Horizontal channel lengths of cloud to ground lightning flashes
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This work utilized data from Lightning Mapping Array (LMA) networks and the National Lightning Detection Network (NLDN) to assess the relationship between lightning channel lengths and lightning current in cloud-to-ground (CG) lightning flashes. The LMA data were used to map and measure the extent of the in-cloud, horizontal channel lengths and to estimate the vertical length (height above ground) from 149 CG lightning flashes. Peak current values from the NLDN were used for direct comparison of peak current and lightning channel lengths, and to estimate an average current for each lightning flash as well as for each stroke within multiple stroke flashes. Additionally, the horizontal channel lengths as measured from LMA data were used to examine the flash area, or footprint, of the in-cloud portion of CG flashes used in this study. The results of this work provided insights into long-observed differences between positive and negative CG flashes, often referred to as polarity asymmetry. Positive CG flashes are typically characterized by a single stroke to ground, whereas negative CGs have been observed to have multiple, discrete strokes to ground. This study presented evidence in support of the theory that the primary factor contributing to polarity asymmetry is the physical differences associated with the development of the in-cloud portions of the lightning channel. For positive CGs, negative leaders constitute the in-cloud portions of the lightning channel. Negative leaders are characterized by higher currents, which are infused into the lightning channel to ground. Positive leaders within the cloud associated with negative CGs are characterized by lower currents than negative leaders, leading to less current being injected into the channel to ground. The overall effect is that the higher currents injected into positive CG lightning channels results in a stable channel to ground and most often a single stroke. Conversely, the lower currents infused into negative CG lightning channels leads to less conductive, less stable lightning channels that may become cut off from the current source within the cloud resulting in multiple strokes.