Influence of solar activity variations on the day-to-day <i>NmE</i> variability during geomagnetically quiet conditions according to the ground-based Dourbes ionosonde data

A study of day-to-day variations in the statistical characteristics of the ionospheric E layer electron number density NmE for each month in the year under geomagnetically quiet conditions at low and middle solar activity was carried out according to the hourly ground-based Dourbes ionosonde measurements of the ionospheric E layer critical frequency during the time periods from 1957 to 2023. The NmE statistical parameters under calculations are the mathematical expectation NmEE, the most probable NmEMP, the arithmetical mean monthly median NmEMED, the standard deviations sE, sMP, sMED, and the variation coefficients CVE, CVMP, and CVMED of NmE relative to NmEE, NmEMP, and NmEMED, respectively. It was shown that the value of NmEE provides the best description of a set of observations of NmE by one parameter due to the lower day-to-day variability of NmE compared to NmEMP or NmEMED. It was proven for the first time that the transition from low to middle solar activity leads to significant changes in the day-to-day variability of NmE with the longest periods of increases and decreases in the studied variability in March and December, respectively.