Revealing the classes of ionospheric disturbances on the basis of multiyear data on the critical frequency of the <Emphasis Type="Italic">F</Emphasis>2 layer

The bases of the classification method of ionospheric disturbances caused by solar-geomagnetic activity on the basis of the critical frequency of the F2 layer are developed. Data for the total solar activity cycle from 1975 to 1986 were used for studying variations in the critical frequency of the ionospheric F2 layer. The critical frequency was measured at the Moscow ionospheric observatory (55°45′N, 37°37′E) at an interval of 1 h. The gaps in the critical frequency values were filled in by the cubic interpolation method. The solar activity level was estimated using the F10.7 index. The geomagnetic disturbance was determined using the Kp · 10, Dst, and AE indices. According to the developed classification, an index of ionospheric activity is introduced. An analysis of the obtained values of the index for years of solar activity minimum and maximum shows that an increase in the absolute values of the index as a rule occurs at an increase in global geomagnetic and/or auroral disturbances. This fact indicates the sufficient information content of the developed index for characterizing ionospheric activity in any season. Moreover, using the sign of the index, one can form an opinion regarding an increase or decrease in the concentration of the ionospheric F2 layer, because the values of the considered index correspond to real oscillations in the critical frequency of the midlatitude ionosphere.     

Geomagnetic Storm Effects at <Emphasis Type="Italic">F</Emphasis>1 Layer Altitudes in Various Periods of Solar Activity (Irkutsk Station)

The influence of geomagnetic disturbances on electron density Ne at F1 layer altitudes in different conditions of solar activity during the autumnal and vernal seasons of 2003–2015, according to the data from the Irkutsk digital ionospheric station (52° N, 104° Е) is examined. Variations of Ne at heights of 150–190 km during the periods of twenty medium-scale and strong geomagnetic storms have been analyzed. At these specified heights, a vernal–autumn asymmetry of geomagnetic storm effects is discovered in all periods of solar activity of 2003–2015: a considerable Ne decrease at a height of 190 km and a weaker effect at lower levels during the autumnal storms. During vernal storms, no significant Ne decrease as compared with quiet conditions was registered over the entire analyzed interval of 150?190 km.     

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.     

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.     

Relation of long-period variations in the critical frequencies of the <Emphasis Type="Italic">F</Emphasis><Subscript>2</Subscript> layer to geomagnetic activity

The relation of the long-period variations in the midnight and noon values of the critical frequency of the ionospheric F ₂ layer at three midlatitude stations (Irkutsk, Moscow, and Boulder) to the daily mean index of geomagnetic activity in years of different solar activity has been studied. It has been found that the correlation coefficients between the above parameters depend on time of day, season, and solar activity level. The correlation coefficients are higher at night than in the daytime, especially at low solar activity. The highest absolute values of the correlation coefficient most often appear during equinoxes: April–May and September–October. It has been shown that the variability of the critical frequencies of the midlatitude ionospheric F ₂ layer depends not only on geomagnetic activity but also (to a considerable degree) on the effect of the lower atmosphere.     

Dynamics of the midlatitude ionospheric <Emphasis Type="Italic">F</Emphasis> region at sunrise

The behavior of the F2 layer at sunrise has been studied based on vertical-incidence ionospheric sounding data in Almaty (76°55′E, 43°15′N). Records with small amplitudes of electron density background fluctuations were selected in order to exactly estimate the onsets of a pronounced increase in the electron density at different altitudes. It has been indicated that the electron density growth rate is a function of altitude; in this case, the growth rate at the F2 layer maximum is much lower than such values at fixed altitudes of ～30–55 km below the layer maximum. The solar zenith angle (χ) and the blanketing layer thickness (h ₀) at the beginning of a pronounced increase in the electron density at altitude h are linearly related to the h value, and these quantities vary within ～90° < χ < 100° and 180 km < h ₀ < 260 km, respectively.     

Comparison of <Emphasis Type="Italic">foF2</Emphasis> values in the daytime and after sunset

The ratio of the values of the F ₂, layer critical frequencies 2 h after the sunset and at 1400 LT has been considered. It is assumed that this ratio is a good indicator of the intensity of wind processes in the thermosphere. For 18 ionospheric stations, the dependences of this ratio on the F _wl, solar activity index have been constructed for the period before 1980. It has been found that the standard deviation (S), from this dependence has been increased after 1980 at all stations. An increase in S, is interpreted as an indication of the existence of long-term trends in th e wind regime after approximately 1980.     

