region are often used synonymously in the following work. Above 90 km the temperature increases, rapidly at first and then more slowly, in the thermosphere.