Preliminary Breakdown Process Research Articles

The Characterization of Electric Field Pulses Observed in the Preliminary Breakdown Processes of Normal and Inverted Intracloud Flashes

We have studied the parameters of preliminary breakdown (PB) pulses in 395 normal and 319 inverted intracloud (IC) flashes observed in Gifu, Japan, and Ningxia, China, respectively, by using a low-frequency mapping system called fast antenna lightning mapping array (FALMA). These parameters are extracted from the first half of the PB pulses. It is found that compared to normal IC flashes, inverted IC flashes exhibited PB pulses with slower rise times (6.8 vs. 3.1 μs), wider half-peak widths (3.8 vs. 2.5 μs), longer zero-crossing times (26.2 vs. 14 μs), and extended fall times (4 vs. 3.2 μs). We further demonstrated that such discrepancies between normal and inverted IC flashes should not be caused by subjective factors, like noise threshold setting, or objective factors, like signal propagation distance. Based on this analysis, finally, we inferred that the discrepancies should be a reflection of the PB channel properties of normal and inverted IC flashes.

A 3D Interferometer-Type Lightning Mapping Array for Observation of Winter Lightning in Japan

We have developed and deployed a 3D Interferometer-type Lightning Mapping Array (InLMA) for observing winter lightning in Japan. InLMA consists of three broadband interferometers installed at three stations with a distance from 3 to 5 km. At each interferometer station, three discone antennas were installed, forming a right triangle with a separation of 75 m along their two orthogonal baselines. The output of each InLMA antenna is passed through a 400 MHz low-pass filter and then recorded at 1 GS/s with 16-bit accuracy. A new method has been proposed for finding 3D solutions of a lightning mapping system that consists of multiple interferometers. Using the InLMA, we have succeeded in mapping a positive cloud-to-ground (CG) lightning flash in winter, particularly its preliminary breakdown (PB) process. A study on individual PB pulse processes allows us to infer that each PB pulse process contains many small-scale discharges scattering in a height range of about 150 m. These small-scale discharges in a series of PB pulses appear to be continuous in space, though discontinuous in time. We have also examined the positive return stroke in the CG flash and found a 3D average return stroke speed of 7.5 × 107 m/s.

Correlation analysis of preliminary breakdown and first return stroke process in negative ground flash as a function of Northeast and Southwest monsoon in tropic

The separation duration of Preliminary Breakdown to the First Return Stroke (PB-RS duration) and the ratio between the highest amplitude pulse of Preliminary Breakdown and the first Return Stroke pulse (PB/RS ratio) were successfully used as a tool by many researchers to determine the relationship between lightning ground flashes and physical changes (latitude, regional and thunderstorm). Malaysia is one of the countries with the highest lightning flash density globally, and the occurrences of ground flashes are mainly due to monsoon seasons. Monsoons in Malaysia are Northeast, Southwest, and two transition periods. However, there is no available information to relate the lightning activities in Malaysia under the influence of all monsoon seasons. The above statements motivated our team to present a new statistical analysis of lightning flashes under two seasons' effects (Northeast and Southwest). 83 out of 332 electric radiation field data (fast fields) from six thunderstorms were selected and analyzed. The data was recorded using a 12-bit high-speed transient recorder (HDO 4024) with a resolution of tens of nanoseconds and sampling of 12.5 MS/s. PB/RS ratio between the monsoons is similar to each other. Whereas, the PB-RS separation, has two factors longer during the Southwest monsoon than during the Northeast monsoon. We classified the PB/RS ratio into four categories (>100%, 50-100%, 20-50%, and < 20%) as an indication of the preliminary breakdown strength. It found that the weakest pulse train (PB/RS ratio is less than 20%) has shown a longer duration of PB-RS and produces higher lightning return strokes in a negative ground flash than the strong one. The results in this study could be evident that the lower strength of preliminary breakdown is correlated to a lower Low Positive Charge Region (LPCR), which means the initial breakdown process only requires low energy to break the LPCR. Hence, these features have shown the highest number of stroke counts compared to pronounced ones. However, it would be more remarkable if one could provide bulky data for this specific study.

