Plasma wave evidence for lightning on Venus

Plasma wave data from the Pioneer Venus Orbiter provide the largest body of data cited as evidence for lightning on Venus. These data are also the most controversial, mainly because of the ambiguity in mode identification due to limited spectral information. We review some of the more recent studies of the plasma wave data at Venus, and we demonstrate that the characteristics of the 100 Hz waves are consistent with whistler-mode waves propagating vertically from below the ionosphere. We further show that in situ instabilities are too weak to generate whistler-mode waves, mainly because the thermal pressure is comparable with the magnetic field pressure in the ionosphere of Venus. The lower hybrid drift instability has also been suggested as an alternative source for the 100 Hz waves. However, the wave properties are more consistent with whistler-mode propagation; the lower hybrid drift instability requires very short gradient scale lengths to overcome damping due to collisions. We also note that an apparent association between Langmuir probe anomalies and 100 Hz waves is much lower than previously reported, once we apply a consistent intensity threshold for identifying wave bursts. The lightning hypothesis remains the most probable explanation of the plasma waves detected at low altitudes in the nightside ionosphere of Venus.     

Wave form and spectral distribution of the electromagnetic field of lightning currents

In a resonant wave guide model of lightning currents, two impulse type standing waves can exist: aperiodic waves of the Bruce-Golde form (type 1), and damped oscillations (type 2). The electromagnetic waves generated by these two types of lightning currents are calculated for various distances and compared with observations. It is shown that the measured wave forms of sferics at distances smaller than about 300 km of return strokes (R strokes) as well as of intracloud strokes (K strokes) are generated mainly by type 2 lightning currents. The channel parameters like channel length and channel diameter derived from the observed sferics are 19 km and 4.6 cm, respectively, for the average R stroke, and 4 km and 1.6 cm for typical K strokes. The large values of the lengths probably correspond to the real lengths of oblique and tortuous channels, rather than to their vertical elevation. The finite electric conductivity of the earth modifies the high frequency component of the wave forms. With decreasing conductivity and/or increasing distance, the rise times of the radiation component to its first maximum increases and the maximum amplitude decreases.Typical rise times for R-strokes are about 3 μs consistent with the observations if the electric conductivity of the earth is of the order of 10⁻³ S/m. The spectral functions of the wave forms are also calculated. The spectral amplitude of the average type 2 R-stroke has its maximum near 4 kHz, and that of the type 2 K-stroke maximizes near 35 kHz. Within the high frequency region at frequencies greater than about 300 kHz. the spectral amplitudes decay proportional to the reciprocal third power of the frequency. The radiation component in the far field contains 7% (16%) of the total electromagnetic energy of type 2 R (K) strokes.     

Some remarks about a ball lightning model

Two criticisms of a model for explaining the nature of ball lightning are discussed. It is shown why these criticisms are groundless. Furthermore, by considering the topics in question in a wider context, the reliability of the model is further substantiated. Comparisons between model expectations and ball lightning reports, including laboratory ball lightning, are considered and again the model turns out to be in agreement with observations.     

On the polarity and continuing currents in unusually large lightning flashes deduced from ELF events

An analysis of ELF events has enabled the magnitude and duration of the continuing currents in 274 unusually large lightning flashes to be estimated. An exponential decay model of the variation of the current moment of the source lightning stroke with time was fitted to the measured spectra using an optimisation algorithm. Of the events analysed, 212 were of positive polarity (i.e. the initial polarity of the electric field was positive) and 62 were negative. The average magnitude of the peak current moment for positive events was 5.1 × 10⁷ Am with a standard deviation of 3.2 × 10⁷ Am while that for negative events was 2.9 × 10⁷ Am with a standard deviation of 0.6 × 10⁷ Am. The average time constant T of our data set is 32 ms with a standard deviation of 6 ms for positive events and 26 ms with a standard deviation of 5 ms for negative events. Positive events are therefore associated with somewhat larger and more variable current moments and slightly longer continuing currents. We believe that these positive events are most likely to be produced by positive cloud to ground lightning.     

A numerical model for gravity wave dissipation in the thermosphere

Two simplified models of internal gravity wave dissipation due to viscosity, thermal conduction and ion-drag, in a multilayered, isothermal thermosphere are developed. Each of these models uses the WKB approximation, ray theory and the time-averaged equation of energy conservation, but whereas one of the models (A) employs all of the gravity wave equations appropriate to a dissipative atmosphere, the other (B) does not. Results derived from these models for one particular wave are compared to each other and also to some previously published results of Klostermeyer, which employed a full-wave, model. A breakdown of the WKB approximation in the lower, non-isothermal thermosphere leads to models A and B underestimating the total dissipation there. In the middle thermosphere model A estimates the dissipation reasonably well, while model B grossly overestimates the dissipation. In the upper thermosphere model A underestimates the total upward energy flux, probably as a result of the neglect of coupling into the dissipative waves at these levels, while no energy remains in model B. Results from model A show that when dissipation due to viscosity and thermal conduction are included correctly and simultaneously, the dissipation due to viscosity can exceed that due to thermal conduction by a factor of three. It is argued that ray theory may either overestimate or underestimate the energy flux reaching the upper boundary of a thermospheric model depending on both its height and the particular thermospheric model used.     

A spherical model of wave generation in the atmosphere by the solar terminator

With the help of equations written in a spherical coordinate system moving with the terminator and which describe acoustic-gravitational wave generation, the general solution for pressure field perturbation is obtained. A spherical model of the solar terminator is presented. Types of nonstationary waves generated by the terminator when it crosses atmosphere inhomogeneities have been studied.     

