Upward leaders from instrumented lightning rods competing to connect a downward leader during a lightning attachment process
Marcelo M. F. Saba1, Paola B. Lauria1, Carina Schumann2, José Claudio de O. Silva3, Felipe de L. Mantovani1
1INPE – National Institute for Space Research - São José dos Campos, Brazil.
2JLRL University of the Witwatersrand – Johannesburg, South Africa.
3APTEMC - São José dos Campos, Brazil.
Corresponding author: Marcelo M. F. Saba (marcelo.saba@inpe.br)
Key Points:
Abstract
In this paper we analyze electric-field and current measurements of competing upward leaders induced by a downward negative lightning flash that struck a residential building. The attachment process was recorded by two high-speed cameras running at 37,800 and 70,000 images per second and the current measured in two lightning rods. Differently from previous works, here we show, for the first time, the behavior of multiple upward leaders that after initiation compete to connect the negative downward moving leader. At the beginning of the propagation of the leaders that initiate on the instrumented lightning rods, current pulses appear superimposed to a steadily increasing DC current. The upward leader current pulses increase with the approach of the downward leader and are not synchronized but present an alternating pattern. All leader speeds are constant. The upward leaders are slower than the downward leader speed. The average time interval between current pulses in upward leaders is close to the interstep time interval found by optical or electric field sensors for negative cloud-to-ground stepped leaders. The upward leaders respond to different downward propagating branches and, as the branches alternate in propagation and intensity, so do the leaders accordingly. Right before the attachment process the alternating pattern of the leaders ceases, all downward leader branches intensify, and consequently upward leaders synchronize and pulse together. The average linear densities for upward leaders (49 and 82 µC/m) were obtained for the first time for natural lightning.
Plain Language Summary
The effectiveness of a lightning protection system depends on its efficiency to intercept the down coming leader of a cloud-to-ground lightning flash. The interception is usually done by an upward connecting leader that initiates from grounded structures, humans, or living beings that protrude from nearby ground. The understanding of the upward connecting leader and of the attachment process with the downward leader plays an important role in the determination of the zone of protection and therefore in the improvement of a lightning protection system. Unconnected upward leaders, i.e., upward leaders that fail to connect the downward leader, are also of great importance in lightning protection. They can be large enough to cause damage to equipment vulnerable to sparks or induced currents, and enough to injure people from who it initiates. In this paper we analyze electric-field and current measurements of competing upward leaders induced by a downward negative lightning flash that struck a residential building. The attachment process was simultaneously recorded by two high-speed cameras, an electric-field sensor, and current sensors installed on two lightning rods. Differently from previous works, here we show, for the first time, the behavior of multiple upward leaders that compete to connect the negative downward moving leader.
1 Introduction
Previously, we have reported high-speed video images of attachment process of three negative downward cloud-to-ground flashes to an ordinary residential building (Saba et al., 2017). As mentioned in the cited paper, the effectiveness of a lightning protection system (LPS) depends on its efficiency to intercept the down coming lightning leader which is related to its efficiency to emit upward connecting leaders (UCL). The understanding of the characteristics of an UCL and of the attachment process with the downward leader plays an important role in the determination of the volume or zone of protection of a LPS and in the improvement of LPS designs. Unconnected upward leaders (UUL), i.e., those events that initiate an upward leader but fail to make contact with the downward leader, are also of great importance in lightning protection. They can be large enough to cause damage to equipment vulnerable to sparks or induced currents, and enough to injure people.
Although a few current measurements of upward leaders have been reported from tall towers higher than 60 m (e.g. Saba et al., 2015; Visacro et al., 2017; Arcanjo et al., 2019; Nag et al., 2021 for towers over mountains), from buildings (Saba et al., 2017), and from small structures (Schoene et al., 2008, vertical conductor of 7 m height), no current measurements of upward connecting leaders from common residential buildings have been reported in the literature. Moreover, no study has ever been done on upward leaders competing to connect a downward leader. Besides, some of these past studies do not have electric-field and current measurements together with high-speed video observations which is crucial to visualize what is happening with the upward and downward leaders involved in the attachment process.
