Broadband interferometry of lightning

Abstract

A lightning interferometer is an instrument which determines the direction to a lightning-produced radio point source by correlating the signal received at two or more antennas. Such instruments have been used with great success for several decades in the study of the physical processes present in a lightning flash. However, previous instruments have either been sensitive to only a narrow radio bandwidth so that the correlation can be done using analog hardware, or have been sensitive to a wide bandwidth but only recorded a short duration of the radiation produced by a lightning flash.

In this dissertation, a broad bandwidth interferometer is developed which is capable of recording the VHF radio emission over the entire duration of a lightning flash. In order to best utilize the additional data, the standard processing techniques have been redeveloped from scratch using a digital cross correlation algorithm. This algorithm can and does locate sources as faint as the noise level of the antennas, typically producing 100,000 or more point source locations over the course of a lightning flash.

At very low received power levels, the likelihood that a signal received at the antenna will be affected by the environmental noise is substantially higher. For this reason, the processing allows for the integration windows of the cross correlation to be heavily overlapped. In this way, the location of each event can be based on a distribution of windows. Further, noise identification techniques which leverage the heavily overlapped windows have been developed based on: the closure delay, the standard deviation, the correlation amplitude, and the number of contributing windows. The filtration techniques have proven to be very successful at identifying and removing mis-located sources, while removing the minimum number of low amplitude sources which are well located.

In the past, lightning interferometers have been limited to using only two perpendicular baselines to determine the direction to each point source. Additional techniques are developed in this dissertation for efficiently computing the image of a point source in the sky using an arbitrary number of antennas in an arbitrary configuration. The multiple baseline techniques further improves the sensitivity and accuracy of the locations provided by broadband interferometers.

To demonstrate the usefulness of broadband interferometers, the activity of 6 flashes spanning a diverse selection of lightning flash types are examined in this dissertation. This includes detailed analysis of negative stepped leaders, positive un-stepped leaders, K-changes, and fast positive breakdown. Initial breakdown pulses which are seen at the beginning of the flash are found to be no different than horizontal negative leader steps seen later in the flash. Evidence is found that positive leaders produce VHF radiation, as opposed to all of the radiation in the positive breakdown region being produced by retrograde negative breakdown. The time resolved three-dimensional velocity of 47 K-changes occurring in two flashes is measured. And finally, fast positive breakdown is characterized and found to be produced by a positive streamer process instead of a leader process.

Observations made with the instrument showcase the capabilities of a continuous sampling broadband interferometer. The instrument makes possible measurements which were difficult or impossible to obtain in the past, and the preliminary observations allude to many exciting scientific findings to come.