We identify and investigate the intermittent propagation of upward positive and negative leaders in rocket-triggered lightning based on comprehensive observations of fine time-resolved optical, current, and electromagnetic fields. There is an abrupt luminous crown blooming due to the achievement of a positive leader step, after which the channel weakens and the head degrades. During the positive leader pausing, residual structure is recognized at area of previous luminous crown blooming, in the form of a floating segment. Our observation point to a possible mechanism that it connects with the positive leader head, causing a forward-step and a sharp current pulse. This is to some extent similar to negative leader stepping, in which the space leader emerges and connection between the space leader and the channel head occurs. The generation of clustered space leaders give rise to negative channel branching, but the residual structure in positive leader just leads to individual step.

The attachment process in cloud-to-ground lightning flashes is a crucial process that eventually switch on the discharge route between the cloud and the ground, acting as a transition from the leader stage with a peak current of 1-2 kA to the return stroke stage with a peak current of several tens of kiloamperes (Pu et al., 2019; Rakov & Tran, 2019). In response to the downward leaders starting from the charged clouds, upward connecting leaders (UCLs) of the opposite polarity are initiated from the grounded objects, extend their channels toward the approaching downward leader and make contact, followed by the collision and return stroke (Saba et al., 2017). The progress of the knowledge of lightning attachment experienced the identification of the existence of UCL at the early stage (Golde, 1967; Orville, 1968), later the investigation of the characteristics of the leaders that determine the connection point or influence the selection of contacted leaders (Jiang et al., 2015; Lu et al, 2013, 2016; Qie & Zhang, 2019), and more recently the exploring of the so called breakthrough phase (BTP) which is one of the most poorly understood processes in lightning physics (Tran & Rakov, 2017).

Improving the specific capacitance and energy density of a fiber-shaped supercapacitor (FSSC) is critical to its applications as an energy storage device for advanced smart wearable electronics. In this paper, a heterogeneous poly(3,4ethylenedioxythiophene):poly(styrenesulfonate)/reduced graphene oxide/molybdenum disulfide (PEDOT:PSS/rGO/MoS 2 ) fiber was prepared to achieve a high-performance and durable electrode for the FSSC. As indicated, pseudocapacitive MoS 2 was in situ grown on a highly conductive acid-treated PEDOT:PSS/rGO assembly with a hierarchical structure. This structural design emphasizes the wrinkled morphology and high conductivity of the PEDOT:PSS/ rGO backbone to maximize the MoS 2 deposition and accelerate its electron transfer, which fully utilizes the pseudocapacitance of MoS 2 and provides additional capacitance contribution to the obtained device. Attributed to the synergy, the prepared fiber electrode in the FSSC exhibits high volumetric/areal specific capacitance (325.8 F cm −3 /405.3 mF cm −2 at 1 A cm −3 or 1.2 mA cm −2 ) and excellent rate performance (82%, 1−10 A cm −3 ). The corresponding device shows an ultrahigh volumetric energy density of 6.9 mW h cm −3 at a high power density of 173.6 mW cm −3 and an areal energy density of 8.5 μW h cm −2 at 215.9 μW cm −2 , together with excellent cycle stability and mechanical flexibility, outperforming most of the previously reported FSSCs. Accordingly, the proposed strategy provides a great opportunity to develop a high-performance FSSC for further wearable electronics.

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