Insights on Space‐Leader Characteristics and Evolution in Natural Negative Cloud‐to‐Ground Lightning.

We present sub‐microsecond‐scale, high‐speed video camera observations of three negative stepped leaders in cloud‐to‐ground flashes with return‐stroke peak currents (estimated by the U.S. National Lightning Detection Network) of −17, −104, and −228 kA. The camera frame exposure times for these observations were 1.8, 1.0, and 0.74 µs, respectively. The 0.74 µs exposure time is the shortest reported to date. We observed the temporal and spatial evolution of space leaders from their inception to their attachment to the pre‐existing leader channel (PELC). For stepped leaders that led to return strokes having higher peak currents, the space leaders appear to have incepted at farther median two‐dimensional distances from their respective PELC‐attachment points. These median distances were 6.1, 16.6, and 17.6 m, respectively, for the three strokes. Our observations indicate that space leader characteristics are likely influenced by stepped‐leader line‐charge‐density, which is expected to be higher in strokes with higher return‐stroke peak currents. Plain Language Summary: Lightning and thunderstorm activity have extremely important socio‐economic impacts including on power transmission and distribution, severe weather prediction, lightning safety and protection, management of outdoor events, airport and maritime activities, air travel, and space launches. The vast majority of lightning occurring between thunderclouds and the Earth's surface effectively transfer negative charge to ground. In such discharges, the first negatively charged channel (called a leader) proceeds from the thundercloud to ground in a stepwise fashion, pausing for fractions of a second after each step, before reaching ground. Many questions remain about the detailed mechanism of negative cloud‐to‐ground stepped‐leader progression and how natural lightning leaders compare to those in long sparks in laboratory experiments. We observed sub‐microsecond‐scale processes that lead to the formation and progression of negative cloud‐to‐ground lightning leader channels. Our study presents a detailed characterization of space leaders, a region of super‐heated plasma just ahead of the negative leader tip, that forges a path forward for the first stroke in cloud‐to‐ground lightning. Results of this study help explain why negative leaders pause between bursts of rapid acceleration. Key Points: Median space‐leader maximum 2‐D lengths and inception‐to‐attachment‐point distances are longer in higher peak‐current return strokesHigher stepped‐leader line‐charge‐densities in strokes with higher return‐stroke peak currents influence space‐leader characteristicsLeader‐step lengths and speeds are affected by the varying electric field environments at different altitudes above ground [ABSTRACT FROM AUTHOR]