Variability of parameters of the <Emphasis Type="Italic">F</Emphasis>2-layer maximum in the quiet midlatitude ionosphere under low solar activity: 1. Statistical properties

Results of statistical analysis of the properties of variability of F2-layer maximum parameters (critical frequency foF2 and the height hmF2) in quiet midlatitude ionosphere under low solar activity in the daytime (1000–1500 LT) and nighttime (2200–0300 LT) hours are presented on the basis of Irkutsk station data for 2007–2008. It is found that the distribution density of δfoF2 could be presented as consisting of two distinctly different normal laws of this distribution, one of which corresponds to weak (|δfoF2| < 10%) fluctuations in foF2 and the other corresponds to strong (30% > |δfoF2| > 10%) fluctuations. Weak fluctuations in foF2 to a substantial degree are related to ionospheric variability at times less of than 1–3 h and determine the δfoF2 variability in the daytime hours. Strong fluctuations in foF2 are mainly related to day-to-day variability of the ionosphere at a fixed local time, the variability increasing by approximately a factor of 3 during the transition from day to night and determining the δfoF2 variability in the nighttime hours. The distribution density of ΔhmF2 is close to the normal distribution law. An interpretation of the different character of the distribution densities of δfoF2 and ΔhmF2 is given.     

Ionospheric disturbances in the years of solar activity minimum and their influence on HF radiowave propagation

The oblique sounding data at the Magadan-Irkutsk and Norilsk-Irkutsk paths together with the vertical sounding at stations located in northeastern Russia were used to analyze ionospheric disturbances in September 2005 and during geophysically active period in December 2006. It is found that during the main phase of magnetic storms, wave disturbances with a period of 2–4 h are registered. These disturbances cause variations in the layer maximum height up to 40–100 km and in the critical frequency up to 1.5–2 MHz. Those variations change substantially values of the maximum observed frequencies (MOF) of the ionospheric radio channel at the paths considered. Such wave disturbances can be caused by generation of AGWs in the auroral zone and their propagation to equatorial latitudes.     

The main ionospheric trough in the East Asian region: Observation and modeling

Research results concerning the main ionospheric trough (MIT) in the afternoon sector are present. Data are used from the meridional chain of stations located in the East Asian region. The analysis of ionospheric storms with different intensities reveals that the depletion in the F2 layer ionization in the afternoon/evening sector can be observed in the subauroral latitudes in the storm recovery phase predominantly during equinoxes and is associated with the formation of the MIT equatorward wall. Model calculations of the evening trough show that its location coincides with the belt of westward drift in the geomagnetic latitudes 55–65° at 13–17 MLT. Hence the simulated results support the assumption that the narrow and deep trough in the afternoon sector is formed by the westward drift with high velocities (～700 m/s). the drift transports the low-density plasma from the night side. The eastward drift with high velocities (～1000–1200 m/s) transports the low-density plasma from the night to morning side forming a trough in the morning sector.     

study of ionospheric response to magnetic superstorms in the East Asian sector

In this paper, we present analyses of the great geomagnetic storms observed during last two cycles of solar activity. This study is based on data from a network of ionosondes located within the longitudinal sector of 80–150°Е. it was found that the superstorms were observed predominantly in equinox. Long-lasting severe decreases of ionization at high and middle latitudes were the most impressive storm effect. A short-time positive phase occurred in response to the onset of both ssc and recovery phases of the magnetic storm in the daytime at high and middle latitudes. Large time-varying rates of foF2 were observed at low latitudes. Modeling results of the ionospheric response to two superstorms are also presented. It is established that the storm effect at middle latitudes was controlled predominantly by disturbed thermospheric composition. At high latitudes, the impact of magnetospheric processes and thermospheric composition on the ionosphere was the same.     

Morphological analysis of the winter nighttime ionosphere at mid-latitudes of the Asian region

We present a study of peculiarities of the winter nighttime maximum in the critical frequencies f ₀ F2 at mid-latitudes of the Asian region. The data of stations located at different longitudes and close latitudes have been used in the analysis: Novosibirsk (54.8°N, 83.2°E), Irkutsk (52.5°N, 104.0°E), and Khabarovsk (48.5°N, 135.1°E). It has been found that the nighttime maximum in f ₀ F2 is observed after midnight (∼0200–0400 LT) and is a stable feature of the quiet ionosphere from the middle of October to the middle of March at low solar activity (SA) at all analyzed stations. This interval decreases with increasing SA. The difference between the maximal and minimal f ₀ F2 values in nighttime hours is the largest in December–January, and its amplitude is almost independent of SA. Variations in the critical frequency of the h _m F2 layer are inversely related to those in the height of the maximum. We have studied periods when the difference between the daytime and nighttime values of f ₀ F2 is less than 2 MHz. The intervals of observations of such events at different longitudes do not coincide. No dependence of the winter nighttime maximum amplitude on magnetic activity has been found.     

