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

The study of the interdiurnal variations in the statistical characteristics of the electron number density NmE of the ionospheric E layer peak for each month M of the year in geomagnetically quiet conditions at low and moderate solar activity was carried out based on hourly measurements of the critical frequency of the E layer of the Huancayo and Jicamarca ionosondes from 1957 to 1989 and 1998-2006, respectively. The authors have calculated the mathematical expectation NmEE(UT,M), NmEA(UT,M) arithmetic mean, the standard deviation σE(UT,M) and the variation coefficient CVE(UT,M) of NmE(UT,M) from NmEE(UT,M), respectively, where UT is the universal time. The calculations showed that the value of CVE(UT,M) that determines the relative interdiurnal NmE variability vary between 4-14 and 3-18% at low and moderate solar activity, respectively. It was found for the first time that the interdiurnal variability of NmE can either increase or decrease when solar activity changes from low to moderate levels. In the first case, the increase in σE(UT,M) prevails over the growth of NmEE(UT,M), in the second case, the growth of NmEE(UT,M) prevails over the increase in σE(UT,M).