Relativistic Runaway Discharges in the Earth's Atmosphere

The proposal aims at advancing the knowledge of spectacular naturally occurring phenomena related to lightning activity in the Earth’s atmosphere. Several of such processes are known to the community as sprites, blue and gigantic jets etc. These have attracted the attention of several citizen scientists all over the globe. One such phenomenon, which is still not completely understood is referred to as terrestrial gamma-ray flashes or TGFs. During these events, bursts of X-rays with high energy are generated. The main goal of this research is to develop theoretical work that will shed light on the sources of TGFs. This inter-disciplinary work combines concepts of plasma physics as well as atmospheric processes. It may be relevant to the development of new X-ray sources with possible implications on medical imaging processes. The proposed work will result in the training of a graduate student and will be part of the doctorate thesis. The main motivation for the proposed work results from some recent observations of TGF’s and exploring the underlying physical processes responsible for these events. The core physical process leading is avalanche multiplication of electrons accelerated by force exerted by electric field. In geophysical context the field is generated either as ambient field due to thundercloud electrification or produced by lightning leaders. These electrons gain energy from the electric field and experience quick multiplication in ionizing collisions. In relativistic regime these electrons can radiate energetic photons (X-rays) as they scatter by nuclei of nitrogen and oxygen atoms in air. The radiation is referred to as braking or bremsstrahlung radiation. These X-rays are radiated in all directions and some fraction is radiated backwards with respect to electron motion. These X-rays can produce seed electrons due to photoelectric effect and this leads to strong amplification of the original avalanche. This process is referred to as feedback process. It was discovered recently that these effects occur on very small spatial scales in good agreement with existing observations. This project investigates the exact physical regimes which lead to observed phenomena. The education and transfer of knowledge will benefit from the proposed activity through the mentoring of students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.