Lidar observations of atmospheric sodium at an equatorial location

Lidar measurements of upper atmospheric sodium were made at Alcântara (2°20′ S, 44°30′ W), Brazil, on 8 days during the month of November 1996. These are the first sodium measurements to be reported from an almost equatorial location. The average sodium distribution at Alcântara was found to be very similar to the annual mean for São José dos Campos (23° S), but about 1 km higher than the November mean value of 92 km for the latter site. Only one, weak, sporadic sodium layer was observed during more than 26 h of observations, an occurrence frequency much less than that seen at low latitude sites in the northern hemisphere. A survey of the observations of sporadic layers at various locations suggests that both magnetic dip and declination angles might influence their rate of occurrence.     

A 14-year monthly climatology and trend in the 35–65 km altitude range from Rayleigh Lidar temperature measurements at a low latitude station

A sodium resonance lidar at 589 nm has been operated in São José dos Campos, Brazil (23°S, 46°W) since 1972 mainly for studies related to the origin, chemistry and dynamics of the mesospheric sodium layer. Beginning in 1993, the improved laser capability has also enabled the processing of the Rayleigh signal from which the temperatures from 35 to 65 km are retrieved on a nightly mean basis. We used these nightly profiles to determine the monthly temperature profiles from 1993 to 2006. The mean temperature characteristics for each year and for the whole period are obtained. Seasonal thermal amplitude is small (6 K peak to peak at 40 and 60 km). Compared with the MSISE-90 model, a large difference is noted, with temperature lower than the model below the stratopause and higher above. Also the seasonal variation has a large difference with better agreement occurring around local winter, but with temperatures higher by 8–10 K at the equinoxes. The semiannual component is dominant over the annual at all altitudes. Linear trends with decreasing temperature of 1.09, 2.29 and 1.42 K/decade are observed at 40, 50 and 60 km, respectively.     

The thermal diffusivity of soil at tropical stations in southern hemisphere

Summary The study of heat transfer or temperature changes in the surface layers of the soil are important in agricultural science at tropical latitudes. The quantity, thermal diffusivity largely determines the temperature changes produced in the layer of the soil when heat is conducted there from an adjacent layer. Therefore, the authors have determined the thermal diffusivity of the soil at the sites Arapongas (23.5°S, 51.7°W) and São José dos Campos (23.3°S, 45.8°W), in Brazil (South America), in accordance with the simple theory of periodic heat flow in a one dimensional, semiinfinite, homogeneous medium. The diffusivity increases with the increase of moisture content in the surface layers of the soil. The results of diffusivity obtained from these methods agree well with those of past workers obtained by other techniques.     

Development characteristics of cloud-to-ground lightning with multiple grounding points

Using the optical images of a cloud-to-ground lightning flash with multiple grounding points obtained by a highspeed video system in the Qinghai Province of China along with synchronous radiated electric field information, the propagation characteristic and the electric field change features of the leaders and the grounding behavior of discharge channels are analyzed.In addition, the two-dimensional velocity of the leader was estimated and its correlation with the time interval of the corresponding subsequent return stroke, and that with the peak current of return stroke are investigated. The results show that the average distance between the three obvious grounded points of the first return stroke channel is about 512.7 m, and the average time interval between the pulses of the corresponding electric field fast changes is 3.8 μs. Further, the average time interval between electric field pulses from the stepped leader is smaller than that of normal single grounding lightning. The observed lightning in our study has two main channels, namely the left and right channels. Based on our observations, it is clear that the dart leader comes close to the ground in case of the left channel after the first return stroke, but it fails to form a return stroke.However, the right channel exhibits a relatively rare phenomenon in that the subsequent return stroke R2 occurred about 2.1 ms after the dart leader arrived at the ground, which was unusually long; this phenomenon might be attributed to the strong discharge of the first return stroke and insufficient charge accumulation near the grounded point in a timely manner. The two-dimensional velocities for the stepped leader of the two main channels are about 1.23×105 and 1.16×105 m s-1, respectively. A sub-branch of stepped leader for the left channel fails to reach the ground and develops into an attempt leader eventually; this might be attributed to the fact that the main branch connects considerably many sub-branches, which leads to the instantaneous decline of the potential difference between the sub-branch and ground. Furthermore, it might also be because the propagation direction of this sub-branch is almost perpendicular to the atmospheric electric field direction, which is not conducive to charge transfer. The two-dimensional velocities for the dart leaders of five subsequent return strokes are all in the normal range, and they positively correlate with the peak current of the subsequent return stroke.     

