Theoretical and Experimental Investigations of the Electromagnetic Fields and Energetic Particles Associated with Thunderstorms and Lightning
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Despite more than 250 years after Benjamin Franklin’s kite experiment, lightning is still one of the mysteries of nature. We still do not understand how lightning works at the most basic level. Traditionally, lightning used to be studied using classical electrodynamics. However, in the last decade or so, the observation of many high energy phenomena associated with lightning and thunderclouds (e.g., X-rays from lightning leaders, TGFs, X-ray and gamma-ray glows from thunderstorms, etc.), started a new path in studying lightning and thunderstorms. It is suggested that energetic radiation in our atmosphere is the result of bremsstrahlung scattering of energetic electrons, called “runaway electrons”, from air molecules and atoms. There are three main mechanisms for the production and propagation of runaway electrons; thermal (cold) runaway electron production, Relativistic Runaway Electron Avalanches (RREAs), and relativistic feedback mechanism. Depending on the voltage difference of the runaway zone and the existence of an energetic seed particle, one of these mechanisms is at work. We have performed theoretical, modeling, and experimental studies of all three runaway electron mechanisms. The Relativistic Runaway Electron Avalanche process seeded by Extensive Air Showers (RREA-EAS) has long been suggested to be responsible for the production of Compact Intracloud Discharges (CIDs). We developed a fluid model based on RREA-EAS and simulated the production of CIDs. We compared our simulation results with observational data from the Lightning Observatory in Gainesville, FL and showed that one can find the fields which can produce Narrow Bipolar Pulses, but these fields are not realistic for thunderclouds and the required energy of EAS is too high. Our results do not support the RREA-EAS hypothesis, as it relates to CIDs. A high-energy spectrometer (ARIS-S) for the detection of energetic radiation from leaders in lightning was designed, constructed and deployed at the International Center for Lightning Research and Testing (ICLRT). X-ray pulses from rocket-triggered lightning events were collected and the results produced the world’s first detailed measurement of X-ray spectrum from lightning. Our analysis showed that the spectrum of X-rays from leaders is much too soft to be consistent with RREA and the cold (thermal) runaway mechanism is a viable candidate for the production mechanism of runaway electrons at leader tips of lightning. Finally, in order to determine the importance of the discharge currents caused by the runaway electrons and the role of relativistic feedback, we successfully designed, developed, and tested a balloon payload for the in situ measurement of energetic electrons and gamma-ray photons from inside thunderstorms including many other environmental measurements (e.g., GPS location, acceleration, orientation, etc.). In 2013 we successfully measured the fair weather background radiation profile as a function of height. The fair weather data agreed well with theoretical predictions. In the summer of 2014 we had five balloon launches with four of them measured gamma-ray glows from thunderstorms and at least one of them entered the runaway electron source region. We also observed two events, on two separate balloon launches, with hour-scale long excess radiation. Furthermore, we analyzed our data in accordance with the Kennedy Space Center Lightning Mapping Array (KSC LMA) data and showed that by combining them we can get a great insight regarding the electric field at the source of the gamma-ray glows and runaway electron avalanches.