Incoherent scatter radar observations of AGW/TID events generated by the moving solar terminator

Observations of traveling ionospheric disturbances (TIDs) associated with atmospheric gravity waves (AGWs) generated by the moving solar terminator have been made with the Millstone Hill incoherent scatter radar. Three experiments near 1995 fall equinox measured the AGW/TID velocity and direction of motion. Spectral and cross-correlation analysis of the ionospheric density observations indicates that ST-generated AGWs/TIDs were observed during each experiment, with the more-pronounced effect occurring at sunrise. The strongest oscillations in the ionospheric parameters have periods of 1.5 to 2 hours. The group and phase velocities have been determined and show that the disturbances propagate in the horizontal plane perpendicular to the terminator with the group velocity of 300–400 m s^–1 that corresponds to the ST speed at ionospheric heights. The high horizontal group velocity seems to contradict the accepted theory of AGW/TID propagation and indicates a need for additional investigation.     

Ranges of AGW propagation in the Earth’s atmosphere

The propagation of atmospheric gravity waves (AGWs) is studied in the context of geometrical optics in the nonisothermal, viscous, and thermal-conductive atmosphere of Earth in the presence of wind shifts. Parametric diagrams are plotted, determining the regions of allowed frequencies and horizontal phase velocities of AGWs depending on the altitude. It is shown that a part of the spectrum of AGWs propagates in stationary air in an altitude range from the Earth’s surface through the ionospheric F1 layer. AGW from nearearth sources attenuate below 250 km, while waves generated at altitudes of about 300 km and higher do not reach the Earth’s surface because of the inner reflection from the thermosphere base. The pattern changes under strong thermospheric winds. AGW dissipation decreases with an adverse wind shift and, hence, a part of the wave spectrum penetrated from the lower atmosphere to the altitudes of F2 layer.     

Propagation of radio waves reflected from the ionosphere during a solar eclipse

Summary The article first describes in chronological order the observations on the propagation of radio waves during the solar eclipses, and the development of the conflicting results on the similar transmissions of radio signals. The general theory of the absorption of radio waves in the ionosphere is briefly described and therefrom a method is derived to estimate the variation of the absorption of obliquely incident radio waves during a solar eclipse.The variation of field-strength can be studied in terms of the relationship between the vertical incident equivalent frequency of the signals and the critical frequencies of the ionospheric layers at the regions of reflection. The total absorption of radio waves consists of the non-deviative absorption in theD region and the deviative absorption in the higher layers of the ionosphere. During the eclipse, theD region absorption decreases in phase with the progress of the eclipse, but the variation of deviative absorption may differ in each observation. The deviative absorption is large when the equivalent frequency is close to the critical frequency of the layer reflecting the waves or of the layer just penetrated by the waves. The changes in the deviative absorption during an eclipse can be estimated on the basis of the variation of the critical frequencies of the ionospheric layers. The resultant changes in the total absorption during a solar eclipse may thus be estimated. The different types of field strength variation expected during an eclipse are given.The observations of the vertical incident absorption of radio waves and the field strength variations of obliquely incident continuous wave radio signals during the solar eclipse are described and the changes are explained on the basis of the above theory.     

Advanced methods of spectral analysis of quasiperiodic wave-like processes in the ionosphere: Specific features and experimental results

Backgrounds of adaptive Fourier transform and analytical wavelet transform have been briefly described in comparison with traditional Fourier transform using a time window. As an example, all three transforms are used to analyze quasiperiodic wave-like processes in the ionosphere, which accompanied the passage of the solar terminator and rocket launch from the Plesetsk site. The advantages of adaptive Fourier transform and analytical wavelet transform as compared to traditional Fourier transform, which make it possible to reliably detect wave-like disturbances against a background of noise at a signal-to-noise ratio not less than 0.1, have been demonstrated.     

