Longitudinal variations in the daytime low-latitude ionosphere according to the data of topside sounding from the intercosmos-19 satellite

Using the data of the topside ionosphere sounding from the Intercosmos-19 satellite, longitudinal variations in foF2 at low latitudes at the daytime hours are considered. It is obtained that these variations in particular days in the majority of cases have a regular wave-like character with periods of about 75°–100° in longitude and amplitudes on the average of 2–4 MHz. In other words, along the valley and crests of the equatorial anomaly, a structure with four maximums and four minimums which have a tendency to be located near certain longitudes (the same in all seasons) is observed. The variations in foF2 along the crests of the equatorial anomaly are usually in anti-phase to variations along its valley. Comparing the characteristics of this wavelike structure at the daytime and nighttime hours, we obtained that the average positions of its extremes at the nighttime hours are shifted eastwards by 10°–50° relative to the daytime extremes. As a cause of formation of such a structure, high harmonics of atmospheric tides are assumed which, uplifting from below to heights of the E region, via the electric currents in this region influence the longitudinal structure of the electrodynamic plasma drift over the equator and by that impact the structure of the entire daytime low-latitude ionosphere.     

Detection of a large-scale irregularity in the region of the main ionospheric trough according to the data of topside sounding on board the Intercosmos-19 satellite

Complicated ionograms of topside sounding on board the Intercosmos-19 satellite, which were registered on November 26, 1980, in the dusk sector (1800 LT) at the latitudes of the equatorward wall (55°–62° ILAT) of the main ionospheric trough (MIT), are analyzed. They are characterized by the presence of two extra traces at distances larger than the main traces. Approaching the MIT minimum, all traces become more scattered, converge, and join into one strongly diffusive trace. An attempt of interpretation of the complicated ionograms on the basis of trajectory calculations performed by the method of characteristics in the “complex” two-dimensional version (in two mutually intersecting planes) is undertaken. The modeling shows that the extra traces could be related to the presence of a large-scale irregularity stretched along the geomagnetic meridian at the equatorward wall of the MIT. The calculations make it possible to estimate the parameters of the irregularity: the intensity is δfoF2 ∼ 30%, the length is several hundred kilometers, the semi-thickness is 50–60 km, and the height is 350 km. The possible formation causes of the irregularity are discussed. The intensification of the diffuseness of all traces is related to the increase in the intensity of small-scale irregularities, which is usually observed when approaching the MIT minimum.     

Irregularities in unstable plasma of the upper atmosphere according to the data of sounding on board the Intercosmos-19 satellite

The profiles of the plasma density in the topside ionosphere, according to the data of sounding on board the Intercosmos-19 satellite, are presented. It is shown that the large-scale fluctuations of the plasma density can be related to the propagation and attenuation of the atmospheric waves (e.g., acoustic gravity waves) in the dynamo region of the ionosphere. In the topside ionosphere, suprathermal particle fluxes can be formed and the plasma density can be modulated at an attenuation of small-scale electrostatic fluctuations of the plasma electron component in plasma pits. Plasma vortices can be formed when polarization fluxes of charged particles escape from regions of heating. The vortex field imparts stability to the inhomogeneous plasma structure, necessary for experimental detection of this structure.     

Wave emission during a plasma density jump in the auroral zone of the topside ionosphere according to the APEX satellite data

The intensity of the wave emission in the 0.1–10 MHz band measured in the ionosphere (the APEX satellite experiment) has been presented. A jump of the plasma density and an increase in the emission intensity at a plasma frequency have been registered at altitudes of ～1300 km in the topside auroral ionosphere. The emission intensity in the whistler-mode band nonmonotonically increased along the satellite trajectory near the plasma jump wall. It has been indicated that waveguides could be formed near the wall during damping of electrostatic oscillations generated by precipitating electron fluxes. A spatially nonmonotonous separation of waveguides from the plasma inhomogeneity stretched along geomagnetic field lines is possible in this case.     

