Line Charge Densities and Currents of Downward Negative Leaders Estimated From VHF Images and VLF Electric Fields Observed at Close Distances

Abstract

The channel charge distribution of a lightning leader is a critical parameter for the study of lightning mechanism. However, there is little experiment-based result on it, mainly due to lacking adequate methods for getting the leader charge distribution with ordinary ground observations. In this paper, a method for retrieving the line charge density and the current of a downward leader based on ground observations of lightning images and electric field changes was proposed and examined. The method was then applied to two downward negative leaders in a rocket-triggered lightning discharge with measurements of broadband interferometric images and electric field changes at close distance. Only the descending channel portion between about 900 to 400 m high of the two leaders were analyzed, where the channel was in free air rather than the trace of triggering wire. The line charge density for both leaders showed a general increasing trend with a big impulsive change around 500 m high, as the leaders moved downward, which was in good agreement with the prediction of existing downward leader models. The range of the line charge density was similar for the two leaders (-0.03 to -0.32 mC/m and -0.02 to -0.33 mC/m, respectively). The impulsive change of the line charge density was corresponding to a sharp turn of the leader channel around 500 m high, which may imply that the sharp turn of the leader channel was probably associated with a pre-existing local positive space charge pocket. The estimated leader current ranged from -20 A to -315 A and from -77 A to -541 A for the two leaders, respectively, which was in good agreement with independent estimations in the literature.

Leader Charges, Currents, Ambient Electric Fields, and Space Charges Along Downward Positive Leader Paths Retrieved From Ground Measurements in Metropolis

“…The transferred line charge density of DPL1 ranges in 0.4 to 8.6 mC/m with the peak appearing at 1,750 m high, and that of DPL2 ranges in 0.4 to 15.2 mC/m with the peak appearing at 2,635 m high. While the transferred line charge density for the upper part of the leader channel is higher, that for the lower part of the leader channel is comparable to most of previous results (e.g., Chen, Zheng, et al, 2013 ; Proctor, 1997 ; Shen et al, 2018). The leader current of DPL1 ranges in 0.7 to 5.4 kA with a mean of 3.7 kA and that of DPL2 ranges in 0.7 to 4.6 kA with a mean of 2.3 kA. The ambient electric field for both leaders has an alternating polarity along the leader path.…”

“…For a lightning leader, the transferred line charge density along the leader channel is defined as λ = dQ/dl , where dQ is the charge transferred from the cloud into a channel segment of length dl . In previous studies (Chen, Zheng, et al, 2013; Shen et al, 2018), a basic assumption was that charges being added to the leader tip were transferred directly from the electric source in cloud/ground and charges along the established channel behind the leader tip kept unchanged as the leader developed, which is adopted in this study too. For the electric field measurement at TOLOG, because the observation site is several kilometers away from the cloud and the lightning channel, the electric charge source in cloud can be simplified as a point charge.…”

“…The average values of leader charge densities were 154.8 and 62.2 μC/m, respectively. The similar method was used to calculate the line charge density of two downward negative leaders to flat ground and the range of the line charge density was from −20 to −330 μC/m (Shen et al, 2018). The average line charge density obtained for positive leaders in long gap experiment ranged in 20 to 50 μC/m (Les Renardieres Group, 1977).…”

On the Initiation of Upward Negative Lightning by Nearby Lightning Activity: An Analytical Approach

“…Physics sign convention for the electric field was used in this paper (upward directed field is positive). It is worth noting that here we present a limited number of cases that are representative of common processes, having in mind uncertainties in input variables, including the input geometry, line charge density (e.g., Gao et al., 2020; Shen et al., 2019) and velocity of leaders (e.g., Campos et al., 2014; Proctor, 1997). An open‐source code with a graphical interface is provided as supporting information.…”

The Strongest Negative Lightning Strokes in Winter Thunderstorms in Japan

“…The peak currents range from −5 to −40 kA. These values are one to two orders of magnitude larger than peak currents of downward stepped leaders inferred from magnetic or electric field observations (Shen et al., 2019; Thomson et al., 1985; Williams & Brook, 1963) and upward negative leaders in triggered lightning or from tall objects (Miki et al., 2014; Pu et al., 2017; Watanabe et al., 2019; Zhou et al., 2012). In fact, strong strokes analyzed in this study are almost always preceded by clear leader pulses as shown in Figures 1a–1j, indicating strong peak currents of stepped leaders.…”