HF radiation emitted by chaotic leader processes

This paper presents direct measurements of narrowband 10 MHz HF radiation from so-called “chaotic leaders” associated with subsequent return strokes. Although the term is controversial and poorly defined, we find that more than 30% of subsequent strokes in close lightning flashes contain electric field characteristics that are best described as “chaotic”. In earlier studies, return strokes have consistently been observed to be the strongest sources of HF radiation, but the results for leader processes are less consistent. We also observe return strokes to be the main HF emitter, and the leaders before the first return stroke in a flash sequence also emit HF though somewhat less intensely. The leaders preceding subsequent strokes typically emit little or no HF radiation, whether they are dart or dart-stepped leaders.However, it was observed that the presence of a chaotic component increases the leader HF intensity dramatically Defining the HF intensity unequivocally can be problematic for processes like chaotic leaders which have a combination of continuous and impulsive phenomena. Two time-domain methods were used to measure the HF intensity, the peak energy and the RMS energy. In the frequency domain these correspond to the energy spectral density (ESD) and power spectral density (PSD), respectively.It was found that the methods are not necessarily compatible. Thus, it is suggested that to clarify future work, leader processes should be characterized by the PSD rather than the ESD.     

Observations on the leader-return stroke of cloud-to-ground lightning with the broadband interferometer

Radio frequency observations of cloud-to-ground lightning (CG) were made in 1999 in Guangdong Province with the broadband lightning interferometer. In this paper, radiation source locations and electric field waveforms are analyzed for different types of breakdown events, including the preliminary breakdown of in-cloud activities, the stepped leaders of initial strokes to ground and activities during and following return strokes. It is shown that the structure and development of lightning discharges and associated breakdown processes can be reconstructed by using this new type of lightning radiation source location system. The detectable radiation of lightning was primarily produced by the negative breakdown process. The channel was concentrated with few branches during the preliminary breakdown stage of CG lightning flashes. The radiation sources appeared generally at the tip of the channel. During the late period of the stepped leader, the radiation sources were dispersed with branches extended away from the main channel. The radiation sources were in a certain length segment of the channel and the altitude of the segment descended along with the propagation of the leader to the ground. During the preliminary breakdown and the stepped leader of initial strokes to the ground, a sequence of fast negative streamers were observed to start continually from or farther away the lightning-initiated region and propagate along the developed leader channel, which may supply negative charge that assisted the leader’s development. The progression speed of fast negative streamers was about ten times faster than the average speed of lightning channel.     

基于小波的地闪首次回击辐射场的多重分形分析

利用2002年夏季青海野外观测慢天线电场变化仪资料,应用小波变换模极大方法对地闪首次回击辐射场的检测及多重分形谱特征进行了分析.发现小波分解在小尺度上的时间变异系数在回击主脉冲峰处（回击点）呈现明显的尖峰,据此可对回击点进行快速可靠的检测;地闪回击辐射场多重分形谱可用推广的多重分形二项倍增串级公式比较精确地拟合,最小标度指数及谱宽度平均分别为-011和15,是重要的回击特征参数,而小波及多重分形应是闪电信号处理的重要工具.     

Transient Electric Field in the Mesosphere above a Γ-shape Lightning Stroke

We describe the space–time distribution of the pulsed electric field in the middle atmosphere above a positive Γ-shaped lightning stroke. The channel of such a discharge contains a vertical and a horizontal section. The current wave moves initially vertically and then turns horizontally so that radiation appears from the vertical electric dipole followed by that from the horizontal dipole. Combined with reflection from the perfectly conducting ground, the source provides three subsequent pulses in the atmosphere, with the lag being determined by the finite velocity of the current wave in the Γ-shaped stroke. The pulses are reproduced by reflections from the air-ground and the air-ionosphere interfaces and the waveform resembles the M-component, which is often noted in the negative strokes (e.g. Yashunin et al., J Geophys Res 112:D10109, 2007). The non-stationary fine structure appears in the spatial distribution of electric field, which persists for 2 ms or even more and exceeds the runaway electron threshold. Estimates support the idea of free electron bunching in the mesosphere by the pulsed electric field. Focusing may occur about 10 km away from the point of electron- field interaction; it is delayed by a few ms from the moment of interaction. The data presented might be helpful in realistic modeling of the red sprite formation.     