This study presents observational data of several positive upward leaders competing to connect a negative leader of a downward cloud-to-ground flash that strikes an instrumented lightning rod of a residential building. It is the first to report current measurements of two upward leaders induced by the same downward leader. The use of high-speed video images and electric field measurements reveal the nature of the physical process that is generating the currents measured on the vertical lightning rods on the top of buildings.
2 Instrumentation
The lightning attachment to the building was observed by two high-speed video cameras Vision Research Phantom v12 and v711 operating at 70,000 and 37,800 frames per second with exposure times of 13.55 µs and 25.85 µs and time intervals of 14.29 µs and 26.46 µs respectively (videos available in Supplementary Information). Image spatial resolution used for the flashes herein was 128 × 360 pixels and 368 × 416 pixels, respectively. They were positioned at 220 m from a pair of identical 14-story apartment buildings, named P1 and P2, located in São Paulo City (23.483°S, 46.728°W), Brazil (Figure 1a). Their steel reinforced concrete structures are used as natural LPS. Each building has a vertical lightning rod, and their tips are at a height of 52 m respective to ground level. All reported distances and speeds given by the analysis of the images from the high-speed videos were measured in 2D and therefore underestimated.
The electric field changes caused by the attachment process was measured by a flat plate antenna with an integrator and amplifier. The antenna was located on top of building P2 only 4 m away from the lightning rod that was struck by a cloud-to-ground lightning flash (see Figure 1a). A fiber-optic link was used to transmit the signal from the integrator/amplifier to the digitizer. The bandwidth of the system ranged from 306 Hz to 1.5 MHz. The physics sign convention is used when referring to the electric field and its change. The approach of a nearby negative leader produces positive electric field change, and a negative CG return stroke produces a negative field change.
One current transformer sensor (Pearson model 301-X) was installed on the lightning rod of each building. This current sensor is capable of recoding current up to 50,000 A with a useable rise time of 200 nanoseconds, a low frequency 3 dB cut-off of approximately 5 Hz and a high frequency 3 dB cut-off of approximately 2 MHz. The output of the sensor is split in two channels (20 dB and 50 dB attenuation over 50 Ω) and sent to a data acquisition system through a pair of fiber optic links. Before installation, both sensors were tested and calibrated in a high voltage facility. The electric field and current were continuously recorded at a sampling rate of 5 MS/s. GPS antennas were used to synchronize all measurements and video images.
Data from lightning location systems (LLS) were used to obtain the polarity, the time, and an estimate of the peak current of the return stroke. A complete study on the accuracy of peak current estimation given by the LLS has not been performed yet. However, for one recent event of a cloud-to-ground flash that struck one of the buildings, the error was within 20% for the strokes that were correctly classified as cloud-to-ground. In that event, four strokes were detected by the LLS and they were directly measured by the current sensor installed in the vertical lightning rod to where the attachment occurred. Further information about these systems and their performance are given in Naccarato et al. (2012 and 2017).
More information about the cameras, the locations of the two buildings and the topography of the terrain can be found in the previous work (Saba et al., 2017).
3 Data
On 1 February 2017 a single-stroke negative cloud-to-ground lightning discharge struck the tip of the lightning rod B on building P2. According to the LLS, its peak return stroke current was -73 kA and occurred at 19:01:10.689307 (UT). During the approach of the stepped leader, a positive UCL was launched from the tip of the lightning rod B on building P2 together with five positive UULs from the vertical air-termination rod A of the other building (P1) and other nearby structures and corners (named C, D, E, F), as shown in Fig. 1. The first upward leader to start a continuous propagation was the UCL leader. It started propagation when the downward leader closest tip was 102 m away from the tip of the P2 lightning rod where it started. The leaders had their origin at different distances from the electric field sensor and at different times (t = 0 s correspond to the attachment and beginning of return stroke in all tables and graphs). The leader types, 2D lengths (measured one frame before the occurrence of the return stroke), their horizontal distances from the electric field sensor, and inception times are shown in Table 1.