Abstract

The work examines diurnal variations in the statistical characteristics of the variability of the electron number density *NmF*2 of the maximum of the ionspheric *F*2 layer during quiet geomagnetic conditions at low solar activity based on hourly ionosonde measurements of the critical frequency of the ionspheric *F*2 layer from 1957 to 2017 over Moscow. The statistical parameters of *NmF*2 are calculated for each month M of each year at the universal time UT = 0, 1, … 23 h: the mathematical expectation *NmF*2*E*; the most probable value *NmF*2*MP*; the average monthly median *NmF*2*MED*; the arithmetic mean *NmF*2*A*; the standard deviations of *NmF*2 from *NmF*2*E*, *NmF*2*MP*, and *NmF*2*MED*; and the coefficients of variation *CV**E*, *CV**MP*, and *CV**MED* of *NmF*2 relative to *NmF*2*E*, *NmF*2*MP*, and *NmF*2*MED*, respectively. It follows from the calculations that the *CV**E*, *CV**MED*, and *CV**MP* values vary within the intervals of 12–43%, 12–60%, and 13–75%, respectively, and, in the vast majority of cases, *CV**E*(UT, M) < *CV**MED*(UT, M) and *CV**E*(UT, M) < *CV**MP*(UT, M). If the coefficient *CV**E* in each month of each year is compared for different time points, the lowest *CV**E* value varies from 12% (July) to 19% (December) and occurs during daytime, while the highest *CV**E* value lies in the interval from 26% (June) to 43% (December). For each UT, the lowest and highest values of this coefficient in the autumn, winter, and spring months are greater than those for the summer months. It was found that the difference of *NmF*2*A*(UT, M) from *NmF*2*E*(UT, M) does not exceed 0.2%.