Global scale annual and semi-annual variations of daytime NmF2 in the high solar activity years |
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| Authors: | Tao Yu Weixing Wan Libo Liu Biqiang Zhao |
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| Institution: | aInstitute of Geology and Geophysics, CAS, Beijing 100029, China;bNational Satellite Meteorological Center, CMA, Beijing 100081, China |
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| Abstract: | The annual and semi-annual variations of the ionosphere are investigated in the present paper by using the daytime F2 layer peak electron concentration (NmF2) observed at a global ionosonde network with 104 stations. The main features are outlined as follows. (1) The annual variations are most pronounced at magnetic latitudes of 40–60° in both hemispheres, and usually manifest as winter anomalies; Below magnetic latitude of 40° as well as in the tropical region they are much weaker and winter anomalies that are not obvious. (2) The semi-annual variations, which are usually peak in March or April in most regions, are generally weak in the near-pole regions and strong in the far-pole regions of both hemispheres. (3) Compared with their annual components, the semi-annual variations in the tropical region are more significant.In order to explain the above results, we particularly analyze the global atomic/molecular ratio of O/N2] at the F2 layer peak height by the MSIS90 model. The results show that the annual variation of O/N2] is closely related with that of NmF2 prevailing in mid-latitudes and O/N2] annual variation usually may lead to the winter anomalies of NmF2 occurring in the near-pole region. Moreover, NmF2 semi-annual variations appearing in the tropical region also have a close relationship with the variation of O/N2]. On the other hand, the semi-annual variations of NmF2 in the far-pole region cannot be simply explained by that of O/N2], but the variation of the solar zenith angle may also have a significant contribution. |
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| Keywords: | Ionospheric annual and semi-annual variation Winter anomaly Atomic/molecular ratio |
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