Electromagnetic Fields Associated With the M‐Component Mode of Charge Transfer

Abstract

In upward flashes, charge transfer to ground largely takes place during the initial continuous current (ICC) and its superimposed pulses (ICC pulses). ICC pulses can be associated with either M‐component or leader/return‐stroke‐like modes of charge transfer to ground. In the latter case, the downward leader/return stroke process is believed to take place in a decayed branch or a newly created channel connected to the ICC‐carrying channel at relatively short distance from the tower top, resulting in the so‐called mixed mode of charge transfer to ground. In this paper, we study the electromagnetic fields associated with the M‐component charge transfer mode using simultaneous records of electric fields and currents associated with upward flashes initiated from the Säntis Tower. The effect of the mountainous terrain on the propagation of electromagnetic fields associated with the M‐component charge transfer mode (including classical M‐component pulses and M‐component‐type pulses superimposed on the initial continuous current) is analyzed and compared with its effect on the fields associated with the return stroke (occurring after the extinction of the ICC) and mixed charge transfer modes. For the analysis, we use a 2‐Dimentional Finite‐Difference Time Domain method, in which the M‐component is modeled by the superposition of a downward current wave and an upward current wave resulting from the reflection at the bottom of the lightning channel (Rakov et al., 1995, https://doi.org/10.1029/95JD01924 model) and the return stroke and mixed mode are modeled adopting the MTLE (Modified Transmission Line with Exponential Current Decay with Height) model. The finite ground conductivity and the mountainous propagation terrain between the Säntis Tower and the field sensor located 15 km away at Herisau are taken into account. The effects of the mountainous path on the electromagnetic fields are examined for classical M‐component and M‐component‐type ICC pulses. Use is made of the propagation factors defined as the ratio of the electric or magnetic field peak evaluated along the mountainous terrain to the field peak evaluated for a flat terrain. The velocity of the M‐component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster traveling wave speed results in larger peaks for the magnetic field. However, the peak of the electric field appears to be insensitive to the M‐component wave speed. This can be explained by the fact that at 15 km, the electric field is still dominated by the static component, which mainly depends on the overall transferred charge. The contribution of the radiation component to the M‐component fields at 100 km accounts for about 77% of the peak electric field and 81% of the peak magnetic field, considerably lower compared to the contribution of the radiation component to the return stroke fields at the same distance. The simulation results show that neither the electric nor the magnetic field propagation factors are very sensitive to the risetimes of...

An Extension of the Guided Wave M‐Component Model Taking Into Account the Presence of a Tall Strike Object

“…Figure 8 shows the effects of the M‐wave speed on the electric fields at the three considered distances. The M‐wave velocity affects the electric fields significantly at a close distance range from the lightning channel, in agreement with (He et al., 2019; Zhang et al., 2011). As expected, the sharp peak shows a larger magnitude for a taller tower.…”

“…Figure 7 shows the effects of the channel height on the electric fields at the three considered distances. It can be seen that a longer M‐wave channel, for instance 5 km or so (e.g., He et al., 2019; Rakov, 2016; Zhang et al., 2011), results in a higher magnitude of the late‐time response for the fast M‐component current waveform at 15 km. The main features of the field are relatively insensitive to the channel length.…”

“…(2018), and He et al. (2019). The occurrence of the M‐components was found to be higher in tower‐initiated upward flashes than in rocket‐triggered lightning.…”

Characteristics of different charge transfer modes in upward flashes inferred from simultaneously measured currents and fields

“…An M-component (MC) is a transient process occurring during the continuing current (CC) following return strokes (RSs), and it has been studied for more than eight decades (e.g. [1][2][3]). MCs have distinct characteristics in terms of their current peak; their rise-time and the associated E-field waveforms, significantly different from those associated with the charge transfer in RSs [2].…”

The laser lightning rod project

“…Since the start of the operation of the modern lightning measurement system at the Säntis Tower in 2010, almost 1000 flashes have been recorded and analyzed [60]. the physics of lightning, among which the study of processes in upward negative cloud-to-ground flashes [66,67], the characterization of upward positive cloud-to-ground flashes [68], propagation of lightning electromagnetic fields over irregular terrain [69], and lightning-ionosphere interactions [70]. In addition, a significant fraction of the activity regards the evaluation and development of lightning detection and forecasting, in particular the performance analysis of the European Lightning Detection Network for Upward Flashes [14,71] and the forecasting of lightning occurrence using machine learning (ML) [72].…”