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.     

Response of the ionosphere to natural and man-made acoustic sources

A review is presented of the effects influencing the ionosphere which are caused by acoustic emission from different sources (chemical and nuclear explosions, bolides, meteorites, earthquakes, volcanic eruptions, hurricanes, launches of spacecrafts and flights of supersonic jets). A terse statement is given of the basic theoretical principles and simplified theoretical models underlying the physics of propagation of infrasonic pulses and gravity waves in the upper atmosphere. The observations of “quick” response by the ionosphere are pointed out. The problem of magnetic disturbances and magnetohydrodynamic (MHD) wave generation in the ionosphere is investigated. In particular, the supersonic propagation of ionospheric disturbances, and the conversion of the acoustic energy into the so-called gyrotropic waves in the ionospheric E-layer are considered.     

Detection of Impact Points of Fragments of Spent Launch Vehicle Stages Using Infrasound Direction-Finding Methods

The paper describes the principles and techniques used to detect signals propagating in the atmosphere in the infrasonic frequency range. Such signals can be generated by different sources: ground and atmospheric explosions, as well as objects moving in the atmosphere at supersonic speed (aircraft, rockets, bolides, fragments of spent stages of launch vehicles). Portable infrasound monitoring stations are described, each of which includes three spaced infrasonic microphones. Each such station makes it possible to determine three basic parameters of the detected infrasound signal, which are subsequently used to solve the direction- finding problem: the time of arrival of an infrasonic wave, the azimuth to the source in the horizontal plane, and the wave approach angle from the source of infrasonic waves to the Earth’s surface in the vertical plane. An acoustic detector used to extract useful signals against a noise background is described. The detector is based on an algorithm similar to the STA/LTA detection algorithm known in seismology. Examples of the operation of an acoustic detector with data obtained during real measurements are given. Passive infrasound direction-finding technology is described. It is based on mathematical modeling of the of infrasonic wave propagation in the atmosphere, which are generated by objects moving along possible trajectories; comparison of theoretical signals with real ones recorded by monitoring stations; and determination of the realized trajectories. The paper gives examples of experimental verification of the effectiveness of passive infrasound direction-finding technology for determining the impact points of the first and second stages of launch vehicles. It is shown that infrasound direction-finding systems makes it possible to reduce the estimated search area for launch vehicle fragments that fall to the Earth, significantly decrease the time and costs for their search and utilization, and mitigate the negative environmental impact of the rocket and space industry.     

The effect of wind on the gravity wave propagation in the Earth’s ionosphere

The effect of ionospheric wind on the gravity wave propagation is studied. These waves arise in the ionosphere due to intensification of their sources near the Earth’s surface during enhanced seismic activity. The influence of the wind on these waves is connected with the Ampere’s force that produces the ion-drag force acting on the atmosphere. This results in the occurrence of the discrete wave spectrum the maximum of which increases in proportion to the numbers of the natural scale. Furthermore, these waves are amplified during propagation from the source region in the direction perpendicular to the wind direction. These peculiarities of the gravity waves can be used for monitoring of seismic activity based on the ionosphere sounding.     

电离层声重波扰动的高频无线电诊断

为了充分发挥现代数字式电离层探测仪在电离层结构与优动研究中的潜力,必须在传统的频高图真高换算的基础上,发展新的反演理论和算法.本文介绍了随时空缓变的各向异性电离层介质中无线电波包传播的广义射线方程组,讨论了计算波包射线几何路径以及计算波包参数,如频移、波矢、群时延等沿射线路径变化的传播正问题,并详细讨论了根据波包参量的测定值反演介质结构和扰动伏态的传播反问题.文中若干实验实例表明,这种反演理论和算法可用于数字式电离层探测仪记录的分析,它使高频无线电波探测技术成为研究全球电离层声重波一类扰动的有力工具。     

Disturbances of the ionosphere of blast and acoustic waves generated at ionospheric heights by rockets

In this paper we present a model, which describes the propagation of acoustic impulses produced by flight of rockets through a model terrestrial atmosphere, and effect of these impulses onto the ionosphere above a rocket. We show, that experimentally observed ionospheric disturbances with duration about 300 s cannot be explained by effect of acoustic impulses onto the ionosphere. We have calculated parameters of a blast wave produced by launch vehicle on the ionospheric heights. It was shown that the blast wave is intense and this wave can generate great disturbance of electron density. The disturbance of electron density can exceed the ambient electron density in 2.6 times. We supposed that the observed ionospheric disturbances might be produced by propagation of delayed magnetoacoustic wave, which, in turn, was produced by the blast wave.     