Plasma inhomogeneities in the topside ionosphere in the region of the geomagnetic equator and wave radiation according to the APEX satellite data

The measurements of the broadband wave radiation in the topside ionosphere in the region of the geomagnetic equator (the APEX satellite experiment) are presented. The region of unstable plasma with increased density was observed in the nightside topside ionosphere. This region could be formed by heating of the ionosphere from below. An asymmetric distribution of the frequency band width and electrostatic radiation intensity relative to the geomagnetic equator was registered. It has been indicated that a substantial effect of the geomagnetic equator on plasma diffusion from the heating region could be related to the generation, propagation, and damping of electrostatic oscillations and large-scale (as compared to the Larmor ion radius) plasma vortices. The anisotropy in the temperature of the plasma electron component can increase in the regions where the transverse electric field of disturbances damps. The intensity of the electromagnetic radiation, caused by the external sources, apparently, of an artificial origin at frequencies higher than the local plasma frequency, decreases to the radiation detection threshold level in the region of increased plasma density.     

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.     

Yearly variations in the low-latitude topside ionosphere

Observations made by the Hinotori satellite have been analysed to determine the yearly variations of the electron density and electron temperature in the low-latitude topside ionosphere. The observations reveal the existence of an equinoctial asymmetry in the topside electron density at low latitudes, i.e. the density is higher at one equinox than at the other. The asymmetry is hemisphere-dependent with the higher electron density occurring at the March equinox in the Northern Hemisphere and at the September equinox in the Southern Hemisphere. The asymmetry becomes stronger with increasing latitude in both hemispheres. The behaviour of the asymmetry has no significant longitudinal and magnetic activity variations. A mechanism for the equinoctial asymmetry has been investigated using CTIP (coupled thermosphere ionosphere plasmasphere model). The model results reproduce the observed equinoctial asymmetry and suggest that the asymmetry is caused by the north-south imbalance of the thermosphere and ionosphere at the equinoxes due to the slow response of the thermosphere arising from the effects of the global thermospheric circulation. The observations also show that the relationship between the electron density and electron temperature is different for daytime and nighttime. During daytime the yearly variation of the electron temperature has negative correlation with the electron density, except at magnetic latitudes lower than 10°. At night, the correlation is positive.     

Pearl diffuse structures on ionograms of Intercosmos-19 associated with irregularities of the low-latitude ionosphere

Unusual complex ionograms obtained by the Intercosmos-19 satellite are considered, in which four diffuse clouds with a characteristic shape are strung like pearls on the main path of the reflected signal. Ray tracing has been used to show that they are associated with 26 layers of irregularities located at altitudes from hmFs2 up to ~900 km. The sizes of the irregularities range from a few kilometers to 100 kilometers, and the intensity of δNe reaches 100%. The heights of irregular layers increase towards the equator, together with a rise of the F2 layer, and are not associated with magnetic field lines. Complex ionograms have been observed on the outer slope and at the top of the crest of the equatorial anomaly. They are probably caused by the processes occurring in the equatorial ionosphere.     

Plasma disturbance sources in the postsunset low-latitude outer ionosphere according to the Intercosmos-Bulgaria-1300 satellite

The satellite low-latitude and midlatitude measurements of the disturbed postsunset plasma density and electron temperature at altitudes of ～900 km have been compared with the data of incoherent scattering and high-altitude rocket launching at the corresponding local time. It has been found that plasma density disturbances are independently caused by the turbulent interaction between atmospheric masses of gas and plasma ascending from heated and not yet cooled ionospheric regions and cooling masses descending from protonospheric altitudes. Plasma regions with an energetically nonequilibrium vertical density distribution of the mixture of heavy ion impurity (O⁺) and major light ions (H⁺) can simultaneously appear, as a result of which the gradient-drift impurity instability is generated. If this instability is sufficiently developed, there appears an anomalous ion drift with the formation of real plasma regions of decreased density. All these phenomena generate different irregularities in a wide range of scales: from several tens or hundreds of meters to several hundreds of kilometers.     