Modeling of thundercloud VHF/UHF radiation on the lightning preliminary breakdown stage

It is well known that there are two basic parts of electromagnetic emissions from a thunderstorm cloud. The first one is due to a return stroke and the second is generated by microdischarges on the lightning preliminary stage and between successive return strokes. The purpose of this paper is to consider the second part of electromagnetic emissions from thunderstorm clouds in a frequency range from one to hundreds of MHz. A new approach is developed, which is based on a three-dimensional computer simulation of microdischarge activity in thunderstorm clouds. We suggest that microdischarges on the lightning preliminary stage are connected with the growth of internal electric cell structures in a thunderstorm cloud. The characteristic scale of cells ranges from ten to hundred meters. The source of these cells can be a beam-plasma-like instability in the thunderstorm cloud medium where microdischarges appear as a saturation mechanism for this instability. Interaction of neighboring cells leads to the formation of dynamic chains of microdischarges. Following step-by-step computer simulations, we calculate radio emissions from every microdischarge and sum up the wave amplitudes from all intracloud volume at the reception point. The standard model for a separate microdischarge current is adopted, and the electromagnetic radiation is estimated in the far zone. We obtain the waveforms of electromagnetic field, the temporal development of radiation and the number of electromagnetic pulses. We have found that signal statistics and calculated frequency spectra exhibit a universal power-law (fractal) behavior. The results of simulations are found to be in satisfactory agreement with the experimental data, because the model waveforms demonstrate a close similarity to the observed ones. Also the temporal development with the duration of pulse trains from ten to hundreds of microseconds and the microdischarge number rate up to hundreds of thousands per second are in agreement with the corresponding experimental data.     

青藏高原那曲地区雷电特征初步分析

通过对2002年夏季青藏高原那曲地区雷暴过程及闪电观测资料的初步分析,发现该地区雷暴电荷结构具有多样性和复杂性,地闪明显偏少. 对高原地闪的一些基本特征参量的统计分析表明,无论正地闪还是负地闪梯级先导前都具有持续时间较长的云内放电过程,地闪以单次回击为主. 与中低纬度地区相比,高原地闪中正地闪比例明显要高,为33髎;负地闪为67髎;正、负地闪回击后常常伴随短时间的连续电流.     

Characteristics of chaotic pulse trains generated by lightning flashes

Pulse trains of irregular pulse characteristics and erratic amplitudes are observed in lightning flashes. These are termed “chaotic pulse trains” in this study. The average width of the individual pulses of these pulse trains is in the range of a few microseconds, with the lower limit of individual measurements extending into the sub-microsecond region. The typical pulse separation lies in the range of 2–. The pulse duration shows a consistent distribution with the most probable value at 400–. Chaotic pulse trains occur most often immediately prior to subsequent strokes. However, they occur also a few milliseconds to several tens of milliseconds prior to return strokes and sometimes do not precede return strokes at all. Twenty-six percent of the total subsequent strokes recorded in this study were preceded by chaotic pulse trains. A few of these pulse trains have occurred in a sequence of several tens of millisecond intervals in between return strokes. We have also found them to occur within very intense cloud flashes. These chaotic pulse trains have features that are different from those of other known pulse trains reported in the lightning literature. Hence, they may be caused by a physical process that has not yet been identified.     

