Using a low‐frequency lightning location system comprising 11 sites, we located preliminary breakdown (PB) processes in 662 intracloud (IC) lightning flashes during the summer of 2013 in Osaka area of Japan. On the basis of three‐dimensional location results, we studied initiation altitude and upward propagation speed of PB processes. PB in most IC flashes has an initiation altitude that ranges from 5 to 10 km with an average of 7.8 km. Vertical speed ranges from 0.5 to 17.8 × 105 m/s with an average of 4.0 × 105 m/s. Vertical speed is closely related with initiation altitude, with IC flashes initiated at higher altitude having lower vertical speed during PB stage. Characteristics of PB pulse trains including pulse rate, pulse amplitude, and pulse width are also analyzed. The relationship between pulse rate and vertical speed has the strongest correlation, suggesting that each PB pulse corresponds to one step of the initial leader during the PB stage. Pulse rate, pulse amplitude, and pulse width all show decreasing trends with increasing initiation altitude and increasing trends with increasing vertical speed. Using a simple model, the step length of the initial leader during the PB stage is estimated. Most of initial leaders have step lengths that range from 40 to 140 m with an average of 113 m. Estimated step length has a strong correlation with initiation altitude, indicating that leaders initiated at higher altitude have longer steps. Based on the results of this study, we speculate that above certain altitude (~12 km), initial leaders in PB stages of IC flashes may only have horizontal propagations. PB processes at very high altitude may also have very weak radiation, so detecting and locating them would be relatively difficult.

Discharge heights of thousands of narrow bipolar events (NBEs) observed in Guangzhou and Chongqing of China are calculated using time delays between the direct wave signals of NBEs and their ionospheric reflection pairs. The result shows that most positive NBEs occur between 8 and 16 km while most negative NBEs occur between 16 and 19 km. Very few negative NBEs are above 19 km or below 14 km. It is inferred that positive NBEs are produced between main negative charge layer and upper positive charge layer while negative NBEs are produced between upper positive charge layer and negative screening charge layer at the cloud top. Variations of NBE discharge heights in two thunderstorms are analyzed. It seems that NBEs can be produced at any position between corresponding charge layers. Positive NBEs are generally higher in the periods when negative NBEs are also occurring. For a given short time period in a single thunderstorm, negative NBEs are always observed to occur at a higher altitude than positive NBEs, indicating a dividing charge layer between positive NBEs and negative NBEs. The possibility of some NBEs as upward discharges from cloud tops mentioned by previous studies is discussed. Supported by multiple evidences, we believe such possibility is very low; instead, NBEs are produced in vigorous convective surges that develop to the height comparable to the discharge height of NBEs. Differences in height distributions in Guangzhou and Chongqing are analyzed and a hypothesis is put forward that both positive NBEs and negative NBEs can only be produced above certain height. The relationship between this hypothesis and the mechanism for NBE production is discussed.

Previous observations show that some narrow bipolar events (NBEs) can initiate intracloud discharges, but the role of NBE as lightning initiation is still unclear. During the summer of 2013, 827 NBEs were detected with a 3-D LF lightning location system in Osaka, Japan. Out of 638 positive NBEs, 103 occurred as the initial events of lightning flashes. These initiator-type NBEs, called "INBEs" in this paper, are always followed by positive pulse trains whose locations show upward propagations probably from the main negative charge region to the upper positive charge region. Most of INBEs develop into intracloud flashes. Only two INBEs develop into positive ground flashes and five INBEs develop into negative ground flashes. Pulse widths and peak amplitudes of electric field change waveforms of INBEs are almost the same as those of normal NBEs. A major difference is that INBEs have much lower discharge heights. Most of INBEs are lower than 10 km while normal NBEs are mainly higher than 10 km. Characteristics of positive pulse trains following INBEs are closely related with discharge heights of INBEs. Higher INBEs are usually followed by weaker, fewer, and less frequent positive pulses with slower upward propagations. As the height increases to above 10 km, NBEs are usually no longer followed by such positive pulses.