A review of atmospheric gravity waves and travelling ionospheric disturbances: 1982-1995

Recent investigations of atmospheric gravity waves (AGW) and travelling ionospheric disturbances (TID) in the Earth’s thermosphere and ionosphere are reviewed. In the past decade, the generation of gravity waves at high latitudes and their subsequent propagation to low latitudes have been studied by several global model simulations and coordinated observation campaigns such as the Worldwide Atmospheric Gravity-wave Study (WAGS), the results are presented in the first part of the review. The second part describes the progress towards understanding the AGW/TID characteristics. It points to the AGW/TID relationship which has been recently revealed with the aid of model-data comparisons and by the application of new inversion techniques. We describe the morphology and climatology of gravity waves and their ionospheric manifestations, TIDs, from numerous new observations.     

Localized packets of acoustic gravity waves in the ionosphere

Using mass-spectrometric measurement data from the Dynamics Explorer 2 satellite, we investigated the distribution of medium-scale acoustic gravity waves (AGWs) at altitudes of the F-region of the ionosphere. It is shown that the planetary field of AGWs contains a regular and a sporadic component. The regular distribution of AGWs involves active polar areas (where the ionosphere is highly disturbed) and a relatively calm equatorial area. Sporadic AGWs are isolated and spatially localized wave packets that are distinguished against the background of the regular distribution of the wave field. We generated a directory containing observations of sporadic AGW for the period January–February 1983 and performed a statistical analysis of their relation to earthquakes.     

GPS monitoring of ionospheric disturbances generated during rocket launches at large distances from launch sites

Wave-like disturbances, caused by the launches of the Soyuz and Proton rockets from the Baikonur site, have been studied using the algorithm of the space-time accumulation of variations in the total electron content (TEC). Ionospheric TEC responses, observed on four GPS arrays at a distance of up to 4000 km from the launch site, represent a quasi-periodic oscillation with a period of 15–20 min, duration of 30–40 min, and amplitude of 0.1 TECU. The propagation velocity of wave-like disturbances is 300–1400 m/s, which corresponds to the range of sonic and supersonic velocities at an altitude of the ionospheric ionization maximum. Wave-like disturbances of TEC are caused by acoustic gravity waves (AGWs) propagating in the Earth’s atmosphere over large distances from a source. It has been established that the rocket launch region and rocket trajectory active legs, when a rocket moves under the action of the second and third operating stages of a propulsion device, are responsible for AGW generation.     

电离层对台风响应的全过程的特例研究

作为特例研究,本文对1988年和1990年两次强台风影响期间的电离层多普勒记录及相应的台风资料进行了细致的相关分析,目的是利用多普勒记录的连续性优点来了解电离层对登陆（或近海）强台风通过声重波响应的演化全过程.分析表明,在这两次台风影响期间,电离层形态中除有明显的波状扰动（中尺度声重波）出现外,还有一些值得注意的新现象：波动的时间演化表现出明显的幅度逐渐增加以及频率由高频向低频转变,在振幅很大的情况下日落后同时出现扩展F（Spread F）现象,显示了声重波在激发电离层不规则结构方面的种子作用.这一演化过程与电离层中TIDs的线性传播理论一致,文中开展了对这一现象的非线性数值模拟,模拟结果基本上也与上述观测现象相吻合.     

Formation of large-scale disturbances in the upper atmosphere caused by acoustic gravity wave sources on the Earth’s surface

The results of a model study of the acoustic gravity wave (AGW) propagation from the Earth’s surface to the upper atmospheric altitudes have been considered. Numerical calculations have been performed using a nonhydrostatic model of the atmosphere, which takes into account nonlinear and dissipative processes originating when waves propagate upward. The model source of atmospheric disturbances has been specified in an area localized on the Earth’s surface. The disturbance source frequency spectrum includes harmonics at frequencies of 0.5ωg-1.5ωg (ωg is the Brunt-Väisälä frequency near the Earth’s surface). The calculations indicated that AGW propagation and dissipation over the source result in the fact that the region of large-scale spatial disturbances of the upper atmosphere mean state is formed at ～200 km altitudes. This region substantially affects AGW propagation and results in waveguide propagation of AGWs with periods shorter than the Väisälä-Brunt period at the altitude of a disturbed atmosphere. The dissipation of AGWs propagating in such a waveguide results in a waveguide horizontal expansion. The extension of the disturbed region of the mean state of the upper atmosphere and, consequently, the waveguide length can reach ～1000 km, if the AGW ground source operates for ～1 h. The physical mechanism by which large-scale disturbances are formed in the upper atmosphere, based on the propagation and dissipation of AGWs with periods shorter than the Väisälä-Brunt period in the upper atmosphere, explains why these disturbances are rapidly generated and localized above AGW sources located on the Earth’s surface or in the lower atmosphere.     

