Behavior of <Emphasis Type="Italic">foF</Emphasis>2 and <Emphasis Type="Italic">hmF</Emphasis>2 after sunset

The time behavior of the foF2 and hmF2 values at the time moment T(ss + 2 h) 2 h after sunset is considered. It is assumed that at this moment, the horizontal winds in the thermosphere in the strongest way influence hmF2 and, therefore, foF2. It is found that a fairly well pronounced and statistically significant change (trend) is observed for the foF2(ss + 2)/foF2(14) ratio, the sign of the change being different for different stations and even different seasons at the same station. A similar picture is obtained for the value of hmF2(ss + 2). It is shown that a positive correlation between the trends of these two values is observed. This confirms the initial concept of the paper that the foF2 and hmF2 trends are caused by long-term trends in the thermospheric dynamics.     

Long-term changes in the relation between <Emphasis Type="Italic">foF</Emphasis>2 and <Emphasis Type="Italic">hmF</Emphasis>2

The change in the dependence of the F2-layer critical frequency on its height hmF2 is considered based on two sources of initial data used earlier by the authors. It is found that the slope k of the foF2 dependence on hmF2 systematically decreases from the earlier (“etalon”) period, 1958–1980, to the later periods of 1988–2010, 1998–2010, and 1998–2014. Since the foF2 value depends on the atomic oxygen concentration in the F region much more strongly than hmF2, the found decrease in k confirms the concept of a decrease in the atomic oxygen concentration in the thermosphere with time previously formulated by the authors.     

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.     

Parameters of the ionospheric <Emphasis Type="Italic">F</Emphasis><Subscript>2</Subscript> layer as a source of information on trends in thermospheric dynamics

The problem of determining trends in thermospheric dynamics parameters (horizontal winds) based on analysis of trends in various combinations of ionospheric F ₂-layer parameters is formulated. The previous attempts of the authors in this direction are briefly described. It is shown that all studied parameters lead to the same result: after the “boundary date” (approximately 1980) a systematic change in these parameters (a long-term trend) is observed, this fact manifesting changes in the dynamical regime of the thermosphere because of cooling and contraction of the entire middle and upper atmosphere. The results of a search for trends in the hmF2 height for the moment (T(ss) + 2 h) are described. These trends are found higher than the hmF2 trends obtained earlier by various authors analyzing the hmF2 behavior at fixed moments of local time.     

Variations in <Emphasis Type="Italic">foF</Emphasis>2 at the end of the 1990s and beginning of the 2000s according to median data

Changes in the values of the critical frequency of the F2 layer from 1990 to 2005–2007, according to median data, are considered. Eleven stations, for which the necessary data are available, have been found in international databanks. The conclusion of the previous publication by Danilov (2011) that at the end of the 1990s and beginning of the 2000s a negative trend in foF2 was observed both after sunset and in the daytime is confirmed.     

Behavior of critical frequency <Emphasis Type="Italic">foF2</Emphasis> at various moments of the day: 1. Dependence on solar activity

Variation with solar activity level of the ratios of the critical frequencies of the F2 layer, foF2, is considered for various pairs of local time moments T1 and T2 for two seasons (winter and summer). It is found that, as a rule, the foF2(T1)/foF2(T2) behavior with the solar activity index F10.7 is different in winter and summer. The variations of foF2(T1)/foF2(T2) with F10.7 obtained from experimental data are considered from the viewpoint of the current theory of F2-layer formation. It is shown that the majority of observed characteristics of these variations are explained using the above-indicated theory. However, special cases of foF2(T1)/foF2(T2) behavior with solar activity are found for which there is still no physical explanation.     

Scatter of <Emphasis Type="Italic">hmF</Emphasis>2 values as an indicator of trends in the thermospheric dynamics

The variability degree of the F ₂-layer height, hmF2, from the 1950s–1960s to the 1990s has been analyzed based on the vertical sounding data for a series of midlatitude ionospheric stations. It has been found that the scatter of the hmF2 values (standard deviation) abruptly increases from the earlier decades to the later ones. This increase is more evident in the spring period of the year and is independent of geomagnetic activity. An increase in the scatter of hmF2 apparently indicates systematic changes (trends) in the thermospheric dynamics, the existence of which was suggested in the recent publications of the authors.     