Spatial field structure and polarization of geomagnetic pulsations in conjugate areas

The ultra-low-frequency (ULF) geomagnetic pulsations observed at two nearly conjugate mid-latitude sites are examined to study their spatial structure and polarization, and learn about the role of ionospheric conductivity in forming their ground signatures. The data of 1999–2002 from Antarctica and New England (L of 2.4) are compared with the numerical results obtained in a simple plane model of ULF wave propagation through the ionosphere and atmosphere. The multi-layered model environment includes an anisotropic and parametrically time-dependent ionosphere, a uniform magnetosphere and a conducting Earth, all placed in a tilted geomagnetic field. The measured diurnal and seasonal variations in the orientation angle of the polarization ellipse are interpreted as effects of hydromagnetic wave propagation through the ionosphere and conversion to an electromagnetic field below. Essentially, the phase, amplitude and polarization of ULF waves observed at the ground are controlled by the wave's spatial structure in the magnetosphere and ionospheric transverse conductivities. The differences shown by the characteristics of simultaneous pulsations in conjugate areas arise mainly from different local ionospheric conditions, while the source waves of the pulsations are common to both sites.     

远程事件次声监测中的信号参数二维子空间计算方法

越来越多的观测发现,在地震、火山爆发、泥石流等重大自然灾害发生前,常产生异常的次声信号,这为地震及其他自然灾害的预报工作增加了一种可能的信息;同时,次声还是监测大气层、浅地表爆炸的有效手段.在自然灾害和爆炸事件次声监测中,慢度和方位角等参数对于源信号传播、定位以及源性质识别等工作具有重要意义.然而,目前的慢度和方位角等参数的算法——频率波数(FK)分析法,尚存在精度和分辨率不高等问题,特别是对多源次声信号的识别能力较差.为提高次声信号的监测精度,基于次声信号和噪声的子空间不相关性,构建了次声信号慢度和方位角二维子空间计算模型,并在此基础上提出了一种高分辨率次声信号二维子空间算法,仿真实验和实际数据的对比分析结果表明:本文提出的方法在精度和分辨率方面明显优于FK法,且能够更好地分离多源次声信号.     

Ionospheric effects of ground motion: The roles of magnetic field and nonlinearity

Transformation of infrasound to magnetic sound upon propagation from ground level up to the ionosphere is considered. It is shown that upon entering the ionospheric layers at altitudes of order 150–170 km, the wave dynamics changes sharply. Nonlinear effects, including shock formation, are also considered. The shocks are typically formed in a relatively narrow range of altitudes, or not formed at all. Generalization of the model to a case of oblique propagation is briefly considered, and the effects of atmospheric profile variation and of finite plasma conductivity are estimated. Along with providing qualitative insight, the model gives some realistic estimates for waves generated by earthquakes.     

Infrasonic Signal Detection and Source Location at the Prototype International Data Centre

