Annual and Semi-annual Variations of the Observed foF2 in a High Solar Activity Year

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 study ing 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 perceiv able in South America. Moreover, the amplitude of daytime foF2 semi-an nual 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 iil March and April. In order to explain the results mentioned above, we studied the atomid molecular ratio [0/N2] and confirmed that the noon foF2 annual variations prevailing in mid-high latitudes are caused largely by the annual variation of [0/N2]. As the noon foF2 semi-annual variations pronounced in far pole

Analysis of the winter and semiannual ionospheric anomalies in 1999–2009 based on GPS global International GNSS Service maps

“…At night the semiannual anomaly is found during high solar activity and the larger values are located at mid to low geomagnetic latitudes with maximum values also recorded in the South American region. Similar conclusions were reached by Li and Yu [2003].…”

“…This causes the variation of the amplitude of the ionospheric equatorial anomaly through a fountain effect and this process induces the semiannual variation of N m F 2 at low latitude. The semiannual variations appear around the globe in daytime, and except in low-latitude regions and in South America sector, there is no obvious semiannual variation of N m F 2 in the nighttime [Li and Yu, 2003]. The amplitude of this variation has close relationship with the solar activity (larger values in maxima of solar activity).…”

Annual and semiannual variations of vertical total electron content during high solar activity based on GPS observations

“…Ma et al (2003) suggested that the semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of the equatorial electrojet, this causes the variation of amplitude of ionospheric equatorial anomaly through fountain effect and this process induces the semiannual variation of the low latitude NmF2. The semiannual variation appears around the globe at daytime, and except in low latitude region and in South American sector (Li and Yu, 2003) there is no obvious semiannual variation of NmF2 in the nighttime. The amplitude of this variation has close relationship with the solar activity, they have asymmetrical structures between the two hemispheres (larger at the Northern Hemisphere) and they have longitude difference (Ma et al, 2003).…”

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.…”