Observations of Lightning Using a Broadband Interferometer

Abstract

A digital broadband interferometer (INTF), based on a system developed at Osaka University [1], but with an upgraded streaming digitizer system, has been deployed at Langmuir Laboratory for the past four summers. The upgraded system allows lightning to be located continuously in time over a broad range of source powers. The data are processed using a cross correlation based algorithm and further enhanced using maximum likelihood windowing functions. The resulting 2D images are highly detailed, having sub-microsecond time resolution as well as good angular resolution that allow the development of positive and negative leaders to be examined in detail. Continuously radiating events such as K-changes and dart leaders are imaged in their entirety, and impulsive events associated with stepping can be isolated in time. The continuous INTF is a highly complementary instrument to the New Mexico Tech Mapping Array (LMA). The LMA locates impulsive VHF radiation events accurately in 3D with up to 10 microsecond time resolution, while the INTF locates both continuous and impulsive radiating events with 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 towards or away from the interferometer are removed. The timing resolution of the LMA is improved to that of the interferometer. Continuously radiating emissions such as K-changes and dart leaders can be mapped in 3D. The combined continuous INTF and LMA system is proving to be a very powerful tool for the study of lightning electrical breakdown processes. A number of previously unobserved phenomena have been discovered using the INTF. Of significant importance is the observation of fast (5 × 107 m/s) positive breakdown which 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 which initiates lightning. Fast positive breakdown, typically several kilometers long, is also observed following return strokes of negative CG ashes, propagating into regions of previously un-ionized air.