Artificially triggered lightning and its characteristic discharge parameters in two severe thunderstorms

The lightning-induced-damages in the mid-latitude regions are usually caused during severe thunder-storms. But the discharge parameters of natural lightning are difficult to be measured. Five lightning flashes have been artificially triggered with the rocket-wire technique during the passage of two severe thunderstorms. The discharge current and close electric field of return stroke in artificially triggered lightning have been obtained in microsecond time resolution by using current measuring systems and electric field change sensors. The results show that the five triggered lightning flashes include 1 to 10 return strokes, and the average return stroke current is 11.9 kA with a maximum of 21.0 kA and a mini-mum of 6.6 kA, similar to the subsequent return strokes in natural lightning. The half peak width of the current waveform is 39 μs, which is much larger than the usual result. The peak current of stroke Ip (kA) and the neutralized charge Q(C) has a relationship of Ip = 18.5Q0.65. The radiation field of return stroke is 5.9 kV·m-1 and 0.39 kV·m-1 at 60 m and 550 m, respectively. The radiation field decreases as r -1.119 with increase of horizontal distance r from the discharge channel. Based on the well-accepted transmission line model, the speed of return stroke is estimated to be about 1.4×108 m·s-1, with a variation range of (1.1―1.6)×108 m·s-1. Because of the similarities of the triggered lightning and natural lightning, the results in this article can be used in the protection design of natural lightning.     

Mesospheric bore events in the equatorial middle atmosphere

An all-sky CCD imager designed to measure wave structure of the OH, O₂b(0,1) and OI557.7 nm airglow emission layers in the mesosphere and lower thermosphere (MLT) region has been operated near the equatorial region at São João do Cariri (Cariri), Brazil, (7.5°S, 36.5°W). A large number of gravity wave was observed from September 2000 to September 2002 and among them 64 wave events were identified as mesospheric bores. The bore front shows a horizontal extension greater than 1000 km, and observed in the airglow layers as a complementary brilliance between the three emissions. At the first time mesospheric bore events were observed and analyzed in the equatorial region. Their predominant characteristics, occurrence, local time dependency, morphology and propagation direction will be presented and discussed.     

Growth and evolution of an emergent tuff cone: Considerations from structural geology, geomorphology and facies analysis of São Roque volcano, São Miguel (Azores)

The syn-eruptive and post-eruptive history of São Roque tuff cone, its geological setting and volcanological features were studied in detail to understand the role played by the different factors that contributed to the morphological evolution of this relatively simple and small volcanic edifice.In addition, attention was also focused on the series of natural changes that affected the tuff cone during the course of the years and that finally led to its structural disassembly. A novel model is proposed to explain this process.The São Roque volcanic centre, located on the island of São Miguel (Azores), consists of two well-consolidated bodies and numerous small islets that formed more than 4700 years ago during the hydromagmatic activity that took place along an intruding dyke, whose NNW–SSE trend is in agreement with the regional tectonic pattern. The eruptive vents probably migrated progressively from SSE to NNW, forming small edifices through the rapid accumulation of sediments during alternating phases of “dry” and “wet” magmatic emissions. Syn-eruptive partial collapses greatly modified the original morphological structure of these edifices, probably allowing sea water to continuously flow into the vents. The complex interaction of these factors controlled the depth of magma fragmentation, producing different types of deposits, in which the ash-lapilli ratio varies considerably. The high-water saturation degree of these deposits caused syn-eruptive and post-eruptive remobilization which resulted in collapses and some small-scale landslides.Post-eruptive, WNW–ESE trending transtensional and extensional tectonic activities operated during the initial dissection of the cone, generating instability. Furthermore, the rapid accumulation of “wet” tephra, and its following consolidation, caused selective collapses that favoured the fragmentation of the deposit and caused the formation of numerous islets separated by radially-arranged channels. Collapses also involved the lava units emplaced in more recent times around the tuff cone, which show that brittle deformation has been significant in the area for a prolonged period.     

Propagation effects on the electric field time derivatives generated by return strokes in lightning flashes

The effects of propagation over finitely conducting ground on the features of radiation component of the electric field time derivatives are investigated. The results show that the peak, the half-width and the risetime of the electric field time derivative change significantly in propagating over finitely conducting ground. Furthermore, any correlation that may exist between various parameters could also change significantly due to propagation effects. Consequently, in return stroke model validations using experimentally measured fields, remote sensing of return stroke current time derivatives using measured electric field time derivatives and in the calculation of induced voltages generated by lightning flashes in electrical installations the distortions caused by propagation effects on the electric field time derivatives cannot be neglected.     

