SHORT-TERM VARIABILITIES OF IONOSPHERIC ELECTRON-CONTENT (IEC) AND PEAK ELECTRON-DENSITY (NP) DURING SOLAR-CYCLE-20 AND SOLAR-CYCLE-21 FOR A LOW-LATITUDE STATION

Hourly values of IEC and of f(0)F(2) (critical frequency) for a low la titude station, Hawaii (21.2 degrees N, 157.7 degrees W), during the s olar maxima (1969 and 1981) and minima (1965 and 1985) years of two co nsecutive solar cycles, 20 and 21, are used to study the day to day va riabilities of the ionospheric parameters IEC and NP. It is found that there is good correspondence in the day to day variations of IEC and NP from one solar cycle to the other for both solar maximum and minimu m years in the two solar cycles. Depending on solar phase and season, while the mean daytime IEC and NP variations range from about 20% to 3 5%, the mean night time values vary from about 25% to 60%. The mean da ytime variations in NP for the solar minimum phase are remarkably high er in all the three seasons compared to the solar maximum phase. Howev er, no such increase is observed in the mean daytime IEC variations, i ndicating the highly variable nature of the daytime ionospheric F regi on compared to the topside during solar minimum For this low latitude station. The winter night time IEC also seems to be a relatively stabl e parameter during the solar minimum. The short term day to day variab ilities of the day time peak values of IEC and NP (ie IEC(max) and NPm ax) are not closely associated with the variations in F-10.7 solar flu x. Contrary to the common expectation, the variabilities in both the p arameters, particularly in NPmax, are somewhat reduced during the sola r maximum (when the variability in F-10.7 solar flux is much higher co mpared to the solar minimum) which is more evident in the stronger 21 solar cycle. A larger number of significant components are seen in the spectra of the percentage variation of both IEC(max) and NPmax during both solar phases of the two solar cycles compared to the correspondi ng F-10.7 solar Aux spectra. The number of additional components for b oth the parameters with periods less than 15 days are more for the low solar activity years than for the solar maximum years.