A possible way to trigger lightning using a laser

To study the possibility of triggering lightning with a laser plasma, laboratory laser-induced discharge experiments have been carried out and the following results were obtained. Both long straight and zigzag laser induced discharges between rod-rod electrodes were realized and the effective guiding effect of laser channels for electrical discharges was confirmed. With plane-plane electrode configuration, the investigation concerning the necessary condition for initiating and guiding a streamer reveals that the necessary electric field for initiating a streamer with a laser plasma depends closely on the absorbed energy used to produce laser plasma channels, the minimum electric field is about 200 kV/m, and that the necessary electric field for guiding a streamer by a laser plasma is lower than that for initiating a streamer and the minimum electric field is about 170 kV/m.We propose to use the local electric field near the top of a grounded tower as the necessary field for a laser-produced channel to trigger an upward leader and have proved that this is feasible.     

A user's guide to scintillation

During the past four decades scintillation methods have been used for remote-sensing distant plasmas and for providing high angular resolution in radioastronomy. This brief review illustrates some of the techniques employed and explains the underlying theory in simple physical terms; it is not intended to be a complete survey of all applications of scintillation.     

Electrical characteristics of winter lightning

To make clear the characteristics of winter lightning flashes, the long term observation of winter lightning on the west coast of Japan has been continued using magnetic links, digital recording systems for the current oscillograms, field mills, still cameras and video camera systems for the images of lightning channels. Of the 66 magnetic links records exceeding 2 kA, 73% were negative in polarity, and 27% were positive. Median peak values for winter flashes were 17 kA for negative flashes and 45 kA for positive flashes, respectively. 145 current waveforms for the winter lightning flashes, which have current amplitudes exceeding 1 kA, have been obtained by the shunt systems and/or the coils system. They show that winter lightning flashes often have a very long duration or continuing current, and sometimes have a very large amplitude exceeding 200 kA in positive flashes. As one example flash on 9 January 1987, the maximum current amplitude was +280 kA, the maximum current derivative 1.0 × 10¹⁰ As⁻¹, the total charge +400 C and the action integral 1.5 × 10⁷ A²s. The winter lightning current waveforms are classified into three types: single stroke flashes, monopolar multiple stroke flashes and bipolar flashes. Moreover, each flash is categorized as positive or negative, single peak or multiple peak, and with or without a continuing current.     

Waveguide model analysis of VLF wave propagation at 16 kHz

Preliminary results of measurement of phase and amplitude of 16 kHz VLF signals propagated in the Earth-ionosphere waveguide are presented. From the measured values of diurnal phase, amplitude changes and sunrise fadings the relevant waveguide parameters are estimated.     

A chronological guide to embossed Lipton Tea tins

Embossed Lipton Tea tin cans are a ubiquitous form of material culture found on archaeological sites internationally wherever Lipton Tea was sold (the present author is based in Alaska). Many of the tins dating from the first half of the 20th century once exhibited paper labels, which almost never survive archaeologically. I instead purchased tins with paper labels, which provide chronological information, on the internet. These, along with dated magazine and newspaper advertisements spanning from the late 19th to mid 20th centuries, allowed for the development of a scheme to date different embossed Lipton Tea tins through time.     

A procedural guide to metacarpal radiogrammetry in archaeology

The technique of metacarpal radiogrammetry was devised in 1960 as a clinical method to aid in the identification of thinned cortical bone, which may help in diagnosis of osteoporosis. Through taking measurements from radiographs, calculations of the amount of cortical bone can be made. The technique is relatively simple and is not destructive. This paper examines the value of the application of this technique to archaeological remains. A standardized methodology of bone assessment is incorporated to enhance the comparability of future research. Statistical tests demonstrate that the technique is robust, as the position of the bone on the film, radiographic enlargement and use of either the left or right metacarpal will not affect the results obtained. However, differences in film type and measurement position can introduce unacceptable variance into the results. Therefore, guidance is given on future applications of the technique. The technique does have value as a widely available, non‐destructive technique for assessing cortical bone loss and the probable prevalence of osteoporosis in archaeological collections. Copyright © 2004 John Wiley & Sons, Ltd.     

基于事故树的布达拉宫雷击火灾风险分析与保护

布达拉宫自兴建以来曾遭受多次不同程度雷击,一旦雷击引发火灾会造成不可挽回的损失,因此需要进行科学合理的雷击火灾分析以有效指导布达拉宫减灾工作。对于布达拉宫雷电可能引起火灾的风险,建立布达拉宫雷击火灾事故树,综合考虑布达拉宫因雷电引发火灾事件发生的可能过程、可能途径。最后对雷击火灾事故树最小割集、最小径集、基本事件的结果重要度进行比较,同时分析布达拉宫地区雷电活动规律。分析结果表明:布达拉宫雷击火灾事故树一共有22个基本事件,有308种可能导致布达拉宫因雷击引起火灾事件发生的途径。布达拉宫雷击火灾防护是一项复杂工程。研究表明,最便于采取的防护措施是加强其雷电防御能力,如清除引雷隐患、安装电涌保护器、进行等电位连接等。     

Current and velocity of the return stroke lightning

An expression for the current flowing through the return stroke channel is derived from the actual flow of charge across the tip of the channel. From the current model a double exponential velocity expression is obtained. The consequence of this double exponential velocity expression for the spectrum of atmospherics is discussed. The variation of current and velocity with increasing length of channel is also discussed and appropriate expressions for the same are obtained.     

Artifacts of the neurosciences: A resource guide to museums and collections

ABSTRACT

Medical museums and collections care for important artifacts relating to the history of the neurosciences across the United States, the United Kingdom, and Europe. This essay highlights the collections and galleries of greatest interest and worth a visit. It also provides a list of online directories of medical museums and bibliography of related publications.