Observation of preliminary breakdown and leader propagation characteristics in a negative cloud to ground flash with two strike points

High-speed video and electric field data were used to analyze a preliminary breakdown process and three different types of leader processes in a negative cloud to ground flash with two strike points. The preliminary leader was multi-branched and significantly brighter than the stepped leader, with the speed varying from 1.4 × 106 m/s to 6.4 × 105 m/s in a decreasing trend. Two types of chaotic leaders were proposed and compared. Type 1 chaotic leaders last for approximately 0.3 ms, with speed greater than 107 m/s, and no change in leader type throughout the whole leader process. Type 2 chaotic leaders with a duration of approximately 0.5 ms and speed on the order of 106 m/s transformed into a dart-stepped leader with a continuously decreasing speed in the leader process, and the pulse amplitude of chaotic leaders tended to decrease during the transition. The attenuation of the pulse amplitude in electric field (E-field) and the insufficient amount of charge may be responsible for such a conversion.

Combination of narrow bipolar pulses and attempted leaders in Melaka, Malaysia

<span>This paper presents the characteristic of the positive attempted leaders preceded by positive narrow bipolar pulses (NPBPs). Attempted leaders are the preliminary breakdown process with no subsequent event (return stroke). On the other hand, narrow bipolar pulse is the lightning event commonly isolated and produces a strong electromagnetic field (in a short period). Attempted leaders hardly occurred in the tropics, and the preceding of the NBP (the combination) should be considered unique. In this present study, we found four samples in which the arithmetic means of duration of NPBPs pulse was 32.19 µs, with separation between the positive attempted leader was 1.86 ms apart. For the positive attempted leader, the arithmetic means of the whole pulse train, individual pulse and interval pulse of positive attempted leaders were 3.47 ms, 29.66 µs and 486.53 µs, respectively. The pulse train in this study seems to fade out fast compared to the isolated positive attempted leader pulse train. Next, the NPBP's pulse duration in this study shows well in agreement with the type of isolated NPBPs indicates that the association does not affect its pulse duration characteristic already present in the title.</span>

Initial Results of Long-term Continuous Observation of Lightning Discharges by FALMA in Chinese Inland Plateau Region

We started a long-term continuous observation of lightning discharges in the Chinese inland plateau region using a fast antenna lightning mapping array (FALMA). During the first year of observation, 2019, we recorded lightning discharges on 25 days in Yinchuan city, the capital of Ningxia. Most of the lightning discharges appeared to occur in the afternoons of individual thunderstorm days in August. We studied the cloud-to-ground (CG) flash percentages, lightning discharge source spatiotemporal distributions, and preliminary breakdown (PB) process characteristics for the two thunderstorm cases that produced the most frequent lightning flashes in 2019 over a wide area. It was found that (1) CG flashes in these two thunderstorms accounted for 28.4% and 32.5% of total lightning flashes, respectively; (2) most lightning discharge sources in these two thunderstorms occurred at temperatures between 5 and −30 °C, with a peak at around −10 °C; and (3) more than 90% of well-mapped PB processes of intracloud (IC) flashes propagated downward. By overlapping the altitudes and the progression directions of the PB processes on the lightning source spatiotemporal distributions, we inferred that the main negative charge of the two storms observed in Ningxia, China, was at a height of around −15 to −25 °C (7 to 9 km) and the main positive charge was at a height of around 5 to 0 °C (2 to 4 km).

Correlation Between the First Return Stroke of Negative CG Lightning and Its Preceding Discharge Processes

AbstractUsing the negative cloud‐to‐ground lightning cases well located by the Fast Antenna Lightning Mapping Array, we have examined the general characteristics of the preliminary breakdown (PB) and stepped leader (SL) processes, and further studied the correlation between the first return stroke (RS1st) intensity and its preceding discharge processes. The RS1st intensity is measured from the range‐normalized radiation field. The parameters used to characterize PB/SL include PB altitude, PB/SL vertical speed, PB/SL pulse rate, and SL duration. The arithmetic mean PB altitude is about 5.6 km and exhibits a weak inverse correlation with the RS1st intensity. As the PB develops into the SL, the arithmetic mean vertical speed decreases from 4.7 × 105 to 1.7 × 105 m/s and the arithmetic mean pulse rate reduces from 5.1 to 2.4 ms−1. In the PB/SL processes, the average vertical speed and pulse rate have positive correlations with the RS1st intensity while the corresponding Spearman's correlation coefficient for SL is considerably larger than that for PB. The average SL duration is 34 ms and has a negative relation to the RS1st intensity, with the coefficient of −0.71. These results demonstrate that on average, lower initiation altitude, faster PB/SL vertical speed, higher PB/SL pulse rate, and shorter SL duration tend to be followed by stronger RS1st. According to the above correlations, we speculate that the combination of a middle negative charge region with a uniform and wide horizontal distribution and a small lower positive charge region is a favorable condition for an intense RS1st.

