Z. Kawasaki

Attachment process in rocket-triggered lightning strokes

In order to study the lightning attachment process, we have obtained highly resolved (about 100 ns time resolution and about 3.6 m spatial resolution) optical images, electric field measurements, and channel-base current recordings for two dart leader/return-stroke sequences in two lightning flashes triggered using the rocket-and-wire technique at Camp Blanding, Florida. One of these two sequences exhibited an optically discernible upward-propagating discharge that occurred in response to the approaching downward-moving dart leader and connected to this descending leader. This observation provides the first direct evidence of the occurrence of upward connecting discharges in triggered lightning strokes, these strokes being similar to subsequent strokes in natural lightning. The observed upward connecting discharge had a light intensity one order of magnitude lower than its associated downward dart leader, a length of 7–11 m, and a duration of several hundred nanoseconds. The speed of the upward connecting discharge was estimated to be about 2 × 107 m/s, which is comparable to that of the downward dart leader. In both dart leader/return-stroke sequences studied, the return stroke was inferred to start at the point of junction between the downward dart leader and the upward connecting discharge and to propagate in both upward and downward directions. This latter inference provides indirect evidence of the occurrence of upward connecting discharges in both dart leader/return-stroke sequences even though one of these sequences did not have a discernible optical image of such a discharge. The length of the upward connecting discharges (observed in one case and inferred from the height of the return-stroke starting point in the other case) is greater for the event that is characterized by the larger leader electric field change and the higher return-stroke peak current. For the two dart leader/return-stroke sequences studied, the upward connecting discharge lengths are estimated to be 7–11 m and 4–7 m, with the corresponding return-stroke peak currents being 21 kA and 12 kA, and the corresponding leader electric field changes 30 m from the rocket launcher being 56 kV/m and 43 kV/m. Additionally, we note that the downward dart leader light pulse generally exhibits little variation in its 10–90% risetime and peak value over some tens of meters above the return-stroke starting point, while the following return-stroke light pulse shows an appreciable increase in risetime and a decrease in peak value while traversing the same section of the lightning channel. Our findings regarding (1) the initially bidirectional development of return-stroke process and (2) the relatively strong attenuation of the upward moving return-stroke light (and by inference current) pulse over the first some tens of meters of the channel may have important implications for return-stroke modeling.

Continuous broadband digital interferometry of lightning using a generalized cross-correlation algorithm

The VHF Broadband Digital Interferometer developed by Osaka University has been improved to allow continuous sampling over the entire duration of a lightning flash and to utilize a generalized cross-correlation technique for determining the lightning source directions. Time series waveforms of 20-80 MHz signals received at three orthogonally located antennas are continuously digitized over multisecond intervals, as opposed to sequences of short-duration triggers. Because of the coherent nature of the measurements, radiation sources are located down into the ambient receiver and environmental noise levels, providing a quantum leap in the ability to study lightning discharge processes. When postprocessed using cross correlation, the measurements provide angular uncertainties less than 1 ! and time resolution better than 1!s. Special techniques have been developed to distinguish between actual lightning sources and noise events, with the result being that on the order of 50,000-80,000 radiation sources are located for a typical lightning flash. In this study, two-dimensional interferometer observations of a classic bilevel intracloud flash are presented and combined with three-dimensional Lightning Mapping Array observations to produce a quasi 3-D map of lightning activity with the time resolution of the interferometer. As an example of the scientific utility of the observations, results are presented for the 3-D progression speed of negative leaders associated with intracloud K-leaders.

Broadband radio interferometer utilizing a sequential triggering technique for locating fast-moving electromagnetic sources emitted from lightning

A broadband radio interferometer to investigate the location of fast-moving electromagnetic sources emitted from lightning discharges has been designed and tested. A sequential triggering technique was applied to the system for recording the data of electromagnetic pulses, once the electromagnetic pulse is detected and its amplitude exceeds a threshold level, the triggering circuit is turned on to record the waveform for 1 microsecond and ready to acquire another pulse afterwards. This technique can record 2000 electromagnetic pulses maximum. We have implemented our system to locate and retrace fast-moving electromagnetic sources emitted from lightning discharges during a field experiment in Australia in December 1997. We succeeded in reconstructing a cloud to ground lightning discharge and displayed it in two dimensions and in time sequences.

Three-Dimensional Localization System for Impulsive Noise Sources Using Ultra-Wideband Digital Interferometer Technique

This paper presents an ultra-wideband (UWB) digital interferometer scheme for locating the impulsive noise radiation from partial discharge (PD) sources in three spatial dimensions. This UWB interferometer localization technique is based on the idea of extracting the phase differences at different frequencies between a pair of the received electromagnetic (EM) signals by the Fast Fourier Transform (FFT). These phase differences are directly related to the direction of arrival of the received signals and used to estimate the angular locations of the source in azimuth and elevation angles. The geometry of the problem is considered as two UWB digital interferometer sites for the experimental measurements. Then, the triangulation scheme was proposed to the 3-D source location. To show the validity and the accuracy of our proposed scheme, a comparison study is shown between time differences of arrival (TDOA) localization scheme which is estimated by conventional cross correlation method and this interferometer proposed localization technique.

