The Diurnal and Seasonal Variation of Electron Densities in the Midlatitude D Region Under Quiet Conditions

Synoptic studies of the D‐region electron densities using the differential‐absorption partial‐reflection experiment at 2.66 and 6.275 MHz are in progress at Ottawa (45°N, 76°W). The measurements are made from about one hour before sunrise to one hour after sunset throughout the winter and for 10 consecutive days each month during the other seasons. The methods employed to analyze data obtained over long periods are outlined in this paper and average electron densities showing diurnal behavior under quiet conditions are presented. Over the greater part of the day (i.e., for solar zenith angles less than about 75°) the electron number densities show regular solar control with values increasing toward local noon. However, there is an asymmetry about local noon of up to 6° in solar zenith angle, the morning electron densities being lower than those at the corresponding time in the afternoon. The diurnal changes for solar zenith angles greater than 75° are more complicated. The formation over dawn of a C layer at heights below 75 km can be observed, and there is evidence that this layer decays after a solar zenith angle of about 90°. This decay continues until arrested by photoionization processes.