FDTD method has been widely used in calculations of electromagnetic fields produced by a lightning return stroke and its effects on other systems. However, the traditional Pi-type distributed resistance (R) – inductance (L) – capacitance (C) model representing a lightning return stroke is no longer suitable for FDTD method. In this study, we propose a new type RLC model that consists of a current source at the lightning channel base feeding a series of passive RLC loads above. The lightning return stroke channel is divided into small segments; each segment is presented by a height-dependent R in series with an L and in parallel with a C. With a current waveform measured for a return stroke in a rocket-triggered lightning discharge as the current source, impacts of different RLC settings on the channel current distribution properties were studied. Results show that the value of R has a significant impact on the attenuation pattern of both the current amplitude and propagation speed along the channel. Values of L and C have more impacts on the rising and falling edge of the current waveform and current propagation speed along the channel. The model was then applied to two sets of channel base currents and multi-station electric field measurements from two return strokes in rocket-triggered lightning experiments. By fitting the simulated electric field with the measured electrical field at each station, an optimal set of lightning channel RLC values for each of the two return strokes were determined, and hence the current propagation speed and amplitude as a function of the channel height were successfully estimated. The results show that both the current speed and amplitude decreased exponentially with the increase of the channel height, which are well consistent with existing optical observations and physical predictions in literature.

Lightning current is a fundamental parameter in lightning research. Previously, Rogowski coils and low-resistance shunts were used to measure lightning current down to a few amperes. Higher value resistors were also used to measure corona discharge current at sub-ampere level, but they were unable to record the increasing current of the lightning leader. As a result, the transition from a corona discharge to a lightning leader process have not been well-addressed. To address this issue, we designed a shock-tolerant multi-range low-noise analog front-end (AFE) for recording lightning current in the range from milli-amps to hundreds of kilo-amps. Large geometry D-MOSFETs were used instead of current limiting resistors to achieve a noise contribution of less than 0.4 nV/√Hz with a protection voltage of up to 100 V, and a noise density of 1.0 nV/√Hz for the AFE was finally achieved. With the system on a tall tower via a 0.25 mΩ shunt, corona current pulses of around 50 mA were identified using digital signal processing, while it is also able to withstand current pulses of over 220kA.

Measurement of lightning current plays a critical role in the field of atmospheric electricity. Traditionally, Rogowski coils or low-resistance shunts were employed for measuring lightning currents in the range from several amperes up to several hundreds of kilo-amperes, and high-value resistors were utilized for measuring corona discharge currents at sub-ampere levels. However, these approaches were not suitable for continuously recording the vast range of lightning currents. For this sake, we have developed a lightning current measurement system equipped with a shock-tolerant low-noise amplifier module. With the system installed on a tall tower, sub-ampere level currents just before the lightning initiation were observed for the first time. To confirm the authenticity of the recorded currents, the background noise of the measurement system and surrounding environment were identified, and a digital multi-frequency notch filter was proposed for de-noising. Results show that the system can achieve a current identification level of 50 mA even in complex electromagnetic environments, while having a measurement capability of 220 kA.