Initial Leader Properties During the Preliminary Breakdown Processes of Lightning Flashes and Their Associations With Initiation Positions

AbstractThe properties of initial leaders (ILs) of 1,056 flashes in two thunderstorms in Guangzhou, China, are analyzed. The median values of IL properties are 11.1 ms for duration, 2.2 km for vertical distance, 2.7 km for 3‐D distance, 1.9 × 105 m/s for vertical displacement speed, 2.4 × 105 m/s for 3‐D displacement speed, 28° for angle between the IL's displacement direction and the vertical direction, 1.1 per ms for pulse rate, 179 m for vertical step length, and 224 m for 3‐D step length. All the IL properties follow lognormal distributions. Strong monotonic relationships of IL duration with speed and pulse rate and pulse rate with speed and step length are revealed. With increasing initiation altitude, IL duration and step length tend to increase, and IL speed and pulse rate tend to decrease. The ILs initiated at approximately the −20 °C isotherm are more vertically orientated on average than those initiated at lower or higher levels. The ILs initiated in the strong kinematic areas generally had a greater speed and pulse rate but smaller duration, distance, and step length relative to those initiated in the weak kinematic areas. The possible impacts of the charge distribution patterns and density on the IL properties in different types of thunderstorms or thunderstorm areas are discussed.

Correlation analysis between initial preliminary breakdown process, the characteristic of radiation pulse, and the charge structure on the Qinghai‐Tibetan Plateau

AbstractUsing synchronous data from a three‐dimensional lightning VHF radiation source mapping system, broadband electric field changes, and the radiation intensity of lightning on the Qinghai‐Tibetan Plateau, we divided the preliminary breakdown process into two processes by subdividing the original definition: an initial preliminary breakdown process and a subsequent preliminary breakdown process. We comparatively analyzed the initial preliminary breakdown process and the initial pulse cluster for different types of lightning in a thunderstorm and studied the correlation between the propagation direction of the initial streamer and the polarity of the initial pulse cluster, as well as the correlation between the propagation path of the initial streamer and the charge structure of the thunderstorm. The statistical analysis shows that the streamer propagation distance of the initial preliminary breakdown process maintained good consistency with the number of the initial pulse clusters generated in the initial preliminary breakdown process. When the initial preliminary breakdown process included multiple pulse clusters, the initial streamer exhibited a discontinuous discharge channel through a stepped development traveling upward or downward. Each step corresponded to a pulse cluster. The amplitude of the radiation and the broad electric field change pulse first increased and then decreased in each pulse cluster. The polarity of the initial pulse cluster was consistent with the propagation direction of the initial streamer in the initial preliminary breakdown process, and the propagation direction of the initial streamer was consistent with the charge structure of the thunderstorms.

A Study of Preliminary Breakdown and Return Stroke Processes in High-Intensity Negative Lightning Discharges

Using an automated data processing algorithm, we examined electric field records of 5498 negative cloud-to-ground flashes reported by the U.S. National Lightning Detection Network (NLDN) within 50 to 500 km of the Lightning Observatory in Gainesville (LOG), Florida. Out of the 5498 flashes, 3496 (64%) had detectable preliminary breakdown (PB) pulse trains. Only 3077 flashes with a single PB pulse train and NLDN-reported first-return-stroke (RS) peak current ≥50 kA were selected for detailed analysis. The arithmetic mean values of PB pulse train duration, PB-RS interval, and PB/RS pulse peak ratio were 2.7 ms, 8.8 ms, and 0.15, respectively. The PB-RS interval was found to decrease with increasing RS peak current (Spearman correlation coefficient was statistically significant and equal to −0.80). The range-normalized PB pulse peak exhibited statistically significant positive correlation with the RS peak current, with Spearman correlation coefficient being 0.48. Thus, it appears that the high-intensity (≥50 kA) negative lightning is characterized by shorter (and, by inference, faster) stepped leaders and more pronounced PB pulse trains.