On the mechanism of X-ray production by dart leaders of lightning flashes

Radiation with energies up to about 250 keV associated with the dart leader phase of rocket-triggered lightning were reported by Dwyer et al. (2004). The mechanism of X-ray generation by dart leaders, however, is unknown at present. Recently, Cooray et al., in pressPlease provide complete bibliographic details for Ref. cooray e al., in press and Saleh et al., in press if available., Cooray et al., 2009a developed physical concepts and mathematical techniques necessary to calculate the electric field associated with the tip of dart leaders. We have utilized the results of these calculations together with the energy dependent frictional force on electrons, as presented by Moss et al. (2006), to evaluate the maximum energy an electron will receive in accelerating in the dart-leader-tip electric field. The main assumptions made in performing the calculations are: (a) the dart leader channel is straight and vertical; (b) the path of the electrons are straight inside the channel; and (c) the decay of the channel temperature is uniform along the length of the dart leader. In the calculation, we have taken into account the fact that the electric field is changing both in space and time and that the gas in the defunct return stroke channel is at atmospheric pressure and at elevated temperature (i.e. reduced gas density). The results of the calculation show that for a given dart leader current there is a critical defunct-return-stroke-channel temperature above which the cold electron runaway becomes feasible. For a typical dart leader, this temperature is around 2500 K. This critical temperature decreases with increase in dart leader current. Since the temperature of the defunct return stroke channel may lie in the range of 2000–4000 K, the results show that the electric field at the tip of dart leaders is capable of accelerating electrons to MeV energy levels.     

The geoid in southeastern Brazil and adjacent regions: new constraints on density distribution and thermal state of the lithosphere

Least-squares collocation technique was used to process regional gravity data of the SE South American lithospheric plate in order to map intermediate (10–2000 km) wavelength geoid anomalies. The area between 35–10° S and 60–25° W includes the Paraná CFB Province, the Southern São Francisco Craton and its marginal fold/thrust belts, the Brazilian continental margin and oceanic basins. The main features in the geoid anomaly map are: (a) Paraná CFB Province is characterized by a 1000 km long and 500 km wide, NE-trending, 9 m-amplitude negative anomaly which correlates with the distribution of sediments and basalts within the Paraná basin. (b) A circular (600–800 km in diameter) positive, 8 m-amplitude geoid anomaly is located in the southern S. Francisco craton and extends into the northeastern border of the Paraná CFB Province. This anomaly partially correlates with Alto Paranaíba Igneous Province (APIP), where alkalic volcanism and tholeiitic dikes of ages younger than 80 Ma are found and where a low-velocity zone in the mantle has been mapped using seismic tomography. This positive geoid anomaly extends towards the continental margin at latitude 21° S and joins a linear sequence of short wavelength positive geoid anomalies associated with Vitoria–Trindade seamounts. (c) A NE-trending, 1000 km long and 800 km wide, 4 m-amplitude, positive geoid anomaly, which is located along the southeastern coast of Brazil, from latitude 24 to 35° S. The northern part of this anomaly correlates with the Ponta Grossa Arch and Florianopolis dyke swarm provinces. The age of this intrusive volcanism is 130–120 Ma. (d) A circular positive anomaly with 9 m of amplitude, located over the Rio Grande and Uruguay shields and offshore Pelotas basin. Few alkaline intrusives with ages between 65 and 80 Ma are found in the region and apatite fission track ages in basement rocks indicates cooling at around 30 Ma. A semi-quantitative analysis of the observed geoid anomalies using isostatic considerations suggests that the mechanism which generated Paraná CFB Province did not change, in a significant manner, the lithospheric thermal structure, since the same geoid pattern observed within this province continues northward over the Neoproterozoic fold/thrust belts systems separating the São Francisco and Amazon cratons. Therefore, this observation favours Anderson’s idea of rapid basaltic outpouring through a pull-apart mechanism along a major suture zone. A thermal component may still be present in the Southern São Francisco Craton and in the Rio Grande Shield and contiguous continental margins, sites of Tertiary thermal and magmatic reactivations.     

