Earthquake responses in the high-latitude ionosphere

The data, obtained using the methods of partial reflections and ionosphere vertical sounding on the Kola Peninsula and in Scandinavia, at Tumannyi (69.0° N, 35.7° E) and Sodankyla (67.37°N, 26.63°E) observatories, have been analyzed in order to detect earthquake responses. The strong earthquakes have been considered: one earthquake with a magnitude of 7.7 occurred at 0819:25 UT on July 17, 2006, on the western coast of Indonesia (9.33° S, 107.26° E), and another earthquake with a magnitude of 6.2 occurred 2253:59 UT on May 26, 2006, on Yava (7.94° S, 110.32° E). These earthquakes, the epicenters of which were located in the same region and at identical depths (10 km), were observed under quiet conditions in the geomagnetic field (ΣK _p = 5.7 and 6.3) and during small solar flares. The response of the ionosphere to these flares was mainly observed in the parameters of the lower ionosphere in the D and E regions. It has been found out that the period of variations in the ordinary component of the partially reflected signal at altitudes of the E region increased before the earthquake that occurred on July 17, 2006. The f _min variations at Sodankyla observatory started 20 h before the earthquake. The periods of these variations were 3–6 h. The same periods were found in the variations in other ionospheric parameters (foEs and h’Es). The variations in the ordinary component of partially reflected signals with periods of 2–5 hours were observed on the day of another earthquake (May 26, 2006). Internal gravity waves with periods of several hours, which can be related to the earthquakes, were detected in the amplitude spectra of the ordinary component of partially reflected signals and in other parameters in the lower ionosphere.     

Earth’s lower ionosphere during partial solar eclipses according to observations near Nizhny Novgorod

The results of observations in the Vasil’sursk Laboratory (56.1° N, 46.1° E) of partial solar eclipses of August 11, 1999, August 1, 2008, and March 20, 2015 are discussed. Ionospheric observations in the eclipse periods and on control days were conducted by the method of resonant scatter of radio waves at artificial periodic irregularities of the ionospheric plasma and the partial reflection method based on radio wave scatter by natural irregularities in the D region. The lower ionosphere reaction to solar eclipses, including variations in the electron concentration and characteristics of the signals scattered by APIs, was studied. An intensification of the lower ionosphere turbulization, an increase in the signal amplitudes backscattered by APIs in the E region, stratification of the D region, and the arrival of scattered signals from mesopause heights were observed during the eclipses. A decrease in the electron concentration of the D region up to a factor of 3–5 was found by the partial reflection method. Above 88 km, the ionospheric response was delayed by 20–25 min relative to the moment of the eclipse maximum phase, whereas this delay in the lower part of the D region was 2–4 min.     

Behavior of the polar lower ionosphere during the solar flares in January 2005 according to the data of the partial reflections method

The results of the observations, using partial reflections, of the lower ionosphere over Tumannyi station in the Murmansk region (69.0° N, 35.7° E) during strong solar flares on January 15–20, 2005, are presented. The structure of the D region of the polar ionosphere and the effects of X-ray flares and fluxes of high energy protons on this region are considered. The anomalous values of electron density in the lower part of the D region, unusually low values of the height of the lower ionospheric boundary, complete or partial absorption of short and medium radiowaves, and bursts of the meter cosmic radio emission were detected during solar disturbances.     

Influence of a tropical cyclone on the upper ionosphere according to tomography sounding data over Sakhalin Island in November 2007

Tomography sounding data for the first half of November 2007 are presented. The sounding was conducted over three points located at the same meridian—Yuzhno-Sakhalinsk (47° N, 143° E), Poronaisk (49° N, 143° E), and Nogliki (51° N, 143° E)—in order to find the possible influence of a tropical cyclone on the upper ionosphere. A change in the foF2 parameter by on average no more than 10–20% is a possible response of the upper ionosphere localized over the tropical cyclone (TC) zone (in the given case, 25°–30° northward and 5°–20° eastward) at a distance of approximately 3800–5500 km from it. A decrease or, vice versa, an increase in foF2 is related to the delay of the measurement moment relative to the beginning of the TC action. The complexity of a morphological analysis of the given event is that a tropical cyclone is a “wideband” (in the longitudinal and, to a lesser degree, in the latitudinal directions) and lasting disturbance source.     

