INTRODUCTION
Ionospheric parameters such as the critical frequency of the F2 layer (fоF2) and the peak height of the F2 layer (hmF2) have been proved to have regional variations [Givishvili et al., 1982; Limberger et al., 2013]. Considering the features of the parameters associated with their geomagnetic latitude ϕ, the ionosphere can be divided into three latitude zones: low-latitude (ϕ<30°), mid-latitude (30°<ϕ<60°), and high-latitude (ϕ>60°) [Hunsucker, Hargreaves, 2003].
The ionospheric parameters also reveal diurnal-seasonal variations. Using electron density (Ne) data from the Indian SROSS C2 satellite for 1994–1997 at low and equatorial latitudes, Bhuyan et al. [2003] found that the electron density had a minimum before sunrise, a diurnal maximum during afternoon hours, a secondary enhancement in sunset hours of the June solstice, as well as seasonal and latitudinal variations. Basing on ionogram data acquired at American latitudes (–65.2° ÷ \(\div\)\(\div\)–18.5°), Ezquer et al. [2004] analyzed the diurnal, seasonal and latitudinal behavior of fоF2 and parameter M(3000)F2 and established that the fоF2 variability is high at low solar activity, larger at night than during a day, and depends on latitude. In the East Asian sector, with digisonde data from the high-latitude station Norilsk (69.2° N, 88.0° E, MLat. 59.8° N), Ratovsky et al. [2013] compared diurnal, seasonal, and solar cycle dynamics of the ionospheric F2 peak density, the peak height, and the thickness of the bottomside ionosphere with the IRI-2007 model and analyzed the winter anomaly manifestation at Norilsk and the peculiar diurnal-seasonal hmF2 behavior under low solar activity (called “polar day effect”). Using digisonde ionogram data from Hainan (19.5° N, 109.1° E, MLat. 9.7° N) for 2002, Zhang et al. [2004] analyzed diurnal and seasonal variations of ionospheric parameters fоF2, hmF2, B0, and B1, and found that in spring fоF2 remained high and at almost constant value between local noon and midnight time, while in summer it had two peaks in the afternoon and around midnight. Using digisonde observation data from low, middle, and high-latitude stations in East Asian sector, Ratovsky et al. [2014] obtained Local Empirical Model (LEM) results on fоF2 and hmF2 dynamics from December 2002 to December 2008 and compared their regular (diurnal, seasonal, and solar cycle) variations. They found that the main variation features given by the local models agreed with previous studies, while the parameters at different latitudes also had some specific features [Ratovsky et al., 2014]. As for the diurnal-seasonal variations of the scale height at hmF2, using the 2002–2004 Hainan ionogram data, Zhang et al. [2006] showed that the scale heights had a noontime peak and pre-sunrise peak, most pronounced in winter. Also they had a clear annual variation with the largest daytime values in summer and the smallest values in winter, and revealed strong solar activity dependence, with daytime values decreasing with solar activity. These results show that the diurnal-seasonal variations of the ionospheric parameters have regional properties.
On the other hand, the ionospheric parameters have also annual and semi-annual variations. For example, for the peak density of the F2 layer NmF2, the winter anomaly or seasonal variation is that at mid-latitudes noontime NmF2 values are larger in winter than in summer. The semi-annual variation is that NmF2 is greater at equinox than at solstice, and the annual or non-seasonal variation is that NmF2 is greater around December solstice than around the June solstice by day as well as by night for both the Northern and Southern hemispheres [Appleton, Naismith, 1935; Zhao et al., 2007]. With TEC or ionosonde data, some authors re-ported that NmF2 annual variations were more significant at high latitudes than at low latitudes, were most pronounced at magnetic latitudes 40–60° with the winter anomaly occurring [Yonezawa, 1971; Yu et al., 2004; Zhao et al., 2007]. Analyzing TEC or ionosonde data, some authors indicated that amplitudes of the NmF2 semi-annual variation were larger at low latitudes than at high ones, larger at noon than at midnight, and larger in solar maxima than in minima [Yonezawa, Arima, 1959; Yonezawa, 1971; Yu et al., 2004]. These results show that annual and semi-annual variations of the ionospheric parameters also have regional properties.
To learn more about variation features at different latitudes, we statically analyze and compare digisonde observation data from Hainan (19.5° N, 109.1° E, MLat. 9.7° N), Irkutsk (52.4° N, 104.3° E, MLat. 42.5° N) and Norilsk (69.2° N, 88.0° E, MLat. 59.8° N) for 2006–2012, acquired at low to high latitudes in East Asian sector. The ionospheric parameters such as the critical frequency of the F2 layer (fоF2), the peak height of the F2 layer (hmF2), and the scale height at hmF2 (HT) were obtained from manually scaled ionogram data. We also discuss common and specific features of diurnal-seasonal variations of these parameters measured at different latitudes during ascending phase of solar activity.