—?This paper describes an automatic and interactive data processing system designed to locate impulsive atmospheric sources with a yield of at least one kiloton by detecting and characterizing the airborne infrasound radiated by the source. The infrasonic processing subsystem forms part of a larger system currently under development at the Prototype International Data Center (PIDC) in Arlington, Virginia where seismic, hydroacoustic, radionuclide and infrasonic methods are used to detect and locate impulsive sources in any terrestrial environment. Infrasonic signal detection is achieved via a coincidence detector which requires both the normalized cross correlation and the short-term-average/long-term-average ratio of a beam in the direction of maximum correlation to exceed predetermined threshold values simultaneously before a detection is declared. The infrasound propagation model currently used to infer travel-time information assumes the horizontal sound speed across the ground to be 320.0?m/s. This crude model is currently being replaced by a model which predicts travel-time information through a ray-tracing algorithm for acoustic waves in an atmosphere with seasonal representations for temperature and wind. A novel feature of the source location process is the fusion of all available arrival information, whether it be seismic, hydroacoustic or infrasonic to locate a single source where it is reasonable to hypothesize a common source. In its final configuration the infrasonic subsystem will routinely process data from the global 60-station International Monitoring System (IMS) infrasonic network currently under development.     

GT0 Explosion Sources for IMS Infrasound Calibration: Charge Design and Yield Estimation from Near-source Observations

Three large-scale on-surface explosions were conducted by the Geophysical Institute of Israel (GII) at the Sayarim Military Range, Negev desert, Israel: about 82 tons of strong high explosives in August 2009, and two explosions of about 10 and 100 tons of ANFO explosives in January 2011. It was a collaborative effort between Israel, CTBTO, USA and several European countries, with the main goal to provide fully controlled ground truth (GT0) infrasound sources, monitored by extensive observations, for calibration of International Monitoring System (IMS) infrasound stations in Europe, Middle East and Asia. In all shots, the explosives were assembled like a pyramid/hemisphere on dry desert alluvium, with a complicated explosion design, different from the ideal homogenous hemisphere used in similar experiments in the past. Strong boosters and an upward charge detonation scheme were applied to provide more energy radiated to the atmosphere. Under these conditions the evaluation of the actual explosion yield, an important source parameter, is crucial for the GT0 calibration experiment. Audio-visual, air-shock and acoustic records were utilized for interpretation of observed unique blast effects, and for determination of blast wave parameters suited for yield estimation and the associated relationships. High-pressure gauges were deployed at 100–600 m to record air-blast properties, evaluate the efficiency of the charge design and energy generation, and provide a reliable estimation of the charge yield. The yield estimators, based on empirical scaled relations for well-known basic air-blast parameters—the peak pressure, impulse and positive phase duration, as well as on the crater dimensions and seismic magnitudes, were analyzed. A novel empirical scaled relationship for the little-known secondary shock delay was developed, consistent for broad ranges of ANFO charges and distances, which facilitates using this stable and reliable air-blast parameter as a new potential yield estimator. The delay data of the 2009 shot with IMI explosives, characterized by much higher detonation velocity, are clearly separated from ANFO data, thus indicating a dependence on explosive type. This unique dual Sayarim explosion experiment (August 2009/January 2011), with the strongest GT0 sources since the establishment of the IMS network, clearly demonstrated the most favorable westward/eastward infrasound propagation up to 3,400/6,250 km according to appropriate summer/winter weather pattern and stratospheric wind directions, respectively, and thus verified empirically common models of infrasound propagation in the atmosphere.     

利用射线追踪方法研究化学物质释放对高频加热的聚焦效应

电离层化学物质释放能导致释放区域电子密度的损耗,从而产生明显的电离层空洞现象.高频电波通过电离层空洞时,由于电子密度不同,对电波产生折射效果进而形成聚焦效应.本文利用射线追踪方法,评估高频电波通过空洞形成聚焦加热的效果.结果表明,释放水分子与SF6都能对电离层产生明显的空洞,空洞半径约为25～50 km,电子密度的损耗...     

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.     

