Features of annual and semiannual variations derived from the global ionospheric maps of total electron content

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 annualto-mean amplitude ratio, A 1 , and the semiannual-to-mean amplitude ratio, A 2 , as well as the latitudinal symmetrical and asymmetrical parts, A and A of A 1 . 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.

Characteristics of the ionospheric total electron content of the equatorial ionization anomaly in the Asian-Australian region during 1996–2004

“…Besides, annual asymmetry, indicated by greater value in January than in July at both the crest regions, is also very clear. This means that no winter anomaly appeared in the southern crest region which is consistent with the result of global ionospheric map (GIM) TEC (Zhao et al, 2007). (2) The latitude of EIA crest (L nc /L sc ) shows a seasonal dependence.…”

Variability of ionospheric TEC during solar and geomagnetic minima (2008 and 2009): external high speed stream drivers

“…1). It is tempting to associate the maxima with the equinoxes (Lal, 1998;Zhao et al, 2007;Lai et al, 2011). However, the equinoxes occurred on ∼DOY 80 and 266, where the second date does not correspond to observed peaks.…”

On the mechanism of seasonal and solar cycleNmF2variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations

“…[2] Seasonal and December anomalies in N m F 2 variations have been studied since the beginning of ionospheric observations [Appleton and Naismith, 1935;Seaton and Berkner, 1939;Yonezawa and Arima, 1959]; however, the mechanisms of their formation are still discussed in publications [Richards et al, 1994c;Rishbeth et al, 2000;Richards, 2001;Kawamura et al, 2002;Yu et al, 2004;Pavlov and Pavlova, 2005;Rishbeth and Müller-Wodarg, 2006;Zhao et al, 2007]. Among different explanations proposed during the decades of the problem analysis [see Rishbeth, 1998], the suggestion by Rishbeth and Setty [1961] that F 2 layer seasonal anomaly is due to neutral composition variation may be considered as generally accepted [Rishbeth et al, 2000].…”