Abstract
Abstract. In the present work we use the NASA-JPL global ionospheric maps of total electron content (TEC), firstly to construct TEC maps (TEC vs. magnetic local time MLT, and magnetic latitude MLAT) in the interval from 1999 to 2005. These TEC maps were, in turn, used to estimate the annual-to-mean amplitude ratio, A1, and the semiannual-to-mean amplitude ratio, A2, as well as the latitudinal symmetrical and asymmetrical parts, A' and A" of A1. Thus, we investigated in detail the TEC climatology from maps of these indices, with an emphasis on the quantitative presentation for local time and latitudinal changes in the seasonal, annual and semiannual anomalies of the ionospheric TEC. Then we took the TEC value at 14:00 LT to examine various anomalies at a global scale following the same procedure. Results reveal similar features appearing in NmF2, such as that the seasonal anomaly is more significant in the near-pole regions than in the far-pole regions and the reverse is true for the semiannual anomaly; the winter anomaly has least a chance to be observed at the South America and South Pacific areas. The most impressive feature is that the equinoctial asymmetry is most prominent at the East Asian and South Australian areas. Through the analysis of the TIMED GUVI columnar [O/N2] data, we have investigated to what extent the seasonal, annual and semiannual variations can be explained by their counterparts in [O/N2]. Results revealed that the [O/N2] variation is a major contributor to the daytime winter anomaly of TEC, and it also contributes to some of the semiannual and annual anomalies. The contribution to the anomalies unexplained by the [O/N2] data could possibly be due to the dynamics associated with thermospheric winds and electric fields.
Highlights
Investigations revealed that the F2 layer is “anomalous” rather than controlled by the solar zenith angle, according to the prediction of Chapman theory (Appleton and Naismith, 1935)
The well-known temporal anomalies observed are the winter anomaly or seasonal anomaly, the semiannual anomaly, the annual or nonseasonal anomaly
The main cause of the winter anomaly is that the vertical wind does change the chemical composition (Rishbeth et al, 1987)
Summary
Investigations revealed that the F2 layer is “anomalous” rather than controlled by the solar zenith angle, according to the prediction of Chapman theory (Appleton and Naismith, 1935). B. Zhao et al.: Annual and semiannual variations from GIM TEC equinoctial asymmetries (the electron density in one equinox being larger than that in the other equinox) in the ionosphere and thermosphere during solar maximum period by using the Japanese MU radar data. Fuller-Rowell (1998) proposed a mechanism named “thermospheric spoon” to interpret the semiannual variation in the ionosphere He suggests that the global-scale, interhemispheric circulation at solstices acts like a huge turbulent eddy in mixing the major species. A database of TEC in the ionosphere and plasmasphere, derived from a worldwide network of global positioning system (GPS) observations, has been used to investigate the local and regional characteristics of various anomalies (Huang and Cheng et al, 1996; Unnikrishnan et al, 2002; Wu et al, 2004). We hope the work could help us in achieving comprehensive insight into the complexities of F2-layer behavior
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