Diurnal Variation of Gravity Wave Activity at Midlatitudes in the Ionospheric F Region

We investigate the short-term fluctuations in the period range from 15 to 180 minutes in the electron density variations of the F region ionosphere. Electron density profiles obtained at the ionospheric stations of Pruhonice (49.9° N, 14.5° E) and Ebro (40.8° N, 0.5° E) at five minute time sampling have been used for this analysis. The diurnal changes of the activity of the acoustic gravity wave fluctuations (AGW) show a clear enhancement during and several hours after sunrise. The periods of such AGW's are about 60 to 75 minutes and these waves propagates vertically through the ionosphere from a source located at an altitude of 180-220 km. The most likely source for these events seems to be passage of the Solar terminator.     

日食电离层效应

本文分析了1987年9月23日日环食期间,我国14个电离层站和1988年3月18日日全食期间两个站的垂测仪和偏振仪记录,并综合50年代以来历次日食电离层效应的观测结果,证实:1.E层和F₁层光食效应明显,F₂层动力学效应显著;2.f₀F₂存在日食日值大于、小于或等于控制日值三种典型情况;3.TEC食变曲线有凹陷和不凹陷两种典型情况,甚至出现日食日值大于控制日值的异常现象. 本文对F₂层和外电离层的动力学特征作了定性讨论,认为:空间等离子体温度急剧下降和沿场扩散是F₂层和外电离层日食效应的最主要因素;而磁赤道上空等离子体的沿场扩散、“喷泉”效应,热层风和全（环）食带方位是影响位于磁赤道异常区各电离层站日食电离层效应的主要因素.     

Ionospheric behavior over Europe during the solar eclipse of 3 October 2005

An annular eclipse occurred over Europe in the morning hours of 3 October 2005. The well-defined obscuration function of the solar radiation during the eclipse provided a good opportunity to study the ionospheric/thermospheric response to solar radiation changes. Since the peak electron density behavior of the ionospheric F2 layer follows the local balance of plasma production, loss and transport, the ionospheric plasma redistribution processes significantly affect the shape of the electron density profile. These processes are discussed here based on a comparison of vertical incidence sounding (VS) and vertical total electron content (TEC) data above-selected ionosonde stations in Europe. The equivalent slab thickness, derived with a time resolution of 10 min, provides relatively good information on the variation of the electron density profile during the eclipse. The computations reveal an increased width of the ionosphere around the maximum phase. As indicated by the available measurements over Spain, the photo production is significantly reduced during the event leading to a slower increase of the total ionization in comparison with the neighboring days. The supersonic motion of the Moon's cool shadow through the atmosphere may generate atmospheric gravity waves that propagate upward and are detectable as traveling ionospheric disturbances at ionospheric heights. High-frequency (HF) Doppler shift spectrograms were recorded during the eclipse showing a distinct disturbance along the eclipse path. Whereas the ionosonde measurements at the Ebro station/Spain in the vicinity of the eclipse path reveal the origin of the wave activity in the lower thermosphere below about 180 km altitude, the similar observations at Pruhonice/Czech Republic provide arguments to localize the origin of the abnormal waves in the middle atmosphere well below the ionospheric heights. Although ionosonde and HF Doppler measurements show enhanced wave activity, the TEC data analysis does not, which is an indication that the wave amplitudes are too small for detecting them via this interpolation method. The total ionization reduces up to about 30% during the event. A comparison with similar observations from the solar eclipse of 11 August 1999 revealed a quite different ionospheric behavior at different latitudes, a fact that needs further investigation.     