Characteristics of a negative lightning with multiple‐ground terminations observed by a VHF lightning location system

AbstractPropagation characteristics of a negative cloud‐to‐ground lightning with four leader‐return stroke sequences exhibiting four ground terminations, referred to as multiple termination flash, are analyzed using a two‐dimensional mapping system of lightning radiation sources in VHF band along with broadband electric field change records. The four stepped leader‐stroke sequences were produced from two different branches of the preliminary breakdown process inside the cloud, and the time intervals between each adjacent two leader‐stroke sequences were 82.9 ms, 81.4 ms, and 152.2 ms, respectively. The first two stroke sequences were multiple termination strokes (MTSs), and each MTS showed two different ground terminations induced by two leader branches. The corresponding electric field changes exhibited two fast‐rising waveforms separated by 1.5 ms and 2.7 ms, respectively, and the second peak was 3–4 times smaller than the first. The second grounding branch in each MTS occurred when its channel was in poor connection conditions to the main stroke channel. The third and fourth leader‐stroke sequences connected to the ground with one of the different grounded channels of the second MTS. The dart leaders before these two strokes exhibited chaotic pulse trains (CPTs) with significant slow electric field variations accompanying strong and continuous VHF radiations. The CPTs only occurred in the dart leader stage which propagated with a speed in an order of 107 m/s. Different from dart leader, attempted leader was associated with regular pulse trains and propagated at a speed order of 106 m/s along the previous channel.

Beijing Lightning Network (BLNET) and the observation on preliminary breakdown processes

Beijing Lightning Network (BLNET), a regional multi-frequency-band lightning detection and location network, has been developed for both research and operational purposes. The network consisted of 15 stations in 2014, covering an area of about 100km×85km in Beijing area. In this paper, we present the configuration of the network and the detailed description on the sensors for the first time. The Chan algorithm and Levenberg–Marquardt method are adopted jointly in the lightning location algorithm. The Monte Carlo simulation reveals that the horizontal and the vertical location errors within BLNET coverage are less than 300m and 600m, respectively. The comparison of total lightning location results with the corresponding radar reflectivity for a thunderstorm case indicates that BLNET has a good capability to track thunderstorms in the Beijing area. Besides, we examine the electric field waveforms of the preliminary breakdown process (PBP) of 111 −CG flashes, 92 +CG flashes, 22 normal-polarity IC flashes and 22 inverted-polarity IC flashes. Statistics on the time interval between the largest PBP pulse and the corresponding first RS pulse for CG flash (PB–RS interval), the ratio of the largest PBP pulse to the corresponding first RS pulse for CG flash (PB/RS ratio), the pulse width for individual pulse in the PBP train (Tw) and the time interval between successive pulses in the PBP train (Ti) are presented. The 3D location result shows that the PBP of a −CG flash initiates at an altitude of ~6km and propagates downward to ~3km with a vertical speed of 5.9×105m/s. The initial 10ms of an IC flash is found to start at ~7.2km and develop upward to ~10km. The vertical propagation speed is estimated to be 4.8×105m/s during the first 6ms. A PBP with relatively clear branched 3D structure is presented and discussed in the end of the paper. The average 3D propagation speed of two branches is estimated to be 2.1×106m/s and 2.6×106m/s, respectively.

Preliminary breakdown of intracloud lightning: Initiation altitude, propagation speed, pulse train characteristics, and step length estimation

AbstractUsing 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.

Characteristics of a Negative Cloud-to-Ground Lightning Discharge Based on Locations of VHF Radiation Sources

AbstractThe lightning very high frequency (VHF) radiation location system based on the short-baseline time-difference of arrival (TDOA) technique provides an effective approach to describe the temporal and spatial development of lightning discharge in two dimensions with high resolution. A negative single-stroke cloud-to-ground (CG) lightning flash was analyzed in detail using the radiation location results and synchronic fast/slow electric field changes. The long-duration preliminary breakdown process appeared to propagate with bi-directional leader channels. The two negative simultaneous discharge channels sloped down with a considerable horizontal component in the lower positive charge region at speeds of about 105 m s-1. The stepped leader was clearly converted from one channel of the preliminary breakdown process and spread downwards with branches. The speeds of the stepped leaders were about 105 m s-1. The K processes after the return stroke could either directly initiate from the start region with ...