Region of permanent generation of large-scale irregularities in the daytime winter ionosphere of the Southern Hemisphere

With the use of data from topside sounding on board the Interkosmos-19 (IK-19) satellite, the region of permanent generation of large-scale irregularities in the daytime winter ionosphere of the Southern Hemisphere is differentiated. This region is characterized by low values of foF2 and hmF2 and occupies a rather large latitudinal band, from the equatorial anomaly ridge to ~70° S within the longitudinal range from 180° to 360°. Irregularities with a dimension of hundreds kilometers are regularly observed in the period from 0700–0800 to 1800–1900 LT, i.e., mainly in the daytime. In the IK-19 ionograms, they normally appear in the form of an extra trace with a critical frequency higher than that of the main trace reflected from the ionosphere with lower density. The electron density in the irregularity maximum sometimes exceeds the density of the background ionosphere by nearly a factor of 3. A model of the ionosphere with allowance for its irregular structure was created, and it was shown on the basis of trajectory calculations how the IK-19 ionograms related to these irregularities are formed. A possible mechanism of the generation of large-scale irregularities of the ionospheric plasma is discussed.     

Characteristics of low altitude ionospheric electric field over Hainan Island,China

A sounding rocket experiment undertaken by the Chinese Meridian Project from a low latitude station on Hainan Island(19.5°N,109.1°E),China,measured the DC electric field during 05:45-05:52 LT on April 5,2013.The data observed using a set of electric field double probes,as part of the rocket's scientific payload,revealed the special profile of how the vectors of the DC electric field vary with altitude between 130 and 190 km.During the experiment,the vertical electric field was downward,and the maximum vertical electric field was nearly 5.1 mV/m near the altitude of 176 km.The zonal electric field was eastward and slightly less than 0.6 mV/m.The plasma drift velocity was estimated from the E×B motion,and the zonal drift velocity was eastward and of the order of 100 m/s.The zonal wind velocity was also estimated using the drift velocity near the maximum density height in the F1-region,and it was found to be nearly 120 m/s.This work constituted the first in situ measurement of the DC electric field conducted within the Fl-region(between 130 and 190 km) in the East Asian Sector.     

Waveguide radio-wave propagation in the equatorial ionosphere according to the Interkosmos-19 data

Additional strongly remote (up to 2000 km) radio-signal reflection traces on Intercosmos-19 ionograms obtained in the equatorial ionosphere have been considered. These traces, as a rule, begin at frequencies slightly lower than the main trace cutoff frequencies, which indicates that an irregularity with a decreased plasma density exists here. The waveguide stretched along the magnetic-field line is such an inhomogeneity in the equatorial ionosphere. The ray tracing confirm that radio waves propagate in a waveguide and make it possible to determine the typical waveguide parameters: ?δN _e ≥ 10%, with a diameter of 15–20 km. Since the waveguide walls are smooth, an additional trace is always recorded distinctly even in the case in which main traces were completely eroded by strong diffusivity. Only one additional trace (of the radio signal X mode) is usually observed one more multiple trace is rarely recorded. Waveguides can be observed at all altitudes of the equatorial ionosphere at geomagnetic latitudes of ±40°. The formation of waveguides is usually related to the formation of different-scale irregularities in the nighttime equatorial ionosphere, which result in the appearance of other additional traces and spread F.     

Model simulation of the equatorial electrojet in the Peruvian and Philippine sectors

Between 100 and 120 km height at the Earth's magnetic equator, the equatorial electrojet (EEJ) flows as an enhanced eastward current in the daytime E region ionosphere, which can induce a magnetic perturbation on the ground. Calculating the difference between the horizontal components of magnetic perturbation (H) at magnetometers near the equator and about 6–9° away from the equator, ΔH, provides us with information about the strength of the EEJ. The NCAR Thermosphere–Ionosphere–Electrodynamics General Circulation Model (TIE-GCM) is capable of simulating the EEJ current and its magnetic perturbation on the ground. The simulated diurnal, seasonal (March equinox, June solstice, December solstice), and solar activity (F_10.7=80, 140 and 200 units) variations of ΔH in the Peruvian (76°W) and Philippine (121°E) sectors, and the relation of ΔH to the ionospheric vertical drift velocity, are presented in this paper. Results show the diurnal, seasonal and solar activity variations are captured well by the model. Agreements between simulated and observed magnitudes of ΔH and its linear relationship to vertical drift are improved by modifying the standard daytime E region photoionization in the TIE-GCM in order to better simulate observed E region electron densities.     