Dependence of geomagnetic disturbances on extreme values of the solar wind <Emphasis Type="Italic">E</Emphasis><Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> component

The dependence of the zonal geomagnetic indices (AE, Ap, Kp, Kn, and Dst) on the solar wind parameters (the electric field E _y component, dynamic pressure P _d and IMF irregularity σB) has been studied for two types of events: magnetic clouds and high-speed streams. Based on the empirical relationships, it has been established that the AE, Ap, Kp, and Kn indices are directly proportional to the E _y value at E _y < 12 mV m^?1 and are inversely proportional to this value at E _y > 12 mV m^?1 for the first-type events. On the contrary, the dependence of Dst on E _y is monotonous nonlinear. A linear dependence of all geomagnetic indices on E _y is typical of the second-type events. It has been indicated that the specific features of geoeffectiveness of magnetic clouds and high-speed solar wind streams are caused by the dependence of the electric field potential across the polar cap on the electric field, solar wind dynamic pressure, and IMF fluctuations.     

Global empirical model of critical frequency of the ionospheric <Emphasis Type="Italic">F</Emphasis>2-layer for quiet geomagnetic conditions

Using the foF2 database obtained from satellites and ground-based ionospheric stations, we have constructed a global empirical model of the critical frequency of the ionospheric F2-layer (SDMF2—Satellite and Digisonde Data Model of the F2 layer) for quiet geomagnetic conditions (Kp < 3). The input parameters of this model are the geographical coordinates, UT, day, month, year, and the integral index F10.7 (day, τ = 0.96) of solar activity for a given day. The SDMF2 model was based on the Legendre method for the spatial expansion of foF2 monthly medians to 12 in latitude and 8 in longitude of spherical harmonics. The resulting spatial coefficients have been expanded by the Fourier method in three spherical harmonics with respect to UT. The effect of the saturation of critical frequency of the ionospheric F2-layer at high solar activity was described in the SDMF2 model by foF2 as a logarithmic function of F10.7 (day, τ = 0.96). The difference between the SDMF2 and IRI models is a maximum at low solar activity as well as in the Southern Hemisphere and in the oceans. The testing on the basis of ground-based and satellite data has indicated that the SDMF2 model is more accurate than the IRI model.     

Statistical study of anomalous nighttime maximums in the <Emphasis Type="Italic">NmF</Emphasis> 2 diurnal variations in the region of appearance of the equatorial anomaly northern crest

The occurrence probabilities of the first and second anomalous nighttime local maximums in the diurnal variations in the electron density at a maximum of the ionospheric F ₂ layer (NmF2) in the region where the crest (hump) of the equatorial anomaly originates in the northern geographic hemisphere have been studied using the data of the stations for vertical sounding of the ionosphere (Paramaribo, Dakar, Quagadougou, Ahmedabad, Delhi, Calcutta, Chongoing, Guangzhou, Taipei, Chung-Li, Okinawa, Yamagawa, Panama, and Bogota) from 1957 to 2004. It has been demonstrated that the anomalous nighttime NmF2 maximums are least frequently formed at ～53° geomagnetic longitude. The calculations have indicated that the studied probabilities are independent of solar activity. Geomagnetic activity weakly affects the rate of occurrence of the first nighttime NmF2 maximum at geomagnetic longitudes of approximately 140° to 358°. At geomagnetic longitudes of approximately 16° to 70° (i.e., in the longitudinal zone of a decreased occurrence frequency of anomalous nighttime maximums), the occurrence probability of the first anomalous nighttime NmF2 maximum under geomagnetically quiet conditions is pronouncedly lower than under geomagnetically disturbed conditions. The dependence of the occurrence probabilities of the first and second anomalous nighttime NmF2 maximums on the month number in a year has been studied.     

