基于三亚VHF雷达的场向不规则体观测研究:1.电离层E区连续性回波

基于三亚(109.6°E,18.4°N)VHF电离层相干散射雷达观测,分析了我国低纬电离层E区场向不规则结构连续性回波的发生特征.研究结果表明:白天,E区连续性回波的多普勒速度范围为-50至25m/s,多普勒宽度主要分布在20至70m/s;连续性回波的高度大约以1km/h的速度缓慢下降,与偶发E层(Es)底部所在高度(hbEs)有很好的相关性,表明在背景电场影响下,Es经梯度漂移不稳定性产生场向不规则结构,引起E区连续性回波.夜间,E区连续性回波的多普勒速度范围为-50至50m/s,多普勒宽度为20至110m/s,回波在时间-高度-强度图上常呈现多层结构,可能与潮汐引起的多个离子层相关;而E区连续性回波的短暂中断,以及120km以上高E区连续性回波的发生,则可能归因于赤道扩展F极化电场的影响.此外,对E区连续性回波多普勒速度与全天空流星雷达风场观测的比较发现,在100km以下,多普勒速度与子午风场有很好的相关性.     

Observations of 50-MHz type-II coherent echoes from within the polar cap

50-MHz type-II coherent echoes at geometrical aspect angles of 11.5○ have been observed in the northern polar cap during pre-noon hours. The echoes had an unusually large Doppler width of 1200–1400 m s⁻¹ and were well correlated with strong magnetic disturbances in the range 500–1000 nT. The dependence of intensity, spectral width and skewness versus radial velocity were similar to those known from previous experiments at lower latitudes and at small aspect angles. It is concluded that echo onset was due to the combination of several factors, including a highly conducting ionosphere, the presence of a very intense electric field, and strong radar wave refraction.     

基于三亚VHF雷达的场向不规则体观测研究:2.东亚低纬电离层E区准周期回波

中低纬电离层E区不规则体准周期雷达回波现象,在地球不同经度区被观测到并开展了有关研究.本文利用三亚(109.6°E,18.4°N)VHF相干散射雷达2011年2月6日的观测,第一次给出了中国低纬电离层E区准周期回波的发生和变化特征.观测结果表明:准周期回波发生在地方时夜间2100—2200LT的110km高度上,与连续性回波可同时发生;准周期回波斜纹在雷达探测的高度-时间-强度(HTI)图上可延伸5～20km,持续时间为5～15min,回波斜纹高度随时间以20～30m/s下降,斜纹在HTI图上彼此间隔10km和10min左右.此外,雷达回波多普勒谱和雷达干涉分析显示不同高度准周期回波的多普勒速度随高度-时间表现出不同的变化趋势,与回波条纹斜率无明显联系,不同高度准周期回波对应的不规则体在东西方向也表现出截然不同的运动特征.分析结果表明,三亚电离层E区准周期回波的发生可能并不是由散块Es随着中性风周期性的经过雷达探测区域所致,而可能和Es中的扰动结构相关.     

Dynamics and electrodynamics of ionospheric inhomogeneities of the midlatitude E region (Review)

Present-day ideas of the formation of inhomogeneities in midlatitude lower-ionosphere plasma are considered. Experimental and theoretical results are described that testify to the existence of anisotropic structures developing at lower-ionosphere altitudes and bring about strong electric fields, narrow echo peaks, and quasi-periodic echoes (detected by coherent radar). The origin of inhomogeneities and structures observed in plasma and neutral components of the lower ionosphere is analyzed.     