利用日本GPS网探测2011年Tohoku海啸引发的电离层扰动

海平面的海啸波会产生大气重力波进而引发电离层扰动.本文利用日本GPS总电子含量数据来探测2011年3月11日Tohoku海啸引发的电离层扰动.观测结果表明,在日本上空的电离层中存在两种重力波信号,分别由海平面的海啸波以及地震破裂过程产生.地震产生的电离层重力波分布在震中周围（包括海洋上空以及远离海洋的区域）,而海啸引发的电离层重力波主要分布在海洋上空.地震产生的电离层重力波具有不同的水平速度,包括约210 m·s^-1以及170 m·s^-1,其频率为1.5 mHz;而海啸引发的电离层重力波水平速度快于前者,约为280 m·s^-1,其频率为1.0 mHz.此外,海啸引发电离层重力波与海平面上的海啸波有相似的水平速度、方向、运行时间、波形以及频率等传播特征.本文的研究将电离层中的海啸信号与地震信号区分开来,进一步确认电离层对海啸波的敏感性.

     

Ionospheric response to the entry and explosion of the South Ural superbolide

The South Ural meteoroid (February 15, 2013; near the city of Chelyabinsk) is undoubtedly the best documented meteoroid in history. Its passage through the atmosphere has been recorded on videos and photographs, visually by observers, with ground-based infrasound microphones and seismographs, and by satellites in orbit. In this work, the results are presented of an analysis of the transionospheric GPS sounding data collected in the vicinity of the South Ural meteoroid site, which show a weak ionospheric effect. The ionospheric disturbances are found to be asymmetric about the explosion epicenter. The received signals are compared, both in shape and amplitude, with the reported ionospheric effects of ground level explosions with radio diagnostics. It is shown that the confident registration of ionospheric effects as acoustic gravity waves (AGWs) by means of vertical sounding and GPS technologies for ground explosions in the range of 0.26–0.6 kt casts doubt on the existing TNT equivalent estimates (up to 500 kt) for the Chelyabinsk event. The absence of effects in the magnetic field and in the ionosphere far zone at distances of 1500–2000 km from the superbolide explosion epicenter also raises a question about the possibility of an overestimated TNT equivalent. An alternative explanation is to consider the superposition of a cylindrical ballistic wave (due to the hypersonic motion of the meteoroid) with spherical shock waves caused by the multiple time points of fragmentation (multiple explosions) of the superbolide as a resulting source of the AGW impact on ionospheric layers.     

基于GPS技术实时监测2009年7月22日日全食期间长三角区域电离层TEC变化

2009年7月22日上午发生的日全食是21世纪持续时间最长的日全食,其全食带覆盖了中国中部的K江流域,为研究日全食对电离层的影响提供了一次难得的机会,为此本文通过卡尔曼滤波算法实现了实时求解TEC和GPS系统硬件延迟,为实时监测日全食期间电离层变化提供了绝对的电离层TEC.采用上海和浙江区域内GPS网的观测数据,建立了...     

一次日食电离层效应模拟研究

用120°E经度链附近台站电离层垂测资料和一个二维低纬电离层理论模式探讨1995年10月24日日食电离层效应．在日食条件下只考虑日食区计算太阳EUV辐射减少．模式结果显示：（1）日食期间较低高度电离层光食效应显著,电子浓度跟随食分迅速变化,在食甚后浓度减少达到最大。较高高度电离层对日食响应延迟.（2）低纬地区日食日f₀.F₂比控制日低,而h_mF₂比控制日高．在低纬度地区日食带来的影响相对较大·（3）赤道附近h_mF₂食甚后有一突变,出现日食F₁.5层。（4）食甚后海口纬度附近F层受日食影响持久,而f₀F₂在赤道附近出现第2次下降．最后对低纬日食电离层效应的动力学因素进行了初步的讨论．     

