Study Of High-frequency Radio Radiation From Lightning Leaders

Thunderstorms produce multiband radio radiations and high-energy radiations,such as Xrays and y rays.The high-frequency radio radiations mainly include the radio radiations in the downward negative cloud-ground lightning and the narrow bipolar events(NBEs)related to the lightning preliminary breakdown process.These electromagnetic radiations cause serious interference in wireless communications and electronic devices,and even harm human health.Therefore,it is important to study the radiation mechanisms in long-distance discharge,which not only promotes the lightning physics but also is of great guidance to the scientific prevention of lightning disasters.Radio radiations and high-energy radiations often arise at the same time in thunderstorms and are closely associated with the stepped leader tip and the streamer during the propagation of stepped leader.These phenomena indicate that the radiation source exists in the leader and streamer.However,the radiation mechanism is still not fully understood.Based on the theory that thermal runaway electrons are generated by the strong electric field at the streamer tip,this thesis proposes the transition radiation as the generation mechanism of high-frequency radio radiation in thunderstorms.Firstly,the lightning plasma leader channel propagates forward in the form of discrete steps,and a very strong electric field is generated at the streamer tip when the main channel is connected to the space leader.Secondly,due to the thermal runaway electron and the relativistic runaway electron avalanches,lots of relativistic runaway electrons are generated at the tip of the streamer.According to the theory of transition radiation,electromagnetic radiation is produced when high-energy electrons cross the interface of different media.Therefore,the relativistic runaway electrons generated in the streamer tip will also produce transition radiation when passing through the interface between the streamer and air.Starting from Maxwell equations,we derived the transition radiation generated at the interface between plasma and air.For spatially Gaussian distributed electron beams with monochromatic and Boltzmann energy distributions,respectively,the incoherent transition radiation(ITR)and coherent transition radiation(CTR)are derived.We got equations of the electric field,energy spectrum distribution,far-field radiation intensity,and total radiation energy for ITR and CTR.On this basis,the theoretical analyzes and numerical calculations of the transition radiation are carried out.It is found that the characteristics of transition radiation generated by the electron beam with Boltzmann energy distribution are similar with that generated by the monochromatic electron beam.The radiation is more concentrated with the higher characteristic energy of the electron beam.And the ITR can be ignored when spatial characteristic parameters are small enough.Meanwhile,we estimated the characteristic parameters of the electron beam at the streamer tip by combining the detection data of the streamer and radiation during discharges,and then put these parameters into the equations of CTR.The calculated results show that the energy spectrum and radiation power of CTR generated by the relativistic runaway electrons in the streamer tip are consistent with those of the experimentally detected radio radiations.Therefore,the transition radiation of relativistic runaway electrons could be treated as a physical mechanism to explain thunderstorm radiations,especially for high-frequency radio radiations detected in discharge leaders and NBEs in thunderstorms.This model fits well with the theory of runaway electrons and bremsstrahlung generated around the streamer tip.It may help explain the simultaneous detected high-energy radiation and radio radiation from the leader.