Influence of Solar Activity Variations on Interdiurnal Variability of NmE in Geomagnetically Quiet Conditions Obtained from Ground-Based Dourbes Ionosonde Data

The article studies interdiurnal variations in the statistical characteristics of the electron number density NmE of the ionospheric E layer peak for each month of the year in geomagnetically quiet conditions with low and average solar activity based on hourly measurements of the critical frequency of the E layer from the ground-based Dourbes ionosonde from 1957 to 2023. The authors have calculated the mathematical expectation NmE_E; the most probable NmE_MP; the arithmetically monthly median NmE_MED; the standard deviations of σ_E, σ_MP and σ_MED of quantity NmE from NmE_E, NmE_MP, and NmE_MED; the variation coefficients CV_E, CV_MP, and CV_MED of quantity NmE with respect to NmE_E, NmE_MP, and NmE_MED, respectively. It is shown that NmE_E provides the best description of the set of measurements of NmE using only the statistical parameter due to the smaller interdiurnal variability of NmE compared to NmE_MP or NmE_MED. For the first time, it is proved that the transition from low to average solar activity leads to significant changes in the interdiurnal variability of NmE, with the longest periods of increase and decrease in the studied variability in March and December, respectively.