Seasonal changes in the amplitude and phase of diurnal and semidiurnal variations in parameters of the midlatitude F2 layer under minimum solar activity conditions according to the data of an ionospheric station in Irkutsk (52.5°N, 104.0°E)

We performed a statistical and spectral analysis of variations in two main parameters of the ionospheric F2 layer: critical frequency (f ₀F2) and peak height (h _m F2), recorded at an ionospheric station in Irkutsk (52.5°N, 104.0°E) in the period from December 1, 2006, to January 31, 2008, under low solar activity conditions. It was found that the f ₀F2 and h _m F2 variations contained quasi-harmonic oscillations with periods T _n = 24/n h (n = 1−7). We studied the seasonal changes in the mean and median values of monthly f ₀F2 and h _m F2 time series, their spectra, as well as the amplitudes and phases of the diurnal (n = 1) and semidiurnal (n = 2) variations. It is shown that the amplitude of the diurnal f ₀F2 variations was maximal in October–March 2007 and minimal in May–August 2007. The diurnal f ₀F2 variations were maximal at noon in the winter months and at 1600 LT in the summer months. The semidiurnal f ₀F2 variations had two maxima: a primary maximum in December and January and a secondary maximum in May–July. The maxima of semidiurnal f ₀F2 variations were shifted from 0000 and 1200 LT in winter to 0900 and 2100 LT in summer.     

Variability of parameters of the F2-layer maximum in the quiet midlatitude ionosphere under low solar activity: 2. Strong fluctuations of critical frequency

This paper presents a qualitative analysis of the properties and particular examples of strong (10% < |δfoF2| < 30%) and very strong (|δfoF2| > 30%) fluctuations in the critical frequency of the F2 layer (foF2) of the quiet ionosphere at midlatitudes under low solar activity according to the Irkutsk station data for 2007–2008. It is found that strong day-to-day fluctuations in foF2 are mainly related to changes in thermospheric parameters, which have a nature of planetary waves and tides. Evidently, very strong day-to-day fluctuations in foF2 are caused by superposition of the effects in the ionosphere caused by changes in the thermospheric parameters and those related to a complex of processes of solar wind interaction with the magnetosphere, including the effects caused by the reversal of the vertical component of the solar wind magnetic field southwards. The increase in foF2 during nighttime hours in winter up to values typical for the daytime maximum in foF2 is the brightest example of very strong changes in foF2 in the quiet ionosphere.     

Geospheric effects of the solar flare of December 13, 2006

Disturbances in the magnetic field and magnetospheric and ionospheric plasma registered on December 14–16, 2006, during a strong magnetic storm caused by a solar flare of 4B/X3.4 class are studied. It is shown that in the north of Yakutia, interactions between the Earth’s magnetosphere and the region of high dynamic pressure of the solar wind led to the formation of sporadic layers in the ionospheric E and F regions, depletion of the critical frequency of the F2 layer, and total absorption. At the end of the magnetic storm’s main phase, anomalously high values of foF2 exceeding the quiet level by a factor of 1.5–1.7 were detected. It was found that the disturbances detected by ground-based observatories had developed on the background of changes in the temperature, density, and the pitch-angle distribution of particles at the geostationary orbit manifesting radial shifts of magnetospheric structures (magnetopause, cusp/cleft, and plasma sheet) relative to the observation points. A conclusion is drawn that in this case, changes in the near-Earth plasma and magnetic field manifest the dynamics of the physical conditions at the magnetospheric boundary and diurnal rotation of the Earth.     

Regular changes in the critical frequency of the <Emphasis Type="Italic">F</Emphasis>2 layer of the quiet midlatitude ionosphere

A method for constructing the empirical model of the F2 layer critical frequency (foF2) under magnetically quiet conditions, aimed at analyzing disturbances of any nature, is proposed. This method has been analyzed, and typical features of regular changes in foF2 of the quiet ionosphere and day-to-day foF2 variability are analyzed using the data from Irkutsk and Slough stations as an example. In particular, it has been obtained that this model differs from the international IRI model, and this difference is mainly caused by the fact that the foF2 values in the IRI model do not correspond to quiet conditions. Therefore, this model gives a larger amplitude of the annual and semiannual variations in foF2 than the IRI model. In addition, this model more accurately reproduces the rate of foF2 annual variations at a fixed local time, especially in equinoxes, when foF2 variations can exceed 1 MHz within one month.     

Electron concentration variations in the F1-region during magnetic storms in a low solar activity period

Vertical sensing data obtained with the help of DPS-4 digisondes in Irkutsk and Yakutsk are used to analyze the response of the F1-layer to magnetic disturbances in May 2005. It is found that the electron concentration is decreased at heights of 150–200 km during magnetic storms, and the amplitude of this negative disturbance increases with height. This effect is more distinct in Yakutsk than Irkutsk. The main gas constituents of the thermosphere during disturbed periods are estimated.