27-day variations in F 2 layer critical frequencies at Huancayo

Abstract

The solar radiation responsible for the production of F 2 layer ionization is already known to change with the 11-year sunspot cycle. The question has therefore been examined whether analogous changes occur in the course of the solar rotation of about 27 days period in cases where the sunspot numbers R show appreciable quasi-persistent periodicities. After correcting for lunar tidal influence, variations of the type in question have been found in the noon values of ƒF 2 for Huancayo, Peru, using the superposed epoch method. Such variations are found to be of the order of 6 to 10%. Changes in ƒF 2 accompanying changes in R appear to be delayed by about two days, but a scatter analysis by means of synchronized harmonic dials throws doubt on the statistical significance of such a lag. The lunar tides in the noon values of ƒF 2, examined for comparison, cause total semimensual changes of more than 10% in southern summer, but only 2 to 3% in southern winter. These seasonal changes in the lunar tidal effect L(ƒF 2) are much larger than those disclosed in the quantitative effects of changes of R on ƒF 2. The use of these results in ionospheric forecasting is briefly discussed.