A Study on Characteristics of Lightning Processes Using Upgraded Continuous VHF Broadband Interferometer

VHF broadband interferometer (VHF DITF) developed by Lightning Research Group of Osaka University is a system to visualize lightning channel with an extremely high time resolution due to its broad bandwidth. The digitizer was upgraded to records the waveforms at 180 MS/s for 2 seconds continuously. VHF DITF was deployed at Langmuir Laboratory in New Mexico for lightning observations during the storm seasons of 2011 and 2012. The use of continuous records changes the criteria which are used to determine whether the signals are due to lightning processes. The signals may be contaminated by radiation sources which are not lightning and by reflections of lightning emissions off nearby objects. These components cause adverse effects on the estimation of the source location of the lightning emissions. Ideally, each radiation source would be located using only frequency components known to have come from lightning processes and using only the waveform components that propagated directly from the source location to the antenna. We suggest the use of a normalized phase difference to pick out the frequency components of the emission that originated from lightning processes. The normalized phase differences within 1 sigma of a linear fit are considered to have originated from lightning. Such a system has been implemented and is believed to be robust to noise. We have successfully located radiation sources with low amplitudes as well as higher amplitude with extremely high spatial and time resolutions. Due to the high sensitivity, we can visualize extremely low amplitude processes such as emissions originating from the positive leader. Observations have also been made of recoil streamers, including recoil streamers traveling from one negative charge region to another negative charge region. We have also observed recoil streamers that passed the vicinity of the initiation point where they appear to slow down and show a reduced amount of emission. We have classified the radiation sources into 3 groups for the negative leader, positive leader and recoil streamer. From these classifications, we can investigate radiation characteristics of the different source types such as peak powers of pulses, frequency characteristics and differences in the time series of the waveforms. This analysis has been done for a number of lightning flashes.