Properties of x-ray emission from natural and rocket triggered lightning as measured by the Thunderstorm Energetic Radiation Array (TERA).

摘要

The Thunderstorm Energetic Radiation Array (TERA) is an experiment designed to measure energetic radiations (x-rays and gamma-rays) from thunderstorms and lightning. It is located at the International Center for Lightning Research and Testing (ICLRT) facility at Camp Blanding, FL. The array is integrated with the Multiple Station Experiment (MSE) which provides simultaneous measurement of electric and magnetic fields from nearby lightning. Since 2003, the facility has been operated jointly by University of Florida and Florida Institute of Technology. The array contains eight plastic, two Lanthanum Bromide (LaBr3) and 37 Sodium Iodide (NaI) detectors distributed over 25 stations covering the &sim .5 km2 facility and centered around the rocket launch tower which is used to trigger lightning. A subset of the MSE/TERA network, consisting of 9 stations, is operated as a time of arrival network. From 2005 to 2007, MSE/TERA recorded 9 successful rocket-triggered lightning events and 16 natural flashes, all negative cloud-to-ground. In this dissertation, I present an analysis of the x-ray emission from rocket triggered and natural lightning to study their spatial and temporal variation during the leader formation. The x-rays are emitted in discrete bursts lasting for about 1 musec and observed to occur during the last 10&sim100 musec of the dart leader and dart-stepped leader phase of triggered lightning and during the last 1 ms of the stepped leader phase of natural lightning, just prior to the time of the return stroke and ceases afterwards. I propose a model for the x-ray emission from a dart-stepped leader which depicts the seed electrons being accelerated near the leader tip clue to the high electric fields and following the path of the dart-stepped leader. Using detailed 3D Monte Carlo simulations, which fully model the bremsstrahlung production, electron and x-ray propagation, the model shows that the seed electrons are emitted isotropically in the azimuthal angle with energies that can extend up to 1 MeV. The luminosity of the runaway electrons suggests that lightning might play a role in the seeding mechanism for Terrestrial Gamma-ray Flashes (TGFs). I also apply the same model to dart leaders and demonstrate that there is a strong correlation between the characteristic energy of the x-ray emission, as well as the intensity, and the peak current of a return stroke. Observations from natural lightning also show that the x-ray emissions from stepped leaders are similar to those from dart-stepped leaders. The emission continues to be consistent with the source being emitted isotropically in the azimuthal direction. However, there is an indication that the emission is favored in the hemisphere along the direction of the leader propagation. I also discuss x-ray observations from laboratory sparks and their implications on the role of the cold runaway process in the production of energetic electrons. These results will help shed light onto the production mechanism of energetic radiation from lightning and provide a new insight into the discharge mechanism.