No abstract

The 2-D transition metal dichalcogenide (TMD) semiconductors, has received great attention due to its excellent optical and electronic properties and potential applications in field-effect transistors, light emitting and sensing devices. Recently surface plasmon enhanced photoluminescence (PL) of the weak 2-D TMD atomic layers was developed to realize the potential optoelectronic devices. However, we noticed that the enhancement would not increase monotonically with increasing of metal plasmonic objects and the emission drop after the certain coverage. This study presents the optimized PL enhancement of a monolayer MoS2 in the presence of gold (Au) nanorods. A localized surface plasmon wave of Au nanorods that generated around the monolayer MoS2 can provide resonance wavelength overlapping with that of the MoS2 gain spectrum. These spatial and spectral overlapping between the localized surface plasmon polariton waves and that from MoS2 emission drastically enhanced the light emission from the MoS2 monolayer. We gave a simple model and physical interpretations to explain the phenomena. The plasmonic Au nanostructures approach provides a valuable avenue to enhancing the emitting efficiency of the 2-D nano-materials and their devices for the future optoelectronic devices and systems.

Fast Antenna Lightning Mapping Array (FALMA), a low‐frequency lightning mapping system comprising an array of fast antennas, was developed and established in Gifu, Japan, during the summer of 2017. Location results of two hybrid flashes and a cloud‐to‐ground flash comprising 11 return strokes (RSs) are described in detail in this paper. Results show that concurrent branches of stepped leaders can be readily resolved, and K changes and dart leaders with speeds up to 2.4 × 107 m/s are also well imaged. These results demonstrate that FALMA can reconstruct three‐dimensional structures of lightning flashes with great details. Location accuracy of FALMA is estimated by comparing the located striking points of successive RSs in cloud‐to‐ground flashes. Results show that distances between successive RSs are mainly below 25 m, indicating exceptionally high location accuracy of FALMA.

[1] Prior studies have found that positive and negative compact intracloud discharges (+CIDs and −CIDs, according to the physics sign convection) occur at distinctly different altitudes, which correspond to different regions in a thunderstorm. On the basis of a large number of CIDs of both polarities recorded by our VLF/LF lightning location network, characteristic differences between +CIDs and −CIDs are discussed in this study. The results reveal that −CID is a more special type of discharge. Compared with +CIDs, −CIDs produce larger electric field changes on average, and they are more isolated from other discharge processes. A locating method based on ionospheric reflection pairs of CIDs is developed, which confirms that −CIDs do occur at higher altitudes. The relationship between CIDs and convective strength is also analyzed. Out of nine storms analyzed in this study, eight produce fewer −CIDs than +CIDs. The percentage of −CIDs seems to increase with the convective strength. Although −CIDs are relatively rare, their occurrences are more temporally compact; that is, a large portion of −CIDs are produced in a very short period. +CIDs also have this characteristic, but it is not as pronounced as that of −CIDs.

Using a low-frequency lightning location system comprising nine stations, we have observed and analyzed 374 large and bipolar electric field change waveforms that occurred during the winter of 2012-2013. Since the waveforms are different from those produced by any well-studied lightning discharge processes, we refer to these source discharge events using a new name: large bipolar events (LBEs). LBEs can be characterized by the following features: (1) All have the same polarity as negative return stroke. (2) All exhibit a single bipolar pulse with a pulse width around 15 μs and similar positive and negative cycles. (3) All are located on the land along the Japan Sea coast, indicating they are probably associated with high grounded objects. (4) Most LBEs produce very large electric field changes that are even larger than that of positive and negative return strokes. (5) Most LBEs are temporally isolated within several milliseconds but are frequently followed by intracloud discharges after tens of milliseconds. (6) Most LBEs produce a single well-distinguished ionospheric reflection pulse, and the time difference between LBE pulse and the corresponding reflection pulse can be used to calculate ionospheric reflection height. It is speculated that LBE is a type of powerful and transient lightning discharge event produced within a compact region of strong electric field formed when the negative charge layer in thunderclouds is very close to the top of a tall grounded object.