Abstract Title: Can reduced air density along the lightning leader path to ground increase X-ray production relative to normal atmospheric conditions?

Abstract Submitted to: ATMOSPHERIC AND SPACE ELECTRICITY

Abstract Text:

Mallick et al. (2012) 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 (even when the NLDN-reported peak current for subsequent stroke was lower than that for the first one). They used the relatively short subsequent-leader durations measured in their electric field and electric field derivative records to argue that their subsequent leaders followed the same path to ground as the first leader, as opposed to deviating from the previously formed channel and forging a new path to ground through cold air. Mallick et al. attributed their finding to the fact that normal subsequent leaders traverse channels whose air density is considerably lower than that of the virgin air in which first-stroke leaders have to develop. Their implicit assumption was that the only difference between the first- and subsequent-leader paths in their data was the air temperature (ambient for first leaders vs. about 3,000 K for subsequent leaders), with the total particle density at 1-atm pressure for the latter being about an order of magnitude lower than for the former (Uman and Voshall, 1968). However, they had no optical records to confirm that both the first-stroke leader and subsequent-stroke leaders in a given flash indeed followed the same path to ground. In this paper, we will present new observations, including optical data that were missing from Mallick et al.’s (2012) paper.

One can expect that low-density channels traversed by subsequent-stroke leaders are more conducive to the occurrence of cold runaway breakdown than the air at normal atmospheric conditions, which first-stroke leaders have to move through. The reason for this should be a longer electron mean free path in low-density air, resulting in a larger number of electrons that are able to run away and contribute to the avalanche growth process. It is possible that some TGFs can occur in a similar manner; that is, via secondary breakdown retracing the low-density remnants of a previously conditioned in-cloud channel section or cloud region (see Tran et al. (2015) and references therein).