Quasi-27-Day Variations of Ionospheric NmF₂

The data observed at 11 ionosonde stations in the East Asia/Australia sector during 1969---1980 and the band-pass filter method have been used to analyze the characteristics of the quasi-27-day variations of daytime NmF₂ and the relativity between NmF₂ and the solar activity index F_10.7. The results are summarized as follows. In most years, the short period (2~70 days) spectra of F_10.7 and NmF₂ have obvious maxima around 27-day period. In the quasi-27-day variations, the standard deviations of F_10.7 and NmF₂ have evident variability year-to-year. The average standard deviation of F_10.7 is 10.9\%. The standard deviations of NmF₂ depend on geomagnetic latitude, the minima appear at equator and they become greater with increasing latitude. The average standard deviation of NmF₂ of 11 stations is about 8.2%. In the quasi-27-day band, F_10.7 and NmF₂ have notable correlation. The probability of the significant correlation at 0.05 significance level exceeds 90%. The average time delay between variations of the NmF₂ and F_10.7 is about 2 days. As a whole, the quasi-27-day variations of NmF₂ primarily attribute to the quasi-27-day variations of the solar EUV flux.