On the Role of Reduced Air Density Along the Lightning Leader Path to Ground in Increasing X-Ray Production Relative to Normal Atmospheric Conditions

Abstract

Mallick et al. (2012, https://doi.org/10.1029/2012JD017555) discovered that subsequent-stroke leaders in natural negative lightning could be more prolific producers of hard X-rays and gamma rays than the first-stroke leader in the same flash. However, they had no optical records to confirm that their subsequent leaders followed the same path to ground as the first leader, as opposed to forging a new path to ground through cold air. In this paper, we present new observations, including optical data, showing that a second stroke produced more detectable X-ray pulses than the first stroke, with both strokes following the same channel to ground. Additionally, we present data for the fifth stroke from a different flash, which show the occurrence of X-ray emission at the onset of the common streamer zone between the hot channels of the downward negative dart-stepped leader and upward positive connecting leader. However, there were no detectable X-rays associated with negative leader steps.