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In this paper we analyze electric‐field and current measurements of 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, current measurements of multiple upward leaders that after initiation propagate 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 2D leader speeds are approximately 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, that is, 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, speed, and current measurements of 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 we show, for the first time, current measurements of multiple upward leaders induced by the negative downward moving leader. Key Points: Current and charge density measurements of two upward leaders induced by the same downward leaderUpward leaders alternate their progression during initial propagationCurrent pulses of upward leaders increase intensity and synchronize right before attachment [ABSTRACT FROM AUTHOR]

Two downward negative flashes (F1612 & F1613) with multiple return strokes struck to the 356-m Shenzhen Tower were analyzed. The high-speed camera captured upward positive connecting leaders (UCL) for the first strokes of both flashes and two subsequent strokes of F1612. The observations also show that none of the downward negative leader (DNL) completely propagated to the tower tip before the occurrence of the return stroke (RS), so that we inferred that the UCL initiated from the tower tip might exist for each RS of the two flashes. The possible height of the UCL and DNL connecting point above the tower tip for each RS was estimated, which showed a positive correlation to the speed of corresponding DNL. The striking distances estimated for 8 subsequent RSs in F1612 were in a range from 14.6 m to 85.4 m and that for 3 subsequent RSs in F1613 were from 24.2 m to 66.2 m. There was a weak positive linear relationship between the DNL speed and striking distance for subsequent RSs in the two flashes. The DNL speed showed a positive correlation to the peak current of corresponding RS. • Two negative flashes with multiple return strokes to a tall tower were studied based on high-speed camera measurements. • Upward connecting leaders from the tower tip were observed for the first time for two subsequent return strokes of a flash. • Heights of upward and downward leader connecting points showed a positive correlation to the downward leader speeds. [ABSTRACT FROM AUTHOR]

A very close high‐speed video observation of lightning attachment to a building revealed novel details regarding the leader streamer zone dynamics. Upward leaders propagate in a steady and unbranched manner, displaying a uniformly luminous corona brush. The exception being the upward connecting leader (UCL) just before connection, when its streamer zone increases in size and develops a more filamentary pattern. Downward negative leaders have 3‐m long multiple streamers emanating from each negative leader tip. On some occasions, plasma formations known as space stems are seen to form in the location previously occupied by negative streamers. Space stems have luminosities comparable to the main leader channel, but are detached from it by 4 m. Some space stems display streamers of their own, including cases where streamers are emanating from both ends. The space stem formation hampered the propagation of the negative leader that was closest to the UCL. Plain Language Summary: A serendipitous close observation of a natural lightning flash revealed novel details of the lightning attachment process to residential buildings in highly populated areas. A staggering total of 31 lightning precursor channels (called leaders) were launched from nearby buildings in an attempt to intercept the down coming negative leaders. The upward positive leaders propagate almost in a straight path manner, do not branch, and display a uniformly luminous "corona brush" at their tips. This contrasts with the negative leaders coming down from the cloud, which present substantial branching and have numerous filaments (called streamers) emanating from their tips. The high‐speed and high‐resolution images obtained also revealed that, in some cases, the negative leaders display a luminous formation that is about 2 m long and is detached from the main channel by about 4 m. These observations consist of one of the rare sightings of these luminous formations known in the peer‐reviewed literature as "space stems." It is understood that space stems play a key role in the stepped propagation of negative leaders. In these observations, it seems that they hamper the leader propagation, making the upward connecting leader intercept a different downward branch, which was originally more distant from the striking point. Key Points: Very close high‐speed video observation of lightning attachment reveals details of corona brush, streamers and space stemsStreamer zone of upward connecting leader increases in size and develops a more filamentary patternSome space stems display streamers emanating from both ends, forming an embryonic bidirectional space leader [ABSTRACT FROM AUTHOR]

Using video data recorded at 50,000 frames per second for nearby negative lightning flashes, estimates are derived for the length of positive upward connecting leaders (UCLs) that presumably formed prior to new ground attachments. Return strokes were 1.7 to 7.8 km distant, yielding image resolutions of 4.25 to 19.5 m. No UCLs are imaged in these data, indicating those features were too transient or too dim compared to other lightning processes that are imaged at these resolutions. Upper bound lengths for 17 presumed UCLs are determined from the height above flat ground or water of the successful stepped leader tip in the image immediately prior to (within 20 μs before) the return stroke. Better estimates of maximum UCL lengths are determined using the downward stepped leader tip's speed of advance and the estimated return stroke time within its first frame. For 17 strokes, the upper bound length of the possible UCL averages 31.6 m and ranges from 11.3 to 50.3 m. Among the close strokes (those with spatial resolution <8 m per pixel), the five which connected to water (salt water lagoon) have UCL upper bound estimates averaging significantly shorter (24.1 m) than the average for the three close strokes which connected to land (36.9 m). The better estimates of maximum UCL lengths for the eight close strokes average 20.2 m, with slightly shorter average of 18.3 m for the five that connected to water. All the better estimates of UCL maximum lengths are <38 m in this dataset [ABSTRACT FROM AUTHOR]

High-speed video images of 24 downward negative lightning flashes terminating on tall structures in Guangzhou are selected to analyse the connecting behaviour of the downward and upward leaders during the attachment process preceding the first return stroke. Three types of leader connecting behaviour have been observed: Type I, the tip of downward leader (DL) to the tip of upward connecting leader (UCL), which accounts for 42% of all the events (10/24); Type II, the DL's tip to the lateral surface of UCL, which accounts for 50% (12/24); and Type III, the combination of Types I and II, which accounts for 8% (2/24). For the two cases of Type III behaviour, each case had two junction points (one with Type I and the other with Type II behaviour) between the downward and upward leaders. Therefore, Types I and II can be viewed as the two basic types of the leader connecting behaviour during the attachment process in negative lightning. No attachment process exhibited the connection of the UCL's tip to the lateral surface of DL. The presence of multiple DL branches and their integrated effect on the development of positive UCL/UCLs are likely to be the main reasons for the Type II behaviour.

A downward negative lightning flash that contained three return strokes and terminated on a 440-m-high building in Guangzhou in 2012 was analyzed. The three-dimensional (3-D) channels of the return stroke and an unconnected upward leader (UUL) were reconstructed using the dual-station optical observation data. The 3-D propagation characteristics of the downward leader, the upward connecting leader (UCL), and the UUL during the attachment process prior to the first return stroke were obtained. For the 3-D propagation speed, both the UCL and the UUL exhibit an increasing trend after their inception, whereas the downward leader shows no clear trend, except for the final 200 μs preceding the first return stroke onset. The speed of the UCL can reach five times that of the downward leader. The two-dimensional propagation characteristics of these leaders during the attachment process were also analyzed using single-station high-speed video recordings and compared with the 3-D results. [ABSTRACT FROM AUTHOR]

The influence of the corona space charge layer created at ground level on lightning attachment to high (20–100 m) slender objects is numerically studied. The shielding effect of corona space charge produced near the object tip on the electric field at ground level is considered. The behavior of a non-stationary streamer-free corona developed from ground irregularities in a time-varying thundercloud electric field is discussed. The initiation of an upward leader from the tip of a high grounded object is simulated in the electric field created by a storm-cloud, approaching downward leader and space charge produced by corona at ground level and near the object tip. The corona space charge produced on the ground surface is shown to affect only slightly the inception of an upward connecting leader from the object when the local electric field near the object tip rises rapidly due to an approaching downward leader. The effect of neutral aerosol particles on lightning attachment to high grounded slender objects is also small. [ABSTRACT FROM AUTHOR]