Abstract
By the Fourier series expanding method, the observed F2 layer critical frequencies (foF2) globally over 70 stations in a high solar activity year of 1958, are used to analyze the annual and semi-annual variations of foF2, and the world wide distribution features of their amplitude and phase in daytime and nighttime are studied in detail. The results for foF2 annual and semi-annual variation are summarized as follows. The midnight (2:00 LT) foF2 annual variations are noticeable in both hemispheres at mid-high latitudes, and their amplitudes are slightly larger in far pole regions than in near pole regions. Generally, at most stations, the midnight foF2 reach the maximum in summer, and no winter anomaly can be discerned. While in daytime (14:00 LT), there are pronounced annual variations with large amplitude in both hemispheres at mid-high latitudes. After carefully studying their phases, we find that these annual variations usually peak in winter, which indicate all the variations are classic winter anomaly. However, the winter anomaly is very weak in the equatorial zone and not even perceivable in South America. Moreover, the amplitude of daytime foF2 semi-annual variation is generally small in near pole regions and large in far poles region of both hemispheres. Compared with their annual component, the semi-annual variations in the tropical region are significant. Their phase distributions reveal that the semi-annual variation usually peaks in March and April.
Highlights
In 1930's, Chapman had predicted that the electron concentrations in the ionosphere vary regularly with the solar zenith angle as the well-known Chapman layer, so any departure from 'solar-controlled' behavior in the ionosphere is originally taken as 'anomaly' (Rishbeth 1998)
In order to explain the results mentioned above, we studied the atomid molecular ratio [0/N2] and confirmed that the noon frequencies of F2 layer (foF2) annual variations prevailing in mid-high latitudes are caused largely by the annual variation of [0/N2]
A1, A2 are the amplitudes of foF2 annual and semiannual components in a certain local time and in units of MHz, and D1, D2 are the phases corresponding to those components and in units of days, zero phase meaning the maximum is on 1 January
Summary
In 1930's, Chapman had predicted that the electron concentrations in the ionosphere vary regularly with the solar zenith angle as the well-known Chapman layer, so any departure from 'solar-controlled' behavior in the ionosphere is originally taken as 'anomaly' (Rishbeth 1998). There are many 'anomalies' that are well known in the F2 layer (Rishbeth 1998, 2000b; Wills et al 1994), such as the 'equatorial F2layer anomaly' of electron concentration, the 'winter anomaly', the 'seasonal anomaly' and the 'semi-annual anomaly' of foF2. Lal (1998) re ported semi-annual variation of the solar wind and the other geophysical phenomena. Ma et al (2002) and Liao et al (2000) analyzed the semi-annual variations of foF2 and Equatorial Electrical Jet by observed data, and discussed their possible physical cause. All of these observing and experimental results developed a better way for us to reveal and under stand the phenomena of annual and semi-annual variations in the ionosphere, and give birth to the following extensive assumptions for those phenomena
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