Ground-based observations of lightning-related X-ray/gamma-ray emissions in Florida: Occurrence context and new insights

Abstract

Ground-based observations of energetic radiation (X-rays/gamma-rays) associated with natural lightning discharges are presented and discussed. The emphasis is placed on relating X-ray/gamma-ray emissions to specific lightning processes. X-rays/gamma-rays have been observed in the following three contexts: (1) final stages of the descending leader, (2) collision of opposite-polarity streamers at the onset of lightning attachments process, and (3) in-cloud processes giving rise to energetic radiation bursts characteristic of Terrestrial Gamma-ray Flashes (TGFs). In all three cases, the X-ray/gamma-ray production involves runaway electrons and can be materially influenced (enhanced) by the presence of previously created but decayed lightning channels. Such channels are characterized by elevated temperature (about 3,000 K vs. 300 K for ambient air), which significantly lowers the friction curve (representing the spatial rate of electron energy loss), so that its peak is an order of magnitude lower than that for cold air. As a result, in the electric field of about 4 MV/m (such and even higher fields are briefly produced near the tips of lightning leaders), ambient electrons can be accelerated over the friction-curve peak to the keV range and further to relativistic energies needed for production of X-ray/gamma-ray emissions. Significant avalanching of runaway electrons seems to be possible. For one very intense (55 kA) subsequent stroke, which was a prolific X-ray/gamma-ray producer, we estimated the spatial extent of strong (>4 MV/m) electric field region associated with the descending leader tip to be about 1.5 m, which is sufficient for multiplication of runaway electrons by a factor of 2 × 104 or so.