Global pattern of the ionospheric response to large-scale internal gravity waves

The global pattern of the ionospheric response to large-scale acoustic gravity waves (LS AGW) has been constructed on the basis of an analysis of the large data set available during the 22 March 1979, magnetic storm. Ground-based ionospheric measurements and in-situ satellite measurements from Cosmos-900 were used in this study together with the Joule heating distribution in the high-latitude ionosphere specifically taken at the maxima of two substorms. The characteristics of the reconstructed planetary pattern of the LS AGW have been analysed in detail. It has been established that the LS AGW effects in the ionosphere in terms of both universal and local time were determined by the pattern of high-latitude atmospheric heating, and that the wave front of the LS AGW during both substorms covered practically all local times, i.e. all longitudes. In addition, it was established that one of the sources of the LS AGW was the thermospheric heating in the day-side cusp region. The local time dependence of the amplitude of the AGW effect in both maximum height, h_mF2, and critical frequency, f_OF2, has been reconstructed for the mid-latitude F2 layer. The AGW effects were clearly separated from the electric field effects related to turnings of the interplanetary magnetic field (IMF) B_Z. In the day-time, electric field effects prevailed over the AGW effects, but during the night-time the amplitudes of these two effects were comparable. In contrast to the common view, f_OF2 variations after the AGW passage had a quasi-sinusoidal character both in the day-time and in the night-time. In the night-time ionosphere a high degree of symmetry was observed for the AGW effects in Northern and Southern hemispheres. During the day-time a significant asymmetry was observed in the American longitudinal sector which was related largely to the peculiarities of the heating pattern in the high-latitude ionospheres of the Northern and Southern hemispheres. These observations demonstrate the complexity of the response of the ionosphere at all latitudes to heating of the auroral region.     

The wave-like structures of ionospheric perturbation associated with Sumatra earthquake of 26 December 2004, as revealed from VLF observation in Japan of NWC signals

The observations of subionospheric VLF waves from the Australian VLF transmitter NWC (frequency=19.8 kHz) at the Japanese receiving stations Chofu, Chiba and Kochi have been utilized to identify a possible precursor of ionospheric perturbations to the huge Sumatra earthquake of 26 December 2004. The VLF amplitude data at Japanese stations have indicated the depression in amplitude and also the enhancement in nighttime amplitude fluctuation before the earthquake. The nighttime fluctuation is composed of wave-like structures, and the wavelet analysis and cross-correlation analyses have been performed for those fluctuations. A significant enhancement in the fluctuation spectra in the period 20–30 min to ∼100 min (the frequency range of atmospheric gravity waves) is observed only before the earthquake. Then, the wave-like structures tend to propagate from the NWC–Kochi path to NWC–Chiba path with the time delay of ∼2 h, and so the wave propagation speed is estimated as ∼20 m/s. This finding might be important when we think of lithosphere–ionosphere coupling mechanism.     

Disturbances in the ionosphere and atmosphere caused by the solar eclipse of August 11, 1999

Results of comparing ionospheric radio noise at wavelength of 2 m at midlatitudes to the data of the ionospheric vertical sounding during the partial phase of the solar eclipse of August 11, 1999, are presented. Disturbances in the ionospheric layers, radio noise of the ionospheric plasma, and variations and fluctuations in the atmospheric pressure at the Earth surface during the eclipse are considered. The parameters of the Lamb wave, which propagated with velocity of 300 m/s from the region of the total phase of the eclipse are determined. The Lamb wave characteristics in the summer midlatitude and auroral ionosphere have been compared.     

A study of the Joule and Lorentz inputs in the production of atmospheric gravity waves in the upper thermosphere

First results of a modelling study of atmospheric gravity waves (AGWs) are presented. A fully-coupled global thermosphere-ionosphere-plasmasphere model is used to examine the relative importance of Lorentz forcing and Joule heating in the generation of AGWs. It is found that Joule heating is the dominant component above 110km. The effects of the direction of the Lorentz forcing component on the subsequent propagation of the AGW are also addressed. It is found that enhancement of zonal E × B forcing results in AGWs at F-region altitudes of similar magnitudes travelling from the region of forcing in both poleward and equatorward directions, whilst enhancement of equatorward meridional E × B forcing results in AGWs travelling both poleward and equatorward, but with the magnitude of the poleward wave severely attenuated compared with the equatorward wave.