Observation Results and Discussion on an Upward Leader Followed by a Long Continuing Current After a Return Stroke

Abstract

An abnormal discharge process after a return stroke (RS) in a negative cloud‐to‐ground flash was studied in detail. Correlating high‐speed video images and electric field data, it was found that the abnormal discharge process was initiated at ground and transported positive charge from ground toward cloud. Moreover, there was a long continuing current (CC) initiated by the abnormal upward discharge process. Compared with the characteristics of RSs, CCs and M components, we conclude that the upward discharge process is a kind of upward positive leader. The 3‐D speeds of the positive leader range from 3.72 × 106 m/s to 14.48 × 106 m/s with average value of 6.5 × 106 m/s. As far as we know, it is the first report that a long CC follows the upward positive leader after RS, which is obviously different from traditional CCs. The discovery is helpful to gain a deeper understanding of the mechanism and physical process of negative cloud‐to‐ground flashes.

Observation of five types of leaders contained in a negative triggered lightning

“…Before the occurrence of SUPL, there was no other obvious discharge process but the residual channel with slowly decaying luminosity recorded within FOV. SUPL is similar with the upward dart leader reported by Li et al 15 in that both of these two leaders initiate without any noticeable inducing factors. SUPL here propagates upward continuously with an average 2-D speed of 1.40 × 10 6 m/s, which matches the speed range of dart leaders observed in natural lightning flashes.…”

“…upward positive stepped leaders and downward negative dart leaders or downward negative dart-stepped leaders. A forth type of leader, upward positive dart leader, is observed for the first time by Li et al 15 . The cases of triggered lightning flashes involve a fifth type of leader, BL, in the residual channel are only reported by Qie et al 16 .…”

“…Li et al 15 found the 3-D speeds of the upward positive dart leader (referred to as UPL* below) had a range from 3.72 × 10 6 to 14.48 × 10 6 m/s, with the mean value of 6.5 × 10 6 m/s. When it propagated between 0 to 2 km above the ground, UPL* had three decreasing 2-D speed values calculated, all of which are between 3 × 10 6 and 5 × 10 6 m/s.…”

“…In negative CG lightning flashes, most of the reported dart leaders propagated downwards from the cloud to initiate the subsequent RS in both downward and upward lightning flashes. The exception that a dart leader can propagate upwards from the ground, in negative CG lightning flashes, is observed by Li et al 15 . In that case, the upward positive dart leader occurred after a RS in the residual channel and propagated continuously to lead a continuing current (CC) process.…”

Orientation of Initial Breakdown Pulses and Leader Discharges by Magnetic Direction Finder

“…The proposed magnetic measurement has been equipped in northern Jiangsu Province, China, from the summer of 2017, along with electric field change meter, VHF, and high‐speed video (Li et al, ). A four‐channel 16 bit digitizer is used to record the magnetic and electric field signals, with a sample rate up to 40 MHz and record length up to 1 s. The signal of the electric field also provides a trigger source for the data acquisition system.…”