Dayside auroras during unusually large positive values of the IMF <Emphasis Type="Italic">B</Emphasis><Subscript><Emphasis Type="Italic">z</Emphasis></Subscript> component

The characteristics of dayside auroras during the large (16–24 nT) positive values of the IMF B _z component, observed on January 14, 1988, during the interaction between the Earth’s magnetosphere and the body of the interplanetary magnetic cloud, have been studied based on the optical observations on Heiss Island. A wide band of diffuse red luminosity with an intensity of 1–2 kilorayleigh (kR) was observed during 6 h in the interval 1030–1630 MLT at latitudes higher than 75° CGL. Rayed auroral arcs, the brightness of which in the 557.7 nm emission sharply increased to 3–7 kR in the postnoon sector immediately after the polarity reversal of the IMF B _y component from positive to negative, were continuously registered within the band. Bright auroral arcs were observed at the equatorward edge of red luminosity. It has been found out that the red auroral intensity increases and the band equatorward boundary shifts to lower latitudes with increasing solar wind dynamic pressure. However, a direct proportional dependence of the variations in the auroral features on the dynamic pressure variations has not been found. It has been concluded that the source of bright discrete auroras is located in the region of the low-latitude boundary layer (LLBL) on closed geomagnetic field lines. The estimated LLBL thickness is ∼3 R _e . It has been concluded that the intensity of the dayside red band depends on the solar wind plasma density, whereas the position of the position equatorward boundary depends on the dynamic pressure value and its variations.     

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.     

Studying the spread <Emphasis Type="Italic">F</Emphasis> effect before earthquakes

The observations of spread F during the nighttime hours (0000–0500 LT) have been statistically analyzed based on data of Tokyo, Akita, Wakkanai, and Yamagawa Japan vertical ionospheric sounding stations for the time intervals a month before and a month after an earthquake. The disturbances in the probability of spread F appearance before an earthquake are revealed against a background of the variations depending on season, solar activity cycle, geomagnetic and solar disturbances. The days with increased solar (Wolf number W > 100) and geomagnetic (ΣK > 30) activity are excluded from the analysis. The spread F effects are considered for more than a hundred earthquakes with magnitude M > 5 and epicenter depth h < 80 km at distances of R < 1000 km from epicenters to the vertical sounding station. An average decrease in the spread F occurrence probability one-two weeks before an earthquake has been revealed using the superposed epoch method (the probability was minimal approximately ten days before the event and then increased until the earthquake onset). Similar results are obtained for all four stations. The reliability of the effect has been estimated. The dependence of the detected effect on the magnitude and distance has been studied.     

Variations in statistical parameters of the <Emphasis Type="Italic">NmF</Emphasis>2 winter anomaly with latitude and solar activity

The maximal R ratios of the winter-to-summer NmF2 values of each ionosonde are calculated for a specified UT under daytime quiet geomagnetic conditions and at approximately equal levels of solar activity, based on foF2 measurement data of 98 ionosondes at mid- and low geomagnetic latitudes of the Northern and Southern hemispheres for 1957–2009. The P(R > 1) conditional probability of NmF2 winter anomaly observations, as well as the most probable RMP and average <R> of R values are calculated for low, moderate, and high solar activity on the base of foF2 measurements during the periods December 22 ± 30 days and June 21 ± 30 days. Variations in P(R > 1), RMP, and 〈R〉 with latitude and solar activity are analyzed.     

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.     

Vertical structure of the midlatitude winter <Emphasis Type="Italic">F</Emphasis> region of the ionosphere during postmidnight enhancements in <Emphasis Type="Italic">NmF</Emphasis>2

The behavior of the vertical structure of the ionospheric F2 layer, including the variations in the heights of the maximum and bottom of the layer, its half-thickness, and electron content at some fixed heights during postmidnight enhancements in the electron density at the F2 layer maximum (NmF2), has been studied based on the data of the ionospheric vertical sounding, conducted in Alma-Ata (76°55′ E, 43°15′ N) in 2005–2006. The analysis of the amplitude and phase relationships between the measured parameters of the layer made it possible to qualitatively complete the existing concepts of the mechanisms by which the discussed effect is maintained. It is shown that the accelerated decrease in the electron density of the layer within a short time interval preceding the beginning of the postmidnight increase in NmF2 is governed not only by recombination processes but also by the plasma redistribution over the increasing thickness of the layer. The regularly observed effect of the delay in the moment of reversal in the motion direction of the layer bottom relative to the corresponding moment for the layer maximum made it possible to conclude that the meridional wind asynchronously reverses its direction from the poleward daytime to the equatorward nighttime in the entire layer: the direction changes later with decreasing height.     