Disturbances in the topside ionosphere during solar flares

The results of studying the ionospheric response to solar flares, obtained from the data of the GPS signal observations and incoherent scatter radars and as a result of the model calculations, are presented. It is shown that, according to the GPS data, a flare can cause a decrease in the electron content at altitudes of the topside ionosphere (h > 300 km). Similar effects of formation of a negative disturbance in the ionospheric F region were also observed during the solar flares of May 21 and 23, 1967, with the Arecibo incoherent scatter radar. The mechanism by which negative disturbances appear in the topside ionosphere during solar flares has been studied in this work based on the theoretical model of the ionosphere-plasmasphere coupling. It has been indicated that the formation of the electron density negative disturbance in the topside ionosphere is caused by an intense removal of O⁺ ions into the overlying plasmasphere under the action of an abrupt increase in the ion production rate and thermal expansion of the ionospheric plasma.     

The earthquake-related disturbances in ionosphere and project of the first China seismo-electromagnetic satellite

Based on the case studies and statistical analysis of earthquake-related ionospheric disturbances mainly from DEMETER satellite, ground-based GPS and ionosounding data, this paper summarizes the statistical characteristics of earthquake-related ionospheric disturbances, including electromagnetic emissions, plasma perturbations and variation of energetic particle flux. According to the main results done by Chinese scientists, fusing with the existed study from global researches, seismo-ionospheric disturbances usually occurred a few days or hours before earthquake occurrence. Paralleling to these case studies, lithosphere-atmosphere-ionosphere (LAI) coupling mechanisms are checked and optimized. A thermo-electric model was proposed to explain the seismo-electromagnetic effects before earthquakes. A propagation model was put forward to explain the electromagnetic waves into the ionosphere. According to the requirement of earthquake prediction research, China seismo-electromagnetic satellite, the first space-based platform of Chinese earthquake stereoscopic observation system, is proposed and planned to launch at about the end of 2014. It focuses on checking the LAI model and distinguishing earthquake-related ionospheric disturbance. The preliminary design for the satellite will adopt CAST-2000 platform with eight payloads onboard. It is believed that the satellite will work together with the ground monitoring network to improve the capability to capture seismo-electromagnetic information, which is beneficial for earthquake monitoring and prediction researches.     

Disturbances of the topside ionosphere caused by typhoons

The measurements on board the Cosmos-1809 satellite of various parameters of the topside ionosphere plasma during more than ten typhoons in various regions are analyzed. It is shown that specific zones of increased pressure of the electron gas, electric field, and intense ion oscillations are formed during the intensification stage. In some cases the “typhoon eye” is formed over the tropical depression zone in the ionosphere, that is, the region with sharply decreased plasma density and pressure is observed a day and more prior to the moment when it happens in the atmosphere.     

Variations of daytime and nighttime electron temperature and heat flux in the upper ionosphere,topside ionosphere and lower plasmasphere for low and high solar activity

A database of the electron temperature (T_e) comprising of most of the available LEO satellite measurements is used for studying the solar activity variations of T_e. The T_e data are grouped for two levels of solar activity (low LSA and high HSA), five altitude ranges between 350 and 2000 km, and day and night. By fitting a theoretical expression to the T_e values we obtain variation of T_e along magnetic field lines and heat flux for LSA and HSA. We have found that T_e increases with increase in solar activity at low and mid-latitudes during nighttime at all altitudes studied. During daytime the T_e response to solar activity depends on latitude, altitude, and season. This analysis shows existence of anti-correlation between T_e and solar activity at mid-latitudes below 700 km during the equinox and winter day hours. Heat fluxes show small latitudinal dependence for daytime but substantial for nighttime.     