Preliminary breakdown, following lightning discharge processes and lower positive charge region

Bipolar pulse trains in wideband electric field records are often attributed to preliminary breakdown (PB) processes (Nag and Rakov, 2009). Using electric field waveforms of lightning discharges observed in Beijing and Guangzhou, the detectable PB pulses and the following discharges are analyzed, and the role of lower positive charge region (LPCR) is investigated. Furthermore, a conceptual discharge model is tested. The detailed results are as follows. (1) The percentages of detectable PB pulses are just as a function of latitude in large range of latitude to a certain extent. But the percentages are similar in a small range of latitude. (2) Four discharge types have been identified. Type I, type II, and type III exhibit PB pulses followed by inverted intracloud (IC) discharge, hybrid flash (an IC discharge followed by a negative return stroke (RS)), and leader-RS, respectively. Type IV only exhibits a leader-RS waveform without detectable PB pulses. It is believed that the charge structures of four discharges are characterized with large LPCR, modest LPCR, small LPCR and little LPCR, respectively. (3) The discharges of type IV are about 65.4% and 63.3% in Beijing and Guangzhou. And the percentages of other discharge types are obviously different. There are higher percentages for type I and type II in Beijing, which may be attributed to larger scale of LPCR. (4) The values of pulse interval (Tpi) rank in increasing order for type I, type II and type III. Compared with type III, type II exhibits longer pulse-return stroke interval (Tp-ri). As for type II, obvious larger amplitude ratio between the maximum peak of PB pulse train and the peak of the first RS (Rp-rr) is found in Beijing. It is speculated that the parameters of different discharge types are related to the scale of LPCR. The analysis results are interpreted by a conceptual discharge model and the rationality of the model is also verified by the results to a certain extent.

Characteristics of a Negative Cloud-to-Ground Lightning Discharge Based on Locations of VHF Radiation Sources

The lightning very high frequency(VHF) radiation location system based on the short-baseline time-difference of arrival(TDOA) technique provides an effective approach to describe the temporal and spatial development of lightning discharge in two dimensions with high resolution.A negative single-stroke cloud-to-ground(CG) lightning flash was analyzed in detail using the radiation location results and synchronic fast/slow electric field changes.The long-duration preliminary breakdown process appeared to propagate with bi-directional leader channels.The two negative simultaneous discharge channels sloped down with a considerable horizontal component in the lower positive charge region at speeds of about 105 m s-1.The stepped leader was clearly converted from one channel of the preliminary breakdown process and spread downwards with branches.The speeds of the stepped leaders were about 105 m s-1.The K processes after the return stroke could either directly initiate from the start region with negative polarity lightning discharge,or initiate from a new region in the cloud as negative recoil streamers.All K processes propagated along the preceding electrified channel,while not all K processes initiated from the tips of positive breakdowns.The speeds of the K processes were about 106–107 m s-1.

Characteristics of positive cloud‐to‐ground lightning in Da Hinggan Ling forest region at relatively high latitude, northeastern China

AbstractIn 2009 and 2010, a multistation network of fast and slow antennas was installed on the edge of a relatively high latitude forest region in Da Hinggan Ling (50.4°N, 124.1°E) of northeastern China. The documented 185 positive cloud‐to‐ground (CG) flashes containing 196 return strokes were analyzed in the paper. It was found that 71.89% of the positive CG flashes contained continuing current. The average duration of continuing current was short with an arithmetic mean value of 33.29 ms and a geometric mean value of 16.74 ms; only one continuing current lasted longer than 150 ms, probably because of the small size of the storm cell in this relatively high latitude region. The vast majority (94.59%) of positive CG flashes was characterized by a single stroke, and the average number of stroke per flash is 1.06. The average charge transferred by positive stroke and continuing current, based upon the analysis of five flashes with well‐documented simultaneous measurements of more than five stations, was +5.2 C from a height of 6.0 km (above ground) and +10.2 C from a height of 6.4 km, respectively. The charge moment for +CG strokes ranged from +13.7 C km to +55.9 C km, while that for continuing current ranged from +29.0 C km to +96.9 C km, respectively. The preliminary breakdown process in positive CG flashes can be classified into three types, namely type S (same), type D (different), and type C (chaotic) according to the disparities in the initial polarity of bipolar pulses from the return stroke, which account for 62.92%, 23.60%, and 13.48%, respectively. According to the electric field waveforms indicative (or not indicative) of intracloud (IC) discharge, positive CG flashes are classified into four types, i.e., ordinary positive CG flash (63.78%), hybrid +CG‐IC flash (21.08%), hybrid IC‐+CG flash (5.41%), and hybrid IC‐+CG‐IC flash (9.73%). About 15.14% of the recorded positive CG flashes were byproduct of IC lightning discharge.

