Using hourly data (1958–1992) from Irkutsk ionosonde station, we analyzed properties of variability of the F2-layer maximum density, Nm, under different levels of the solar and geomagnetic activity. The standard deviation s(x) of Nm fluctuations relative to the quiet level (x=(Nm/Nm0–1)·100 %), and the average shift of these fluctuations x-ave were used as characteristics of this variability. For this purpose, the empirical model of the F2-layer maximum density for quiet magnetic conditions, Nm0, was constructed. The Nm variability was found to depend weakly on the solar activity level; in the first approximation, this dependence can be neglected. The Nm variability dependence on geomagnetic activity is among the principal ones, along with the dependences on local time and season. In general, dispersion of Nm fluctuations under quiet conditions is smaller than that during the periods of high geomagnetic activity. During the periods of high geomagnetic activity, however, the dispersion does not increase with the further growth of this activity, while the absolute value of shift x-ave (which is mainly negative under high geomagnetic activity) still increases with growing geomagnetic activity. As a result, if this activity is high enough, the condition |x-ave|>|s(x)| can be fulfilled.

An analysis of properties and peculiarities of the nighttime winter foF2 increases (NWI) in the East Siberia is made on data of ionospheric station Irkutsk in the periods 1958–1992 and 2002–2009 and the empirical model of the F2 layer critical frequency under the geomagnetic quiet conditions deduced from these data (model Q-F2). It is revealed, that the NWI is the stable regularity of the quiet ionosphere over Irkutsk. The amplitude of the NWI (the difference between maximum and minimum foF2 values at night hours) is the greatest in December–January and nearly the same at low and middle solar activity. It is a peculiarity of the quiet ionosphere in the East Siberia. Maximum in night foF2 under quiet geomagnetic conditions is observed mainly after midnight (02-04 LT) and is shifted to predawn hours as solar activity increases. At low solar activity the quiet ionosphere at ∼02–04 LT shows the following properties: (a) the fluctuations of foF2 and hmF2 are in the reverse correlation but this dependence is weak; (b) very strong fluctuations of foF2 (|δfoF2|>30%) occur seldom (∼4% of events) and almost all of them are positive; an example of very strong fluctuations of foF2 up to 60% can be an extreme increase in the foF2 on 19.12.2008; (c) the very strong enhancements of foF2 in the NWI maximum can be observed at the low geomagnetic activity, they occur more often during substorms but very seldom during geomagnetic storms. Possible reasons of these properties of NWI are discussed.

Statistical and spectral analyses are performed to investigate variations of two ionosphere F2 layer key parameters, the critical frequency (foF2) and the peak height (hmF2), that were measured over Irkutsk (52.5°N, 104.0°E) from December 2006 to January 2008 under solar minimum. The analyses showed that both parameters contain quasi-harmonic oscillations with periods of Tn=24/n hours (n=1–7), among which the diurnal (n=1) and semidiurnal (n=2) ones are the strongest. Seasonal variations are explored of mean and median values, spectrum, amplitude, and phase of the diurnal and semidiurnal components of foF2 and hmF2.

A critical question in ionospheric physics is the state of the ionosphere and relevant processes under extreme solar activities. The solar activity during 2007-2009 is extremely prolonged low, which offers us a unique opportunity to explore this issue. In this study, we collected the global ionosonde measurements of the F-2 layer critical frequency (f(o)F(2)), E layer critical frequency (f(o)E), and F layer virtual height (h'F) and the total electron content (TEC) maps produced by the Jet Propulsion Laboratory, which were retrieved from dual-frequency GPS receivers distributed worldwide, to investigate the ionospheric phenomena during solar minimum of cycle 23/24, particularly the difference in the ionosphere between solar minima of cycle 23/24 and the preceding cycles. The analysis indicates that the moving 1 year mean foF2 at most ionosonde stations and the global average TEC went to the lowest during cycle 23/24 minimum. The solar cycle differences in f(o)F(2) minima display local time dependence, being more negative during the daytime than at night. Furthermore, the cycle difference in daytime f(o)F(2) minima is about -0.5 MHz and even reaches to around -1.2 MHz. In contrast, a complex picture presents in global h'F and foE. Evident reduction exists prevailingly in the moving 1 year mean h'F at most stations, while no huge differences are detected at several stations. A compelling feature is the increase in foE at some stations, which requires independent data for further validation. Quantitative analysis indicates that record low foF2 and low TEC can be explained principally in terms of the decline in solar extreme ultraviolet irradiance recorded by SOHO/SEM, which suggests low solar EUV being the prevailing contributor to the unusual low electron density in the ionosphere during cycle 23/24 minimum. It also verifies that a quadratic fitting still reasonably captures the solar variability of f(o)F(2) and global average TEC at such low solar activity levels.

This paper presents the results of investigations of mid-latitude upper atmosphere response to tropical cyclones, which were observed over the Northwest Pacific Ocean in September-October 2005. It is accepted that internal atmospheric waves (IAWs) are one of physical mechanisms of the interaction between the troposphere and ionosphere. It is also assumed that IAWs travel upward along oblique trajectories and manifest themselves in the upper atmosphere as traveling ionospheric disturbances (TIDs). Such TIDs can be registered at distances of some thousands of kilometers from the IAWs tropospheric source. To check up this hypothesis we have carried out statistical and harmonic analyses of data of the vertical sounding of the upper atmosphere in the East Siberian and Far Eastern regions of Russia, in the China and in the Australia. The disturbances, which most probably can be associated with cyclones, were picked out from variations of ionospheric F-region parameters: the increase of oscillation amplitude in the range of periods 1.5-6 h; essential deviation of F-region parameters from their medians.

Results of studies of variations in the ionospheric parameters in October-November 2003 are presented based on data of the network of ionospheric stations located in the longitudinal band 90degrees-130degrees E at latitudes from the auroral region to the equator. Differences in the ionospheric response to geomagnetic storms at different latitudes are shown. During intense storms, the events typical of the region of the main ionospheric trough were observed at a geomagnetic latitude of 40degrees. In the vicinity of the equator, the critical frequencies of the F2 layer demonstrated large-amplitude oscillations. The substorm effect was weakened south of the subauroral region.