The global ionospheric asymmetry in total electron content

Summer‐winter hemispheric asymmetry of the sudden increase in ionospheric total electron content and of the O/N2 ratio: Solar activity dependence

“…As shown by Figures 3b and 3d and Table 1, [20] The dayside ionosphere at midlatitudes is formed by the balance between the production rate due to the photoionization of atom oxygen O and molecular N 2 from the E region through the F region and the loss rate due to the chemical reaction of molecular N 2 and oxygen ion O + in the F region under the strong influence of ambipolar diffusion processes. The F region electron density and TEC at midlatitudes are well known to be correlated with O/N 2 ratio in the ionospheric F region [e.g., Rishbeth et al, 2000;Mendillo et al, 2005]. Sudden increases in the EUV radiation (assumed to be coincident with the X-ray) during solar flares cause immediate enhancements of ion/electron density in the lower F region and the E region rather than around the F2 region.…”

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

“…The value of the global average of an "Asymmetry Index" (AI) (AI=(December-June)/(December+June)), used to characterize the amplitude of the annual anomaly is far greater than the value of 0.035 that corresponds to the annual variation of the solar irradiance due to the Sun-Earth distance by using GIM data of the year 2002 (Mendillo et al, 2005). Our study shows the same results and found that the annual anomaly exists both by day and by night and is least evident in the sunrise and sunset sectors.…”

“…The GPS system and the JPL GIMs derived from its data have become a standard ionospheric diagnostic tool, and are particularly useful for our study. Mendillo et al (2005) have found the annual anomaly in TEC to be a global characteristic by using GIMs data of the year 2002. Here, to some extent, we are about to expand their work and explore the global feature of the principle F2-layer anomalies, including the winter anomaly, the semiannual anomaly and the annual anomaly.…”

A new concept regarding the cause of ionosphere semiannual and annual anomalies

“…However, there are some phenomena that were called anomalies in the early days of ionospheric exploration and have remained as such until the present. Two of the most widely known [see Fuller-Rowell, 1998;Rishbeth, 1998Rishbeth, , 2000Li and Yu, 2003;Mendillo et al, 2005] of these are (1) the semiannual anomaly, which produces larger f o F 2 values for equinoxes than for solstices; and (2) the annual anomaly (also called the annual asymmetry), which is described by a larger f o F 2 value during the December than during the June solstice (larger than the 7% that would be expected considering the change in the Sun-Earth distance). Another manifestation of the annual anomaly is the so-called winter anomaly [Appleton, 1938]: f o F 2 values measured during June over the Northern Hemisphere that are actually lower than (or comparable to) the December values, which is in opposition to the accepted dependence on the Sun's zenith angle.…”

“…The first reports concerning these so-called anomalies of the electron concentration in the upper atmosphere date back to the 1930s, to the pioneering works by Berkner et al [1936] and Appleton [1938]. Since then several authors have proposed different approaches to understanding, modeling, and predicting these anomalies [Fuller-Rowell, 1998;Rishbeth, 1998Rishbeth, , 2000Li and Yu, 2003;Mendillo et al, 2005]. However, the processes seemed to be so complex that no unique theory was able to model both effects, and many early theories could only partially explain one or the other.…”