Observations of burst-like VLF sferic activity in association with sprites

Recent observations have revealed the occurrence of burst-like VLF activity, sometimes lasting several hundred ms, near the onset of many sprites. These "sferic bursts" are thought to be due to intra-cloud lightning activity, since they have been observed to only propagate short distances (a few hundred km) in the Earth- ionosphere waveguide and are generally not reported by the National Lightning Detection Network (NLDN). Similar types of sferic bursts have also been previously observed in association with Early/fast VLF events. The possible involvement of intra-cloud lightning in sprite production has been previously suggested based on the observed long delays between causative CG discharges and sprite events. In this work, we investigate the correlation between sprites and sferic bursts using VLF and optical data from the past decade of sprite observations. In particular, a wideband (10 Hz - 20 kHz) VLF receiver was deployed in the summers of 1995 - 2000 at Yucca Ridge Field Station in Fort Collins, CO, while sprite observations were made from the same location. With VLF data available for thousands of sprite observations through many different dates and storms, we compare the occurrence of sferic bursts in association with sprites. Preliminary results idicate that most sprites, but not all, are found in association with bursts of sferic activity. The data set in hand also allows comparative correlations with such metrics as VLF burst intensity and energy content; and sprite type, size and intensity.