Instability of the state of the night-time topside ionosphere

It is common knowledge that the night-time ionosphere at the middle and moderately high latitudes is sustained by the charged particle flux arriving from plasmasphere along magnetic field lines. The downward directed particle velocity in this flux decreases with the decreasing height. It provides a potential source for plasma instability. This should be interpreted in such a way that the initial density perturbations increase with the time, and are enhanced as they propagate in space. In this paper we have carried out investigations of topside ionosphere plasma stability on the basis of solving the dispersion equation for low-frequency waves in a weakly inhomogeneous medium. Also analysis have been made of propagation of the waves within ray approximation. Is shown, that of irregularities, extending down accrue on intensity up to heights of a maximum of layer F2, and are loss in low layers of ionosphere.     

顶部电离层Pc3压缩波的波动特征

基于2014年至2017年SWARM三颗卫星四年的磁场观测数据,我们挑选出Pc3压缩波事件,研究了顶部电离层Pc3压缩波活动强度的磁纬-磁地方时分布、随磁地方时变化的季节特征以及地理经纬度分布,并进一步分析了Pc3压缩波发生频次的分布特征.结果表明,中高纬和低纬的Pc3压缩波可能存在不同的激发机制.中高纬地区的扰动强度随高度衰减,且高纬地区几乎所有地方时都可以观测到Pc3压缩波,可能主要是由入射的阿尔芬波与电离层相互作用产生;而低纬地区扰动强度没有明显的高度依赖性且主要发生日侧,可能主要是上游波以快模波的形式直接传播到电离层所致.同时,该研究结果给出了入射阿尔芬波和电离层相互作用耦合产生Pc3压缩波理论模型的观测证据.     

Response of the topside ionosphere over Arecibo to a moderate geomagnetic storm

We analyze the data obtained using the Arecibo incoherent scatter radar to examine the response of the topside ionosphere to a moderate geomagnetic storm that occurred during the period March 7–11, 2008. During this time period a magnetic storm with a non-monotonic main phase decrease in the Dst index occurred. The recovery phase also exhibited a secondary Dst decrease. During the initial phase of the storm, Te and Ti increased coincident with the arrival of the solar wind. The main phase registered an increase in proton concentration proportional to Ne while temperatures reached the lowest values. Variations in O⁺ concentration were not significant but a reduction in helium fraction was observed. Soon after the peak of the storm, the transition height between the topside ionosphere and the protonosphere, where H⁺ ions dominate composition, was lower than would be expected during quiet conditions and this behavior lasted for approximately 12 h.     

Specific propagation of radiowaves from the Intercosmos-19 satellite in the region of the nighttime equatorial anomaly crest

It has been indicated how a complex ionogram of topside sounding near the outer slope of the winter southern crest of the equatorial anomaly, where a large NmF2, gradient and a deep hmF2, minimum are observed, is formed. The model latitudinal cross-sectio n of the ionosphere, used to perform trajectory calculations, has been constructed based on the corrected Intercosmos-19 data. The ray trajectories have been modeled using the method of characteristics. It has been indicated that a complex Intercosmos-19 ionogram is formed by an oblique reflection from the equatorial anomaly crest slope (the main trace) and by a strongly oblique reflection from the crest bottom as a result of the wave capture by a large-scale inhomogeneity (the additional trace).     

Studying large-scale traveling ionospheric disturbances according to the data of oblique-incidence sounding

The morphological features of wave-like ionospheric disturbances with periods of 1–2 h and the spatial extent exceeding 1000 km are studied. Oblique-incidence sounding data of the ionosphere, obtained in eastern Siberia during several continuous monthly experiments on three radio paths from 2006 to 2010, have been used. Large-scale traveling ionospheric disturbances generated during magnetic storms and large-scale wave-like ionospheric disturbances registered during geomagnetically quiet periods are considered. Small-scale ionospheric structures were also observed against a background of large-scale traveling iono-spheric disturbances considered in this study.