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 latitude station, Hawaii (21.2 degrees N, 157.7 degrees W), during the solar maxima (1969 and 1981) and minima (1965 and 1985) years of two consecutive solar cycles, 20 and 21, are used to study the day to day variabilities 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 minimum 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 35%, the mean night time values vary from about 25% to 60%. The mean daytime variations in NP for the solar minimum phase are remarkably higher in all the three seasons compared to the solar maximum phase. However, no such increase is observed in the mean daytime IEC variations, indicating the highly variable nature of the daytime ionospheric F region compared to the topside during solar minimum For this low latitude station. The winter night time IEC also seems to be a relatively stable parameter during the solar minimum. The short term day to day variabilities of the day time peak values of IEC and NP (ie IEC(max) and NPmax) are not closely associated with the variations in F-10.7 solar flux. Contrary to the common expectation, the variabilities in both the parameters, particularly in NPmax, are somewhat reduced during the solar maximum (when the variability in F-10.7 solar flux is much higher compared 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 corresponding F-10.7 solar Aux spectra. The number of additional components for both the parameters with periods less than 15 days are more for the low solar activity years than for the solar maximum years.