Lightning VLF/LF waveforms at distances of hundreds to thousands of kilometers and their implications on lightning location systems

The Los Alamos Sferic Array (LASA) geolocates and characterizes lightning discharges by measuring the electric field changes in the frequency range of 100Hz to 500 KHz. It routinely detects return strokes, narrow bipolar events (NBE), and sometimes intra-cloud pulses over hundreds to a couple of thousands of kilometers from its stations. Our current geolocation algorithm is based on waveform cross-correlation which finds the best match among multiple-stations and then time-tags the corresponding arrival time at each station. When the discharge is located more than several hundred kilometers away, the field change waveforms differ significantly as compared to those detected at a closer range. This causes difficulties in finding accurate arrival times using the waveform cross-correlation method. In this presentation, we will first examine the physical reasons behind the waveform distortion with a ground-ionosphere propagation model. At VLF/LF and within the range of hundreds to a couple of thousands of kilometers the ground and ionosphere-reflected wave both contribute to the detected waveforms. This problem is addressed by ray-theory over a spherical earth. The modeled results agree well with the actual measurement. Secondly, we will discuss the applications of the model to LASA's geolocation algorithm. By using the predicted waveforms from the model, we can identify the key signatures for a distorted waveform and correct the arrival time to a reasonable accuracy, therefore improving the location accuracy.