Narrow positive bipolar radiation from lightning observed in Sri Lanka

Narrow positive bipolar pulses (NPBPs), whose origin largely remains unknown as yet, have occasionally been noticed in Sri Lanka. These discharges are found to be opposite in polarity to that of negative return strokes, and are found to occur at the beginning and active stage of thunderstorm activities in Sri Lanka. They are emitted from the thunderstorms that produce other activities also and are relatively narrower, bipolar and isolated in nature. They are neither preceded by leader-type pulses nor succeeded by the subsequent activities and hence, could not be associated with any other known activity. Similar radiations have previously been identified and termed as NPBPs. Their features are indeed unique and different from the other known thunderstorm electrical processes. These events have been found to be accompanied by the HF radiations at 5 and 10 MHz, at the trailing part of the pulse. However, such narrow, bipolar and isolated events have not been observed in Sweden, during a lightning measurement campaign at Uppsala, in 2006. So, it is speculated that the meteorological conditions to be responsible for the thunderstorm activity to produce such pulses. Features of such pulses recorded on different thunderstorm days in 2005 and 2006 have been analyzed and presented in this study.The average rise time (10–90%) (Tr), of the pulses was found to be 2.6 μs, the average zero crossing time (Tz) was found to be 5.85 μs, the average duration of slow front (Ts) was found to be 1.86 μs, and the average ratio of amplitude of overshoot to the corresponding peak amplitude (Os/Pa) of these pulses was found to be 0.39.     

Some observations of bipolar flashes during summer thunderstorms near Warsaw

Lightning discharges monitored by the SAFIR network system in Poland have been additionally identified over the 100×100 km area near Warsaw by single-point independent recordings of electric field and Maxwell current rapid changes. The data collected in summer thunderstorm days of 2002 showed some untypical properties of the lightning discharges which are rarely observed. Especially remarkable was a number of ground multi-stroke flashes with the return strokes (RS) which transported to the earth charges of opposite signs. Bipolar flashes (BF) of this kind were mostly involved in the events in which the nearby intracloud (ic) and cloud-to-ground (c-g) discharges were very closely associated in time. Events of such a close collocation of two different types of lightning discharges, previously called the complex lightning discharge events (CLDE), were quite often observed during summer thunderstorms in Poland. The events of this kind, i.e. 8 flashes, identified by the SAFIR detection system as BF’s present the multiple stroke flashes of the mean horizontal separation distance between striking points of particular RS equal to (2.8 ± 2.1) km and of the mean time interval between strokes of (46.8 ± 74.4) ms. The time separation between the observed BF and the adjacent ic flashes was from 0.1 to 335 ms, and horizontal separation distance between them ranged from 1.8 to 14.5 km. The multiplicity of the recorded BF’s ranged from 2 to 4 strokes. Four of these BF’s followed the ic discharge, but the other three preceded the ic and one was alone with no close ic.     

基于闪电双极性窄脉冲事件波形观测日出日落电离层D层变化

利用闪电作为辐射源来探测电离层D层是近年来国外学者研究的热点.本文基于江淮流域六站同步闪电观测站网,实现了一种利用闪电双极性窄脉冲事件（Narrow Bipolar Events,NBE）来探测电离层D层等效高度的方法.基于此方法,对两次分别发生在日出和日落阶段的雷暴分析结果显示,上述两个阶段D层反射高度变化特征呈现显著的不对称性：日出期间D层反射高度随时间线性降低速率为5.9 km/h;而日落期间D层反射高度随时间线性递增速率为8.6 km/h.综合日间太阳耀斑期间D层反射高度剧烈波动的观测事实,与日出、日落期间D层特征变化,结果表明太阳辐射电离中性大气分子的电子生成机制在日间D层电子密度变化中占主导地位.本文结果展现了利用NBE事件监测电离层D层变化特征的可行性,这一方法与基于地闪回击波形的D层探测方法结合在一起,有望把现有具有闪电事件定位和电磁波波形记录能力的闪电观测站网扩展为实时对电离层D层时空变化监测的网络.