Statistical Characteristics of the Mid-latitude NmF2 Day-to-Day Variability During Geomagnetically Quiet Conditions at Low Solar Activity Obtained from the Dourbes and Juliusruh Ionosonde Observations

The statistical characteristics of the day-to-day variability of the F2-layer peak electron number density, NmF2, measured by the mid-latitude Dourbes and Juliusruh ionosondes are studied for each month, M, in a year for geomagnetically quiet conditions at low solar activity during the time periods from 1957 to 2017. The NmF2 statistical parameters under study are the mathematical expectation NmF2(E), the most probable NmF2(MP), the arithmetical mean monthly median NmF2(MED), and the variation coefficients CVE(UT,M), CVMP(UT,M), and CVMED(UT,M) of NmF2 relative to NmF2(E), NmF2(MP), and NmF2(MED), respectively, where UT is the universal time. It is found that the values of CVE(UT,M), CVMED(UT,M), and CVMP(UT,M) are changed in the intervals of 14-48%, 14-62%, and 14-120%, respectively. In the predominant majority of cases, the coefficient CVE(UT,M) does not exceed the coefficients CVMED(UT,M) or CVMP(UT,M). If CVE(UT,M) > CVMED(UT,M) or CVE(UT,M) > CVMP(UT,M) then the difference between CVE(UT,M) and CVMED(UT,M) or between CVE(UT,M) and CVMP(UT,M) is not significant in comparison with their values. Thus, as a quantitative characteristic that provides the best description of a set of observations of NmF2 by one parameter, one should use the mathematical expectation of NmF2, but not the monthly median of NmF2 or the most probable NmF2. Empirical models of NmF2 should be based on the mathematical expected NmF2, but not on the monthly median of NmF2 or the most probable NmF2.