Ionospheric response to the partial solar eclipse of March 29, 2006, according to the observations at Nizhni Novgorod and Murmansk

The results of observations of the solar eclipse ionospheric effects on March 29, 2006, are presented. The observations were conducted using the partial reflection method near Nizhni Novgorod and the vertical sounding method at the automatic ionospheric station near Murmansk. It has been obtained that the electron density at altitudes of 77 and 91 km decreases by a factor of more than 4; in this case the response of the ionosphere at an altitude of 91 km lags behind the eclipse maximum phase on the Earth by approximately 20 min. It has been established that the eclipse in the E and F1 regions of the polar ionosphere causes a change in the electron density by 15–20%. The delay time of this effect varies from 12 to 24 min depending on the altitude. It has been registered that the reflection virtual altitude at altitudes of the ionospheric F region increases in Murmansk and Nizhni Novgorod.     

Height profiles of the amplitudes of large-scale traveling ionospheric disturbances

Nighttime height profiles of the amplitudes of large-scale traveling ionospheric disturbances (LSTIDs) obtained from the data of vertical sounding in Almaty (76°55′ E, 43°15′ N) for the period 2000–2007 are analyzed. The height profiles are plotted using the time variations in electron density N _h (t) at a series of heights for the F region in the ionosphere with a height step of 10 km. In total, observations were conducted during 1166 nights, among which 581 nights are characterized by wave activity. Nights with the maximum amplitude of variations in N _h (t) exceeding 25% are selected for analysis. The total number of such nights is 63; LSTIDs have been recorded in both magnetically quiet and active periods. The regressive ratios between the height of the F-region maximum and the height that corresponds to the maximum absolute amplitude of a wave, as well as between the values of the maximum amplitude at a height profile and the value of the amplitude of variations in N _m F(t) at the layer maximum, are obtained.     

Detection of the wave-like structures in the F-region electron density: Two station measurements

Comparative studies of short-term ionospheric variability in the F region ionosphere during rapid sequence sounding campaign “HIRAC/SolarMax” (23–29 April 2001) are presented. The ionospheric short-term fluctuations have been studied in detail using measurements from vertical sounding at Ebro (40.8 °N, 0.5 ° E) and Průhonice (49.9 °N, 14.5 °E) in the period range from 15 minutes to 2 hours. The electron density measurements contain variations that indicate the possible presence of propagating gravity waves. Regular wave-like bursts were found during quiet days at both stations in electron concentration in F region, with an increase of the oscillation activity after sunrise and then during late afternoon, and at sunset and after sunset. Solar Terminator is assumed to be one of the sources of the regular wave bursts detected in the ionosphere during campaign HIRAC. As expected, substantial intensification in longer period gravity waves was found to occur during the disturbed period on April 28. Particular enhancement of the wave-like activity during disturbed day is discussed, being significant evidences of a change of the wave-like activity pattern at a height around 200 km.     

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.     

Scattered reflections and multiple traces in the range of 7–10 MHz on ionograms of the interkosmos-19 satellite

The scattered reflections and multiple traces regularly recorded on the topside sounding ionograms of the Interkosmos-19 satellite in the frequency range of 7–10 MHz are considered. The reflected radio signals in this frequency range appear both above and below the critical frequency of the regular layer F2. They are observed at all altitudes of the topside ionosphere from hmF2 to a satellite altitude of 1000 km. It is shown that these phenomena regularly appear at high latitudes (≥60° ILAT) and, less often, in the equatorial region. The scattered reflections indicate the presence of small-scale irregularities, and continuous traces are a consequence of total internal reflection from large-scale irregularities. Small-scale irregularities evidently form within a large-scale irregularity. Ray tracing shows that the size of large-scale irregularities is hundreds of kilometers in height and tens of kilometers in latitude. The appearance of scattered reflections and multiple traces at high latitudes is nearly independent of local time; in the equatorial region, they appear only in the interval of 20–08 LT. All of this agrees well with other observations of irregularities in the ionospheric plasma of different scales.     