Electric field changes generated by the preliminary breakdown for the negative cloud-to-ground lightning flashes in Malaysia and Sweden

We present the study of the electric field changes generated by the preliminary breakdown for negative cloud-to-ground lightning flashes in Malaysia and Sweden concerning the association of slow field changes associated in preliminary breakdown process. We examined the total of 1685 negative cloud-to-ground lightning flashes from the total of 39 thunderstorms by recording the slow electric field, fast electric field and narrowband radiation field at 3 and 30MHz signals simultaneously. Our results show that there is a pre-starting time, i.e. the duration between the first preliminary breakdown pulse and slow field changes starting point, which is found to be after the first preliminary breakdown pulse. The pre-starting time has the arithmetic and geometric mean range from 1.4–6.47 and 1–3.36ms, respectively. The mean values of pre-starting time in Malaysia are greater than the values observed in Sweden by more than a factor of 3. From the two data sets it shows that the slow field changes never start before the preliminary breakdown. Furthermore, the use of single-station electric fields measurement with high resolutions of 12 bits transient recorder with several nanosecond accuracy allow one to distinguish the slow field changes generated by preliminary breakdown, which preceded the negative first return stroke, between tens to hundreds of milliseconds of pre-return stroke duration.

Lightning discharge process based on short-baseline lightning VHF radiation source locating system

A lightning VHF radiation source location system based on short-baseline technology is used to study the location of VHF radiation source in the lightning discharge process. The observation was conducted in Daxing'anling during summer of 2009. Positive cloud-to-ground (CG) lightning, negative CG lightning, and intra-cloud (IC) lightning are analyzed based on the locating results and simultaneous electric field changes. The long lasting preliminary breakdown process of CG flash appears as two-level structure of discharge channel in cloud. There are obvious recoil streamers during the final stage of preliminary breakdown process. The preliminary breakdown process propagates at a speed of about 104 m/s. The preliminary breakdown seems to be necessary for the initiation of stepped leader. The stepped leader starts from the source region of preliminary breakdown and propagates downwards. The stepped leader produces intensive VHF radiation. The propagation speed of stepped leader is about 105 m/s. The K process is caused by the streamers propagating backwards and forwards along the electrified channel. Characteristics of discharge processes before and after the return stroke of positive CG flash are different from those of negative CG flash obviously. During long continuing current after the return stroke of positive CG flash, discharges in cloud produce intensive VHF radiation. The energy spectrum of the VHF radiation produced by lightning is studied statistically. It is found that the energy spectrum of VHF radiation decreases quickly with the frequency at a rate of about f-2.9 between 125 and 200 MHz.

Properties of preliminary breakdown processes in Scandinavian lightning

Lightning flashes are usually preceded by preliminary breakdown processes (PBPs) before a stepped leader is initiated. These breakdown processes are not well understood. An early model, the so-called BIL model, has been called into question in later studies. However, we have found that the BIL model is quite successful in describing initial processes at least in high-latitude Scandinavian lightning. We present results from one summer of measurements in Finland, during which the vertical electric field was measured with a standard broadband plate antenna system. Lightning flash locations were provided by a lightning detection network and magnetic fields were measured with an experimental narrowband detection system. The relationship between the preliminary breakdown and the first return stroke (RS) is studied for 193 flashes at distances of 5–70 km. We can identify a preliminary breakdown in at least 90% of the flashes. The peak electric field of the RS is on average four times as intensive as the highest peak of the PBP. However, in 25% of the cases the PBP peak is more intensive. On the other hand, we show that this method of comparing intensities is physically arbitrary, since the PBP is continuous and the RS is impulsive. The narrowband measurement allows a physically consistent definition for intensities as the root-mean-square (RMS) sum of the most intense parts of signals. The PBP and RS are shown to have almost equal intensities at small distances. At larger distances, the PBP weakens more rapidly. This is suggested to be due to different propagation regimes, with the PBP signal changing from space-wave to ground-wave propagation with increasing distance, while the RS is predominantly ground wave at all distances. The result may have practical applications in narrowband detection of lightning. The BIL model suggests a characteristic signal in the narrowband signal, which could be used to identify the start of a lightning flash. The change in the RS–PBP ratio as a function of distance is statistically significant, but is too weak to significantly improve ranging methods.