基于三亚VHF雷达的场向不规则体观测研究：3. 距离扩展流星尾迹回波

距离扩展流星尾迹回波 （RSTE,range spread trail echo）观测通常采用高功率大孔径雷达.本文利用三亚 （18.4°N,109.6°E） VHF雷达观测,分析研究了小功率VHF雷达观测的RSTE特征.结果表明,三亚VHF雷达在相干散射模式观测的RSTE,其发生和演化的主要形态特征类似高功率大孔径雷达的观测结果.通过2011年8月期间,三亚VHF雷达相干散射模式观测的RSTE,与同时的全天空流星模式观测的镜面流星尾迹（SE,specular trail echo）数据,统计分析了RSTE和SE随地方时以及高度的变化,发现短持续时间RSTE（小于等于15 s）与SE具有类似的时间-高度变化特征,而长持续时间RSTE（大于等于15 s）发生的峰值时间,相比SE和短持续时间RSTE向晨侧偏移,且长持续时间RSTE对高度的依赖性比短持续时间RSTE显著.此外,全天空流星模式观测SE 的数量远多于相干散射模式观测的RSTE数量,可能与不同模式下的观测角度、波束宽度等因素相关.     

On the diurnal variation of the E-region coherent HF echo occurrence

We present a statistical study of the diurnal variations of the E-region backscatter occurrence observed by 15 SuperDARN radars over 3 years. The diurnal variation of echo occurrence observed by each radar is examined for three levels of geomagnetic activity. It is found that the E-region echo occurrence for individual radars exhibits very similar diurnal variations for low geomagnetic activity, with the differences between radars increasing with activity. This indicates that local processes become more important in the generation of the E-region echoes during disturbed periods. It is also shown that the majority of E-region backscatter structures take the form of diffuse echo bands rather than discrete echo patches, with the dominance of diffuse echoes increasing with activity. Finally, a hypothesis that the echo occurrence is controlled by the radar's coverage of the auroral oval is tested using the electron and ion oval models derived from the DMSP satellite observations.     

Studying the fine structure of coherent echo spectra using data from Irkutsk incoherent scatter radar

Studying the processes generating different-scale inhomogeneities is one of the challenging problems of ionospheric physics. Plasma instabilities are one of the physical mechanisms by which small-scale inhomogeneities are formed. The main forms of instability in the ionospheric E-layer are two-stream and gradient-drift ones. The inhomogeneities generated by them lead to an abnormally intense radio scattering of different wavelengths (known as coherent echo (CE) or radio aurora) in the E-layer. Therefore, the method of radiowave backscattering is among the widely used methods for studying such inhomogeneities. The CE phenomenon has been investigated most intensely at high and equatorial latitudes, where the conditions for the CE origination are formed rather regularly. For the last decade, CE has also been intensely studied at midlatitudes, where it is observed less frequently and its formation conditions are less known. In 1998–2006, the purposeful studies of the midlatitude CE peculiarities were performed at the Irkutsk incoherent scatter (IS) radar, with a particular emphasis on its coherent properties. It was for the first time found out that the spectra of some data sets had a fine comb-shaped structure, which generated well-known single-humped CE spectra as a result of statistical averaging. In the scope of this study, unique coherent methods for processing individual data sets of CE signals were developed, making it possible to reveal the peculiarities of unaveraged CE-signal spectra. To describe these peculiarities, we proposed a new model of the inhomogeneity spectrum, which is the superposition of the discrete set of spatial harmonics with close wave numbers. The model was shown to adequately describe the scattered signal characteristics observed experimentally.     

On auroral dynamics observed by HF radar: 1. Equatorward edge of the afternoon-evening diffuse luminosity belt

Observations and modelling are presented which illustrate the ability of the Finland CUTLASS HF radar to monitor the afternoon-evening equatorward auroral boundary during weak geomagnetic activity. The subsequent substorm growth phase development was also observed in the late evening sector as a natural continuation of the preceding auroral oval dynamics. Over an 8 h period the CUTLASS Finland radar observed a narrow (in range) and persistent region of auroral F- and (later) E-layer echoes which gradually moved equatorward, consistent with the auroral oval diurnal rotation. This echo region corresponds to the subvisual equatorward edge of the diffuse luminosity belt (SEEL) and the ionospheric footprint of the inner boundary of the electron plasma sheet. The capability of the Finland CUTLASS radar to monitor the E-layer SEEL-echoes is a consequence of the nearly zero E-layer rectilinear aspect angles in a region 5/10° poleward of the radar site. The F-layer echoes are probably the boundary blob echoes. The UHF EISCAT radar was in operation and observed a similar subvisual auroral arc and an F-layer electron density enhancement when it appeared in its antenna beam.     

Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude

We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are shortlived and occur in single 2- or 10-s data dumps. They are offset by 125–130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental.     

Range resolution dependence of VHF radar returns from clear-air turbulence and precipitation

With employing 1.5 h of the data observed by the Chung-Li VHF radar, the range resolution dependences of the VHF backscatter from refractivity fluctuation and precipitation are investigated in this article. It indicates that the atmospheric layer structure of refractivity seems to play a role in governing the range resolution dependence of clear-air turbulent echoes. Observations shows that the VHF clear-air echo power ratios for 4 to 2 μs pulse lengths are close to 3 dB in the middle or bottom side of the layer, while the ratios are significantly greater than 3 dB in the top side of the layer. The analysis of the precipitation echo power ratio for 4 to 2 ms pulse lengths shows that basically the ratios above 3.0 km are close to 3 dB, but enormously smaller than 3 dB below 3.0 km. The feature of extraordinarily small echo power ratios below 3.0 km is also observed for the radar returns from refractivity turbulence. The radar recovery effect is thought to be a primary factor responsible for the severe diminution of the echo power ratios at the lower altitudes. In addition, statistical analysis reveals that the range resolution effect on the first and second moments of the Doppler spectra for the radar echoes from clear-air turbulence and precipitation is insignificant and negligible. The dependences of the coefficient A and power B in the power-law approximation V_t=AP^B_r to the terminal velocity V_t and range-corrected echo power P_r are examined theoretically and experimentally. The results show that the coefficient A (powers B) is inversely (positively) proportional to the range resolution, in a good agreement with the observations. Because of the strong dependence of coefficient A and power B on the radar pulse width, it suggests that great caution should be taken in comparing the power-law expressions V_t=AP^B_r established from the radar returns obtained with different range resolutions.     

Midday reversal of equatorial ionospheric electric field

A comparative study of the geomagnetic and ionospheric data at equatorial and low-latitude stations in India over the 20 year period 1956–1975 is described. The reversal of the electric field in the ionosphere over the magnetic equator during the midday hours indicated by the disappearance of the equatorial sporadic E region echoes on the ionograms is a rare phenomenon occurring on about 1% of time. Most of these events are associated with geomagnetically active periods. By comparing the simultaneous geomagnetic H field at Kodaikanal and at Alibag during the geomagnetic storms it is shown that ring current decreases are observed at both stations. However, an additional westward electric field is superimposed in the ionosphere during the main phase of the storm which can be strong enough to temporarily reverse the normally eastward electric field in the dayside ionosphere. It is suggested that these electric fields associated with the V × Bz electric fields originate at the magnetopause due to the interaction of the solar wind and the interplanetary magnetic field.     

Acoustic seabed segmentation for echosounders through direct statistical clustering of seabed echoes

A new method is presented for inferring seabed type from the properties of seabed echoes stimulated by echosounders. The methodology currently used classifies echoes indirectly through feature extraction, usually in conjunction with dimensional reduction techniques such as Principal Components Analysis. The features or principal components derived from them are classified by statistical clustering or other means into groups with similar sets of mathematical properties. However, a simpler technique is to directly cluster the echoes themselves. A priori modelling or curve fitting, feature extraction, and dimensional reduction are not required, simplifying the processing and analysis chain, and eliminating data distortions. In effect the echoes are treated as geometrical entities, which are classified by their shapes and positions. Direct clustering places the analysis focus on the actual echoes, not on proxy parameters or mathematical techniques. This allows simple and direct evaluations of results, without the need to work in abstract mathematical spaces of unknown relation to echo properties. The direct clustering method for seabed echoes is demonstrated with echosounder data obtained in Balls Head Bay, Sydney Harbour, Australia, an area with mud, sand, and shell beds.     