Correction of Electron Density Profiles in the Low Ionosphere Based on the Data of Vertical Sounding with the IRI Model

The method of correcting the daytime vertical profiles of electron plasma frequency in the low ionosphere from International Refererence Ionosphere (IRI) model in accordance with the measured data of the virtual heights and absorption of signal radiowaves (method А1) reflected from the bottom of E-region at vertical sounding (VS) is presented. The method is based on the replacement of the IRI model profile by an approximation of analytical dependence with parameters determined according to VS data and partially by the IRI model. The method is tested by the results of four joint ground-based and rocket experiments carried out in the 1970s at midlatitudes of the European part of Russia upon the launches of high-altitude geophysical rockets of the Vertical series. It is shown that the consideration of both virtual reflection heigths and absorption makes it possible to obtain electron density distributions that show the best agreement with the rocket measurements made at most height ranges in the D- and E-regions. In additional, the obtained distributions account more adequately than the IRI model for the contributions of D- and E-regions to absorption of signals reflected above these regions.     

S-wave velocity structure in Tangshan earthquake region and its adjacent areas from joint inversion of receiver functions and surface wave dispersion*

Using the seismic records of 83 temporary and 17 permanent broadband seismic stations deployed in Tangshan earthquake region and its adjacent areas (39°N–41.5°N, 115.5°E–119.5°E), we conducted a nonlinear joint inversion of receiver functions and surface wave dispersion. We obtained some detailed information about the Tangshan earthquake region and its adjacent areas, including sedimentary thickness, Moho depth, and crustal and upper mantle S-wave velocity. Meanwhile, we also obtained the v_P/v_S structure along two sections across the Tangshan region. The results show that: (1) the Moho depth ranges from 30 km to 38 km, and it becomes shallower from Yanshan uplift area to North China basin; (2) the thickness of sedimentary layer ranges from 0 km to 3 km, and it thickens from Yanshan uplift region to North China basin; (3) the S-wave velocity structure shows that the velocity distribution of the upper crust has obvious correlation with the surface geological structure, while the velocity characteristics of the middle and lower crust are opposite to that of the upper crust. Compared with the upper crust, the heterogeneity of the middle and lower crust is more obvious; (4) the discontinuity of Moho on the two sides of Tangshan fault suggests that Tangshan fault cut the whole crust, and the low v_S and high v_P/v_S beneath the Tangshan earthquake region may reflect the invasion of mantle thermal material through Tangshan fault.     

Latitudinal dependence of the seasonal variation of particulate extinction in the UTLS over the Indian longitude sector during volcanically quiescent period based on lidar and SAGE-II observations

The altitude profiles of particulate extinction in the upper troposphere and lower stratosphere (UTLS) obtained from SAGE-II in the latitude region 0–30°N over the Indian longitude sector (70–90°E) are used to study the latitudinal variation of its annual pattern in this region during the volcanically quiescent period of 1998–2003. The SAGE-II data is compared with the lidar measurements from Gadanki (13.5°N, 79.2°E) when the satellite had an overhead occultation pass over a small geographical grid centered at this location. The particulate optical depth (τ_p) in the UT region shows a general decrease with increase in latitude and a pronounced summer–winter contrast with relatively low values during winter and high values during summer. In general, these variations are in accordance with the latitudinal variation of convective available potential energy (CAPE) and thunderstorm activity, which are good representative indices of tropospheric convection. While the particulate extinction (and τ_p) in the 18–21 km (LS₁) region is relatively low in the equatorial region up to 15°N, it shows an increase in the off-equatorial region, beyond 15°N. While the annual variation of τ_p in the LS₁ region is almost insignificant near the equator, it is rather well pronounced in latitude region between 10 and 15°N with relatively high values during winter and low values during summer. Beyond 20°N, this shows a prominent peak during summer. At a higher altitude, the 21–30 km (LS₂) region, the latitude variation of τ_p shows a different pattern with high values near the equator and low values in the off-equatorial region confirming the existence of a stratospheric aerosol reservoir. Low values of τ_p at lower regime (LS₁) near the equator could be due to rapid transport of particulates from the near equatorial region to higher latitudes, while the equatorial high at upper regime (LS₂) could be due to lofting and subsequent accumulation.     

