Fast Initial Continuous Current Pulses vs Return Stroke Pulses in Tower-initiated Lightning

We present a study focused on pulses superimposed on the initial continuous current of upward negative discharges. The study is based on experimental data consisting of correlated lightning current waveforms recorded at the instrumented Santis Tower in Switzerland and electric fields recorded at a distance of 14.7 km from the tower. Two different types of pulses superimposed on the initial continuous current were identified: (1) M-component-type pulses, for which the microsecond-scale electric field pulse occurs significantly earlier than the onset of the current pulse, and (2) fast pulses, for which the onset of the field matches that of the current pulse. We analyze the currents and fields associated with these fast pulses (return-stroke type (RS-type) initial continuous current (ICC) pulses) and compare their characteristics with those of return strokes. A total of nine flashes containing 44 RS-type ICC pulses and 24 return strokes were analyzed. The median current peaks associated with RS-type ICC pulses and return strokes are, respectively, 3.4 kA and 8 kA. The associated median E-field peaks normalized to 100 km are 1.5 V/m and 4.4 V/m, respectively. On the other hand, the electric field peaks versus current peaks for the two data sets (RS-type ICC pulses and return strokes) are characterized by very similar linear regression slopes, namely, 3.67 V/(mkA) for the ICC pulses and 3.77 V/(mkA) for the return strokes. Assuming the field-current relation based on the transmission line model, we estimated the apparent speed of both the RS-type ICC pulses and return strokes to be about 1.4 x 10(8) m/s. A strong linear correlation is observed between the E-field risetime and the current risetime for the ICC pulses, similar to the relation observed between the E-field risetime and current risetime for return strokes. The similarity of the RS-type ICC pulses with return strokes suggests that these pulses are associated with the mixed mode of charge transfer to ground.

Electromagnetic field associated with lightning return strokes to a tall structure: influence of channel geometry

José-Luis Bermudez

The paper is a contribution to the studies of factors that can affect electric (E) and magnetic (H) fields associated with lightning return strokes to tall structure. Herein, the influence of lightning channel inclination upon vertical E-field and azimuthal H-field is analyzed. Input parameters of the simulation (injected current, properties of the tower and of the channel, etc.) are derived from experimental observations. Current distribution is calculated adopting a one-section transmission line model (1TL). For a few characteristic cases of non-vertical channel orientation, E- and H-fields produced at 2 km distance by a lightning strike to the CN tower (546-m height) are computed, and compared to those of the vertical-channel case ("A"). For considered conditions, E-field first peak values differ from the basic case "A" by +9 to - 22 %, while at late-time period (within 10 μs) E-values can differ by +152 to -75 %. For H-field, corresponding deviations are smaller: +7 to -16 % and +87 to -61 %

2003

On Lightning Electromagnetic Field Propagation Along an Irregular Terrain

Mohammad Azadifar, Dongshuai Li, Stefan Metz, Mario Paolone, Davide Pavanello, Marcos Rubinstein, Zhaoyang Wang, Qilin Zhang

In this paper, we present a theoretical analysis of the propagation effects of lightning electromagnetic fields over a mountainous terrain. The analysis is supported by experimental observations consisting of simultaneous records of lightning currents and electric fields associated with upward negative lightning flashes to the instrumented Santis tower in Switzerland. The propagation of lightning electromagnetic fields along the mountainous region around the Santis tower is simulated using a full-wave approach based on the finite-difference time-domain method and using the two-dimensional topographic map along the direct path between the tower and the field measurement station located at about 15 km from the tower. We show that, considering the real irregular terrain between the Santis tower and the field measurement station, both the waveshape and amplitude of the simulated electric fields associated with return strokes and fast initial continuous current pulses are in excellent agreement with the measured waveforms. On the other hand, the assumption of a flat ground results in a significant underestimation of the peak electric field. Finally, we discuss the sensitivity of the obtained results to the assumed values for the return stroke speed and the ground conductivity, the adopted return stroke model, as well as the presence of the building on which the sensors were located.

Institute of Electrical and Electronics Engineers2016

Bipolar Lightning Flashes Observed at the Säntis Tower: Do We Need to Modify the Traditional Classification?

Mohammad Azadifar, Mario Paolone, Davide Pavanello, Marcos Rubinstein

We present and discuss current waveforms associated with 13 bipolar lightning flashes recorded at the Santis Tower during the period from June 2010 to January 2015. During this period, a total of 427 flashes were recorded, of which 13 (3%) were classified as bipolar flashes. The majority of the recorded flashes (10 out of 13) exhibited a polarity reversal during the initial continuous current, therefore belonging to Category I according to the classification proposed by Rakov (2003). Of the three remaining flashes, two were characterized by different polarities of the initial stage current and the following return stroke or strokes (Category II), and one flash involved return strokes of opposite polarity within the same flash (Category III). In Category I bipolar flashes, the polarity reversal is usually assumed to be associated with in- cloud processes. However, two of our 10 Category I flashes appeared to be each a sequence of two upward discharges of opposite polarity, initiated from the tower within tens of milliseconds of each other. This is the first time that such a sequence has been observed from the same tower. We suggest that the traditional classification of bipolar flashes should be modified to distinguish between two types of Category I flashes: those in which the polarity reversal during the initial stage is associated with in- cloud processes (Category Ia) and those in which the polarity reversal is due to initiation of two opposite- polarity leaders from the tower (Category Ib).

American Geophysical Union2016