中山站高频雷达回波的日变化特征及地磁活动的影响

利用新建中山站高频雷达2010年4月到2011年2月的观测数据,研究了中山站高频雷达回波的日变化特征以及地磁活动的影响. 研究结果表明,中山站高频雷达回波具有明显的日变化特征且受地磁活动影响较大. 雷达回波发生率的峰值在地磁活动较小时处于日侧;随着地磁活动的增强,峰值减小并向夜侧移动. 另外,平均多普勒视线速度具有明显的昼夜分布,夜侧主要为正向速度,即朝向雷达,日侧主要为负向速度,即远离雷达;随着地磁活动的增强,平均回波强度和平均多普勒视线速度的峰值都会增加,而多普勒谱宽则会减小.     

Autoscala: an aid for different ionosondes

Autoscala is a software to automatically scale ionospheric characteristics from an ionogram. Initially it was only applied to the ionograms recorded by the AIS-INGV ionosondes installed at Rome and Gibilmanna (Italy), and Tucumán (Argentina), that are not able to record the polarization of the received echo. Recently, Autoscala was also applied to the ionograms recorded by the AIS-Parus ionosonde installed at Moscow (Russia), that is not able to tag the received echo in terms of polarization, and by the VISRC2 ionosonde installed at Warsaw (Poland) that is instead able to perform the polarization tagging of the ordinary and extraordinary echoes. This work shows different examples of processing performed on ionograms recorded by all these three different types of ionosondes.     

Letter to the editor Complete maps of the aspect sensitivity of VHF atmospheric radar echoes

Using the MU radar at Shigaraki, Japan (34.85°N, 136.10°E), we measure the power distribution pattern of VHF radar echoes from the mid-troposphere. The large number of radar beam-pointing directions (320) allows the mapping of echo power from 0° to 40° from zenith, and also the dependence on azimuth, which has not been achieved before at VHF wavelengths. The results show how vertical shear of the horizontal wind is associated with a definite skewing of the VHF echo power distribution, for beam angles as far as 30° or more from zenith, so that aspect sensitivity cannot be assumed negligible at any beam-pointing angle that most existing VHF radars are able to use. Consequently, the use of VHF echo power to calculate intensity of atmospheric turbulence, which assumes only isotropic backscatter at large beam zenith angles, will sometimes not be valid.     

Improvement of MT data processing using stationary and coherence tests

The great majority of data processing methods for magnetotelluric measurements are based on an average Fourier spectral analysis to estimate either the transfer function or the coherence function. This assumes that the time-series data are stationary. The aim of this paper is to present a preprocessing method able to extract the stationary and coherent part of the original signals. The practitioner can then apply the magnetotelluric method of his choice to these new data. This preliminary data sorting is done in four steps: (i) slow drifts are eliminated using a high-pass filter whose cut-off frequency is determined by an iterative procedure. Based on run tests, this procedure is also able to remove segments with non-independent samples in the time series; (ii) non-stationary segments are eliminated after band-pass filtering; (iii) non-coherent segments are eliminated before spectral analysis; (iv) the impedance tensor value is then retained, at a given frequency, only if the signals are coherent. This preprocessing method was tested on the simplest, but still used, magnetotelluric method which uses only two field components, and it was found that the average resistivity standard deviation decreased significantly from 14.6 Ωm without sorting to 8.6 Ωm after sorting.     

Determining the parameters of the layer of scattering irregularities,forming coherent echo,according to the Irkutsk IS Radar data

The method for determining the parameters of the layer of scattering irregularities, form coherent echo, based on the Irkutsk Incoherent Scatter (IS) Radar data is presented. It has been indicted that the method has the necessary accuracy. The processing of experimental data for the periods of September 25–26, 1998, and July 15–16, 2000, indicated that our results are in good agreement with the data obtained by other researchers. The analysis of the experiments with a high time resolution made it possible to reveal the time variation in the determined parameters. The time variations in the layer thickness and height do not contradict the data obtained by other researchers, and the time variations in the angular sensitivity of irregularities has been observed for the first time and requires further studies.     

