OBSERVATIONS OF LIGHTNING USING A BROADBAND INTERFEROMETER

摘要

A digital broadband interferometer INTF based on a system developed at Osa a University 1 but ith an upgraded streaming digiti er system has been deployed at Langmuir Laboratory for the past four summers. The upgraded system allo s lightning to be located continuously in time over a broad range of source po ers. The data are processed using a cross correlation based algorithm and further enhanced using ma imum li elihood indoing functions. The resulting 2D images are highly detailed having sub-microsecond time resolution as ell as good angular resolution that allo the development of positive and negative leaders to be e amined in detail. Continuously radiating events such as -changes and dart leaders are imaged in their entirety and impulsive events associated ith stepping can be isolated in time. The continuous INTF is a highly complementary instrument to the Ne Me ico Tech Mapping Array LMA. The LMA locates impulsive VHF radiation events accurately in 3D ith up to 10 microsecond time resolution hile the INTF locates both continuous and impulsive radiating events ith sub-microsecond resolution in 2D. The combined datasets provide more complete observations and understanding of lightning processes than obtained by either system alone. Ambiguities in the interferometer data arising from sources moving to ards or a ay from the interferometer are removed. The timing resolution of the LMA is improved to that of the interferometer. Continuously radiating emissions such as -changes and dart leaders can be mapped in 3D. The combined continuous INTF and LMA system is proving to be a very po erful tool for the study of lightning electrical brea do n processes. A number of previously unobserved phenomena have been discovered using the INTF. Of significant importance is the observation of fast × 7 m s positive brea do n hich radiates strongly in the VHF. Fast positive streamers about 500 m long are often observed as the first event in a lightning flash and may be the mechanism hich initiates lightning. Fast positive brea do n typically several ilometers long is also observed follo ing return stro es of negative CG ashes propagating into regions of previously un-ioni ed air.