Source location variability and volcanic vent mapping with a small-aperture infrasound array at Stromboli Volcano,Italy

Stromboli Volcano in Italy is a persistently active, complex volcanic system. In May 2002 activity was confined to 3 major summit craters within which several active vents hosted multiple explosions each hour. During a 5-day field campaign an array of 3 low-frequency microphones was installed to investigate the coherent infrasound produced by degassing from these vents. Consistent phase lags across the 3 stations indicate distinct sources that are subsequently investigated to determine the associated vent location, apparent depth, and origin time. The cross-correlation routine allows for variations in comparison window length, waveform filtering bandwidth, and correlation and consistency thresholds, allowing for improved detection of certain types of degassing sources. Identification of activity at the various vents could be subsequently corroborated with 3 channels of synchronously acquired thermal data and video. During the May 2002 experiment persistent, energetic infrasound was observed from a passive degassing source within the Central Crater (CC) and transient infrasound, produced by discrete Strombolian explosions, was identified at 4 additional vents. The continuous infrasound produced by the CC exhibits variable frequency-dependent correlation lag times that are interpreted as a diffraction effect due to the acoustic radiators recessed location within a steep-walled crater. Such dispersion has important implications for accurate eruption source modeling because it indicates that infrasonic waveforms may be significantly filtered during propagation. Transient explosion signals from the Northeast Crater (NEC) and Southwest Crater (SWC) vents also exhibit dynamic correlation lag times, but this scatter may be more reasonably attributed to variable epicentral locations. Explosions from the NEC west vent, for instance, appear to emanate from a diffuse zone with a lateral extent in excess of 10 m.Editorial responsibility: R. Cioni     

甚低纬哨声低电离层透射过程的数值模拟

利用全波解算法模拟哨声波束在甚低纬地区黎明前低电离层透射的三维能量分布，依据波场能量和偏振分布及其对波参量和电子浓度剖面的依赖特征，分析了哨声透射、反射及与大地－电离层波导耦合过程．结果表明，哨声模波存在于９０ｋｍ以上高度，吸收、反射、波束扩展及波模转换主要发生于电离层底部８０－９０ｋｍ区间；到达地面的透射能量密度衰减２０ｄＢ以上，透射衰减随频率变化不大，但随波入射角呈不对称变化；距透射区１５０ｋｍ以外区域的测向方位角有很大偏差；入射波能量的很少一部分（对５ｋＨｚ约为－２５ｄＢ）被反射并激发起哨声模波，反射波束能量集中于入射波束附近，并随频率下降而迅速增强．计算也表明，地面接收到的甚低纬哨声回波可能与使回波向极侧偏移的电离纬向梯度有关．     

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.     

Modification of electron concentration in the ionosphere in the pump-wave plasma resonance region

We discuss the propagation of sounding radio waves in the inhomogeneous ionosphere, in the reflection area of which there are small-scale artificial magnetically-positioned irregularities. The propagation of radio waves in such an area, where the lateral dimensions of strongly elongated artificial irregularities are smaller than the wavelength, has a diffraction nature. It is shown that the calculation of diffraction parameters makes it possible to derive the amplitude of density irregularities and their relative area perpendicular to the magnetic field direction. Comparison of theoretical calculations with experimental studies on modification of the electron density altitude profile by heating of the ionosphere with midlatitude stand Sura showed that the relative area of the negative density perturbations can reach several percent.     

Recent investigation on the coupling between the ionosphere and upper atmosphere

Scientific attention has recently been focused on the coupling of the earth’s upper atmosphere and ionosphere. In the present work, we review the advances in this field, emphasizing the studies and contributions of Chinese scholars. This work first introduces new developments in the observation instruments of the upper atmosphere. Two kinds of instruments are involved: optical instruments (lidars, FP interferometers and all-sky airglow imagers) and radio instruments (MST radars and all-sky meteor radars). Based on the data from these instruments and satellites, the researches on climatology and wave disturbances in the upper atmosphere are then introduced. The studies on both the sporadic sodium layer and sporadic E-layer are presented as the main works concerning the coupling of the upper atmosphere and the low ionosphere. We then review the investigations on the ionospheric longitudinal structure and the causative atmospheric non-migrating tide as the main progress of the coupling between the atmosphere and the ionospheric F2-region. Regarding the ionosphere-thermosphere coupling, we introduce studies on the equatorial thermospheric anomaly, as well as the influence of the thermospheric winds and gravity waves to the ionospheric F2-region. Chinese scholars have made much advancement on the coupling of the ionosphere and upper atmosphere, including the observation instruments, data precession, and modeling, as well as the mechanism analysis.