Doppler radar sounding of volcanic eruption dynamics at Mount Etna

Besides their common use in atmospheric studies, Doppler radars are promising tools for the active remote sensing of volcanic eruptions but were little applied to this field. We present the observations made with a mid-power UHF Doppler radar (Voldorad) during a 7-h Strombolian eruption at the SE crater of Mount Etna on 11–12 October 1998. Main characteristics of radar echoes are retrieved from analysis of Doppler spectra recorded in the two range gates on either side of the jet axis. From the geometry of the sounding, the contribution of uprising and falling ejecta to each Doppler spectrum can be discriminated. The temporal evolution of total power backscattered by uprising targets is quite similar to the temporal evolution of the volcanic tremor and closely reproduces the overall evolution of the eruption before, during and after its paroxysm. Moreover, during the sharp decrease of eruptive activity following the paroxysm, detailed analysis of video (from camera recording), radar and seismic measurements reveals that radar and video signals start to decrease simultaneously, approximately 2.5 min after the tremor decline. This delay is interpreted as the ascent time through a magma conduit of large gas slugs from a shallow source roughly estimated at about 500 m beneath the SE crater. Detailed analysis of eruptive processes has been also made with Voldorad operating in a high sampling rate mode. Signature of individual outburst is clearly identified on the half part of Doppler spectra corresponding to rising ejecta: temporal variations of the backscattered power exhibit quasi periodic undulations, whereas the maximum velocity measured on each spectrum displays a sharp peak at the onset of each outburst followed by a slow decay with time. Periodicity of power variations (between 3.8 and 5.5 s) is in agreement with the occurrence of explosions visually observed at the SE vent. Maximum vertical velocities of over 160 m s^–1 were measured during the paraoxysmal stage and the renewed activity. Finally, by using a simplified model simulating the radar echoes characteristics, we show that when Voldorad is operating in high sampling rate mode, the power and maximum velocity variations are directly related to the difference in size and velocity of particles crossing the antenna beam.Editorial responsibility: A. Woods     

A new type of daytime high-frequency VLF emissions at auroral latitudes (“bird emissions”)

This paper is concerned with a new, previously unknown type of high-frequency (above 4 kHz) VLF emissions that were detected during winter VLF campaigns in Kannuslehto (L ~ 5.5), Finland. These previously unknown emissions have been discovered as a result of the application of special digital filtering: it clears the VLF records from pulse signals of intensive atmospherics, which prevent other kinds of VLF emissions in the same frequency range from being seen on spectrograms. As it appears, aside from wellknown bursts of auroral hisses and discrete quasiperiodic emissions, a previously unknown type of daytime right-hand polarized VLF waves is also present at frequencies above 4 kHz. These emissions can persist for several hours as series of separate short discrete wideband (from 4 to 10 kHz and higher) signals, each with a duration between one and several minutes. It has been found that such signals can be observed almost daily in winter. These emissions sound like bird’s chirping to a human ear; for that reason, they were called “bird emissions.” The dynamic spectra of individual signals often resemble flying birds. The signals are observed during daytime, more often in magnetically quiet conditions preceded by geomagnetic disturbances. As a rule, the occurrence of these bird emissions is accompanied by a slight increase in electron density in the lower ionosphere, which is evidence of the precipitation of energetic (>30 keV) electrons. This raises a number of questions as to where and how the VLF bird emissions are generated and how such emissions, at frequencies greatly exceeding half the electron equatorial gyrofrequency at L ~ 5.5, can reach the Earth’s surface.     

从电离层垂直反射的二次回波的統計特性

在地面上用无线电方法研究电离层时,通常只利用电离层反射的一次回波,但是利用二次回波来研究电离层也是可能的。利用二次回波提供的补充知识,特別是利用一次和二次回波的同时观测结果的相互对比,在某些情况下,对电离层物理和电波传播的研究,具有特别的意义。在这篇文章中,将对垂直採测时从电离层反射的二次回波波場和振幅的统计特性,以及一次和二次回波之间的统计联系,进行一些理论上的计算;并根据这些计算结果,讨论一下利用二次回波研究电离层和电波传播吋的若干问題。