Overview and early results of the Global Lightning and Sprite Measurements mission

Global Lightning and Sprite Measurements on Japanese Experiment Module (JEM-GLIMS) is a space mission to conduct the nadir observations of lightning discharges and transient luminous events (TLEs). The main objectives of this mission are to identify the horizontal distribution of TLEs and to solve the occurrence conditions determining the spatial distribution. JEM-GLIMS was successfully launched and started continuous nadir observations in 2012. The global distribution of the detected lightning events shows that most of the events occurred over continental regions in the local summer hemisphere. In some events, strong far-ultraviolet emissions have been simultaneously detected with N2 1P and 2P emissions by the spectrophotometers, which strongly suggest the occurrence of TLEs. Especially, in some of these events, no significant optical emission was measured by the narrowband filter camera, which suggests the occurrence of elves, not sprites. The VLF receiver also succeeded in detecting lightning whistlers, which show clear falling-tone frequency dispersion. Based on the optical data, the time delay from the detected lightning emission to the whistlers was identified as ∼10 ms, which can be reasonably explained by the wave propagation with the group velocity of whistlers. The VHF interferometer conducted the spaceborne interferometric observations and succeeded in detecting VHF pulses. We observed that the VHF pulses are likely to be excited by the lightning discharge possibly related with in-cloud discharges and measured with the JEM-GLIMS optical instruments. Thus, JEM-GLIMS provides the first full set of optical and electromagnetic data of lightning and TLEs obtained by nadir observations from space.

Human Head Modeling for Handset Antenna Design at 5 GHz Band

This paper discusses the human head modeling for the design of handset antennas at 5GHz band. The effect of the human head heterogeneity and shape on the antenna characteristics is investigated thoroughly. In addition, an attempt is also made to realize a compact structure for the human head model. For this purpose, the radiation characteristics of a dipole antenna in the presence of several head models are compared. The numerical investigations reveal that the effect of the heterogeneity and shape of the human head model on the antenna characteristics at 5GHz band is marginal. Particularly, reasonable result can be obtained even for a model of rectangular parallelepiped with the dimensions (24 mm thickness × 120 mm × 120 mm). This volume is 96% smaller than the original realistic model.

Numerical and Experimental Study on Three-Dimensional Localization For Ultra-Wideband Impulsive Noise Sources

This paper presents a three-dimensional (3-D) localization system for ultra-wideband (UWB) impulsive noise sources. The localization algorithm is in terms of time difference of arrival (TDOA). Firstly, a feasibility study of the 3-D localization scheme is carried out numerically. Then, the effectiveness and limitation of the scheme are revealed. Our proposal is validated by measurement in an open site.

Time-domain mathematical model of impulsive EM noises emitted from discharges

Impulsive electromagnetic (EM) noises emitted from discharges have been measured and characterized in ultra-wideband VHF channel. Statistical characteristics of EM pulses emitted from single discharge source are investigated as fundamental study. Firstly, the reproducibility of impulsive EM noises is confirmed by calculating their auto-correlations. The frequency spectrums of such pulses are found to be unique for each source. Next, statistical analysis is performed on the amplitude and inter-arrival time of received EM pulses. It is found that the probability distributions of amplitude and inter-arrival time of EM pulses follow normal and exponential distributions, respectively. Taking into account these ultra-wideband characteristics of impulsive EM noises from discharges, a mathematical model of EM pulses emitted from discharges is developed in the time domain. This is represented as a simple analytic formula with three parameters, which are derived from measurements. This model has been validated via measurement where multiple discharge sources exist.

Leader process in 3D observed by VLF/LF broadband interferometer

We have been designing and developing a 3D lightning location system based on the broadband digital interferometry technique in VLF/LF band. The VLF/LF broadband digital interferometer (VLF/LF DITF) is able to locate lightning discharges such as return strokes, K events, and compact intracloud lightning discharges (CIDs), which are energetic breakdowns within thunderclouds several hundred kilometers away from the VLF/LF DITF. From July 2011 to now, a simultaneous observation of a VLF/LF band DITF and VHF broadband digital interferometers (VHF band DITF) were conducted in Kansai, Japan. We show the time series variation of an intracloud discharge initiated by a preliminary breakdown pulse train (PB), negative cloud to ground flash and intracloud discharge including K-event. VLF/LF DITF successfully located not only energetic breakdowns within thunderclouds but also leader processes.

A VHF broadband interferometer for lightning observation

A VHF broadband interferometer is a system to locate sources of the radiation events at VHF band by extracting the differences of phases at various frequency components of Fourier spectra between a pair of antennas. Lightning images are derived by sensing the electromagnetic waves radiated from lightning discharge process such as negative stepped leader. Using the system, lightning observations have been carried out firstly in Hokuriku area Japan during the onset of the winter season for rocket triggered lightning experiment, then, in Darwin Australia, and so on. Based on the successful results, the interferometer system is deployed on the GLIMS (Global Lightning and sprIte MeasurementS) mission in the International Space Station to detect and locate the VHF impulses emitted by lightning from space.