Variations in <Emphasis Type="Italic">foF</Emphasis>2 at the end of the 1990s and the beginning of the 2000s according to the SPIDR system data

Variations in the critical frequency of the F2 layer at 22 ionospheric stations within the period 1990–2010 according to the SPIDR system data are considered. A confirmation of the negative trends in foF2 for seven stations considered by one of the authors earlier on the basis of median data is obtained. It is found that both negative (a decrease in foF2 with time) and positive (growth in foF2 with time) trends of the critical frequency are observed. During the later part of the considered period (after 1997), negative trends dominate. This fact manifests itself, apparently, in an increase in the role of the decrease in the thermospheric neutral temperature in the formation of foF2 trends.     

Variations of the critical frequency <Emphasis Type="Italic">foF</Emphasis>2 at middle latitudes during strong disturbance in the electric field observed in the <Emphasis Type="Italic">F</Emphasis> region over saint-santin

Morphological analysis of variations of the critical frequency foF2 in the midlatitude ionosphere at various sectors of local time is carried out on the basis of data from ground-based stations of vertical sounding of the ionosphere in the period when during use of the incoherent scatter radar at Saint-Santin an anomalously strong increase in the electric field was observed at heights of the ionospheric F region in the period of enhanced geomagnetic activity (4+ < Kp < 6−). The obtained picture of the space-time distribution of disturbances in foF2 makes it possible to assume that they could be caused by penetration to middle latitudes of the large-scale electric field of the magnetospheric convection directed westward in the nighttime and morning hours and eastward in the noon and evening sectors.     

Reaction of the critical frequency of the <Emphasis Type="Italic">F</Emphasis>2 layer to a sharp depletion in atmospheric pressure

Changes in the critical frequencies of the F2 layer at several midlatitude stations of ionospheric vertical sounding during a sharp depletion in atmospheric pressure under quiet solar and geomagnetic conditions are analyzed. It is shown that in such periods, the observed foF2 values differ from the mean values by approximately 10–15% and the deviations from the mean could be both negative (in the daytime hours) and positive (at night). Such variations in foF2 could be referred to the known class of ionospheric disturbances observed under a quiet geomagnetic situation, that is, to the so-called “Q-disturbances.” Analysis of wavelet spectra of foF2 variations shows the presence in the F region of oscillations of various periods (from 0.5 to 10 days). The decrease in the amplitude of daily variations during pressure depletion is found. Presumably, the observed effect is caused by the dynamic impact of waves formed in the lower atmosphere on the ionospheric F2 layer.     

Relationship between the daytime critical frequencies of the <Emphasis Type="Italic">F</Emphasis><Subscript>2</Subscript> layer and thermospheric characteristics

The relationship between the critical frequency of the F ₂ layer and the atmospheric characteristics has been obtained in a general form. It has been shown that this relation makes it possible to sufficiently accurately describe the daytime values of foF2 while comparing them with the observed monthly median values. Such comparisons were performed, first, for the data of measurements in Irkutsk using the DPS-4 digital ionosonde in 2003–2006 and, second, based on the annual variations in the noon foF2 values at 24 stations of the Northern Hemisphere in 1984. The calculations were performed using the MSIS-86 thermospheric model [Hedin, 1987].     

Long-term trends in the ratio of the daytime and nighttime values of <Emphasis Type="Italic">foF</Emphasis>2

The consideration of the relation between the daytime and nighttime values of the critical frequency F2, foF2 of the ionospheric F2 layer, started in the previous publication of the authors, is continued. The main regularities in variations in the correlation coefficient R(foF2) characterizing this relation are confirmed using larger statistical material (more ionospheric stations and longer observational series). Long-term trends in the R(foF2) value are found: at all stations the negative value of R(foF2) increases with time after 1980.     

Variations in the fractal dimension of fluctuations of the ionospheric <Emphasis Type="Italic">F</Emphasis>2 layer critical frequency

The 40-year period of observations of short-term variations (with characteristic times of up to 1–2 days) in the critical frequency of the ionospheric F2 layer (foF2) is analyzed. The continuous (with a step of 1 h) series of fluctuations (F) of the foF2 critical frequency (with eliminated daily variations) has been calculated using the hourly variations in foF2 at Moscow stations. The fractal dimension (FRH) of the fluctuations, characterizing short-term variations in foF2, has been determined and analyzed on a 30-day interval, using the Higuchi method. It has been established that FRH estimates substantially change in time. The 11-year cycle, which is in antiphase with the solar cycle, and the total annual and semiannual variations, similar to the variations observed in the normalized critical frequency of the E region and in the electron density of the D region, are clearly defined in these changes. Thus, the parameters of fast variations in the ionospheric F2 layer are affected by the phase of the 11-year solar cycle and by the position of the Earth in the orbit or seasonal variations in the atmosphere.     