Upper mantle structure beneath central Western Europe from data on phase velocities of surface waves

The deep structure of the upper mantle is determined from data on phase velocities of Love and Rayleigh waves measured by a differential method on traces between two stations in central Western Europe. One-dimensional velocity structures are first constructed from data of each pair of stations, after which two-dimensional distributions of SH and SV velocities are calculated by the method of two-dimensional tomography from S wave velocities at fixed depths. The results are presented in the form of 2-D vertical structures of the average S wave velocity (S = (SV + SH)/2) constructed along profiles crossing the region in directions of the best resolution. The main structural features are a higher velocity zone at depths of 60–80 km in the area (48°–50°N, 9°–11°E) and a lower velocity zone in the western part of the region at depths of 100–150 km, probably extending farther beyond the studied area.     

Disturbances in the <Emphasis Type="Italic">F</Emphasis>2 region critical frequency before the earthquake of September 11, 2008 off the coast of Hokkaido,Japan, and during a moderate magnetic storm based on data from ground-based vertical ionosphere sounding stations

Based on data from ground-based vertical sounding stations, the behaviors of the ionosphere F region before a strong M 6.8 earthquake off the coast of Hokkaido, Japan, and during the moderate magnetic storm before this earthquake are compared. It was found that the critical frequency of the ionosphere F region (foF2) above the Wakkanai ground-based ionosphere vertical sounding station, which was located in the preparation zone of this earthquake, suffered a long-term disturbance of slightly more than an hour nearly half a day before the earthquake. The magnitude of earthquake-induced disturbance is comparable to that caused by a magnetic storm.     

An overview and synthesis of plasma irregularities in equatorial spread F

A unified picture of plasma irregularities in equatorial spread F is developed from the analysis of satellite, sounding rocket, and coherent scatter radar observations. The coherent scatter data are analyzed using a new in-beam radar imaging technique that permits direct comparison between radar data, in situ data, and computer simulations of the irregularities. Three varieties of irregularities, all produced by ionospheric interchange instabilities, are found to occur. Thin bottom-type layers are composed of waves with primary transverse wavelengths less than about 1 km and with significant parallel wavenumbers. These exist on magnetic flux tubes controlled by the E region dynamo and drift westward in the postsunset ionosphere. A nonlocal analysis is used to calculate their linear growth rate. When the F region dynamo takes control of the flux tube, bottomside irregularities can emerge. These are more robust irregularities with longer primary wavelengths and which exhibit greater vertical development. Nonlinear analyses explain the appearance of steepened structures in rocket observations and solitary waves in satellite observations of bottomside layers. The one-dimensional spectra of these irregularities obey power laws but are anisotropic and have variable spectral indices and spectral breaks. Very strong polarization electric fields can eject large regions of deeply depleted plasma through the F peak and form topside irregularities. Theoretical calculations supported by satellite data show that ion inertia may become important for topside irregularities. The one-dimensional spectra of irregularities in the inertial regime obey a k^−5/3 power law, but strong plasma inhomogeneity implies that Kolmogorov weak turbulence is not the explanation. Topside depletions are shown to bifurcate and also to pinch off from the bottomside.     

Response of the midlatitude ionosphere to extreme geomagnetic storms of the 23rd solar cycle

The ionospheric response in the Irkutsk region (52.3° N, 104.3° E) to the extreme geomagnetic storms of solar cycle 23 was studied based on the data of the Irkutsk incoherent scatter radar (ISR) and DPS-4 vertical sounding digital ionosonde. The deviations of parameters from the undisturbed level, i.e., from the monthly medians or the values obtained on a quiet day, were considered as an ionospheric response. Values of the electron concentration maximum (N _mF2) and electron temperature (T _e) at a height of 350 km were chosen as parameters. The ionospheric response is interpreted in the scope of the concept of a thermospheric storm and penetration of the magnetospheric electric field.