Estimating the source parameters for the M<Subscript><Emphasis Type="Italic">W</Emphasis></Subscript> 7.6 April 20, 2006 Olyutorskii earthquake from long period <Emphasis Type="Italic">P</Emphasis>-wave records made by the worldwide network

The source parameters of the M _W = 7.6 Olyutorskii earthquake were estimated using the moments of the slip rate function with degrees 1 and 2. The moments were estimated from broadband P-wave records at 52 stations of the worldwide network. The first step was to find a function S(t) for each station; this function is an apparent source time function, i.e., the P-wave slip as radiated by the source toward a station under consideration. The method of empirical Green’s functions was used to estimate S(t). The next step was to calculate the moments of S(t) of degrees 1 and 2 over time and to set up relevant equations to be solved by least squares for the unknown source moments. The horizontal linear source was used as a nonparametric model for calculating the source moments. Haskell’s parametric model was used for further interpretation of the source moments. The resulting estimates are as follows: the source centroid was 13–25 km southwest of the epicenter, the source was 105–120 km long, the source strike was 222°–228°, the rupture velocity was 2.7–3.0 km/s, and the total radiation duration was 24–27 s. These estimates indicate a bilateral rupture dominated by a southwestward sense of rupture propagation. The source characteristics are consistent with the aftershock area geometry and with the focal mechanism, as well as with surface breakage as observed by geologists in the field.     

The instability of the <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> and <Emphasis Type="Italic">M</Emphasis><Subscript>L</Subscript> comparison for earthquakes in Greece for the period 1969 to 2007

We use 576 earthquakes of magnitude, M _w, 3.3 to 6.8 that occurred within the region 33° N–42.5° N, 19° E–30° E in the time period 1969 to 2007 to investigate the stability of the relation between moment magnitude, M _w, and local magnitude, M _L, for earthquakes in Greece and the surrounding regions. We compare M _w to M _L as reported in the monthly bulletins of the National Observatory of Athens (NOA) and to M _L as reported in the bulletins of the Seismological Station of the Aristotle University of Thessaloniki. All earthquakes have been analyzed through regional or teleseismic waveform inversion, to obtain M _w, and have measured maximum trace amplitudes on the Wood–Anderson seismograph in Athens, which has been in operation since 1964. We show that the Athens Wood–Anderson seismograph performance has changed through time, affecting the computed by NOA M _L by at least 0.1 magnitude units. Specifically, since the beginning of 1996, its east–west component has been recording systematically much larger amplitudes compared to the north–south component. From the comparison between M _w and M _L reported by Thessaloniki, we also show that the performance of the sensors has changed several times through time, affecting the calculated M _L’s. We propose scaling relations to convert the M _L values reported from the two centers to M _w. The procedures followed here can be applied to other regions as well to examine the stability of magnitude calculations through time.     

Seismicity anomalies before the great earthquake of <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>=8.1 in the Kunlun Pass and its significance to earthquake prediction

A great earthquake of M _S=8.1 took place in the west of Kunlun Pass on November 14, 2001. The epicenter is located at 36.2°N and 90.9°E. The analysis shows that some main precursory seismic patterns appear before the great earthquake, e.g., seismic gap, seismic band, increased activity, seismicity quiet and swarm activity. The evolution of the seismic patterns before the earthquake of M _S=8.1 exhibits a course very similar to that found for earthquake cases with M _S≥7. The difference is that anomalous seismicity before the earthquake of M _S=8.1 involves in the larger area coverage and higher seismic magnitude. This provides an evidence for recognizing precursor and forecasting of very large earthquake. Finally, we review the rough prediction of the great earthquake and discuss some problems related to the prediction of great earthquakes.     

Causes of <Emphasis Type="Italic">NmF</Emphasis>2 longitudinal variations at mid- and subauroral latitudes under summer nighttime conditions

The main factors controlling NmF2 longitudinal variations at mid- and subauroral latitudes have been studied. The data of the Intercosmos-19 topside sounding, obtained at high solar activity for summer nighttime conditions, have been used in the analysis. The contributions of the solar ionization, neutral wind, and temperature and composition of the thermosphere to NmF2 longitudinal variations have been estimated based on ionospheric models. It has been indicated that NmF2 variations in the unsunlit midlatitude ionosphere mainly depends on the residual electron density and its decay under the action of recombination. At subauroral latitudes under summer nighttime conditions, the ionosphere is partially sunlit, and ionization by solar radiation mainly contributes to NmF2 longitudinal variations, whereas the effect of the neutral wind is slightly less significant. These results also indicate how the contribution of different factors to NmF2 longitudinal variations changes at different latitudes.