Relationship between variations in the characteristics of the high-latitude ionosphere and magnetic <Emphasis Type="Italic">PC</Emphasis> indices

It is shown in a joint analysis of ionospheric vertical sounding data at the arctic Heiss Island and antarctic Vostok stations and the geomagnetic PC index, which characterizes the geoefficient component of the interplanetary magnetic field, that, during a disturbed geomagnetic period when PC > 2 in years of solar activity (SA) maxima in the winter season, positive phases of ionospheric disturbances are predominantly observed. In the nighttime hours, an increase in the critical foF2 frequencies by a factor of 2–3 can occur. In a disturbed geomagnetic period at the PC > 1.5 level in the summer season, negative phases of ionospheric disturbances are mainly observed. In years of maximum and moderate SA, the decrease in foF2, as compared to their median values, happens at night (∼30%). In years of low SA, the decrease value is much lower. At a substantial decrease in the PC index level, in the region of the geomagnetic pole at the Vostok station, in some cases, a substantial increase in the electron density level in the F region occurs with a delay of 0.5 h. At the same time, a significant correlation (r = −0.57) is observed between variations in the PC index and foF2.     

Mechanisms of stratification of the <Emphasis Type="Italic">F</Emphasis>2 layer and formation of the <Emphasis Type="Italic">F</Emphasis>3 and <Emphasis Type="Italic">G</Emphasis> layers in the equatorial ionosphere

The paper is dedicated to the studies of formation mechanisms of additional layers in the equatorial ionosphere carried out using numerical simulations with use of the Global Self-Consistent Model of the Thermosphere, Ionosphere, and Protonosphere (GSM TIP) modified in the part of the solution of the electric field equation in the Earth’s ionosphere. Calculations were preformed for quiet geomagnetic conditions using the MSIS-90 model for the calculation of thermospheric parameters. The obtained spatio-temporal pattern of thermospheric circulation and the variations in the dynamo electric field obtained on its basis make it possible to reproduce the stratification effect of the F2 layer and the appearance of the F3 layer in the equatorial ionosphere due to the action of the nonuniform in height zonal electric field at the geomagnetic equator. On the basis of the earlier presented results of calculations using the modified GSM TIP model, the appearance of a maximum in the vertical profile of the electron density at a height of ∼1000 km formed by H+ ions, which we called the G layer, has been predicted. Numerical simulations showed that this layer is formed by the meridional component of the thermospheric wind and is related to the formation of the nighttime midlatitude maximum at heights of the ionospheric F region.     

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.     

Increase in the critical frequency of the ionospheric <Emphasis Type="Italic">F</Emphasis> region prior to the substorm expansion phase

Specific variations in the critical frequency of the ionospheric F ₂ layer during magnetospheric substorms have been found based on the data of vertical sounding stations in Europe and North America. Maximal attention has been paid to the positive peaks of ΔfoF2 with a duration of 6–8 h before the beginning of the substorm expansion phase (T ₀). The possible physical mechanisms by which these peaks are formed (related to the impact of fast particles in the foreshock region of the solar wind on the Earth’s magnetosphere and different for middle and high latitudes) have been considered. The positive peaks of ΔfoF2 can be used in a short-term prediction of the ionospheric disturbance onset and space weather on the whole.     

Critical frequencies <Emphasis Type="Italic">foF</Emphasis>2 as an indicator of trends in thermospheric dynamics

The time variations in three parameters during the last decades are considered. R(foF2) is the correlation coefficient between the nighttime and daytime values of foF2 for the same day. Stable trends are found for the minimum (R(foF2)(max)) and maximum (R(foF2)(min)) values of R(foF2) during a year. The foF2(night)/foF2(day) ratio demonstrates both, negative and positive trends, and the trend sign depends on the inclination I and declination D of the magnetic field. The correlation coefficient r(h, fo) between foF2 and the 100 hP level in the stratosphere demonstrates a decrease (in the years of maximum and minimum solar activity) from the 1980s to the 1990s. The trends in all three groups of data are considered under the assumption of long-term changes in the circulation in the upper atmosphere.