Charge moment change and lightning-driven electric field associated with negative sprites

Previous observations have shown that most of sprites are produced by positive cloud-to-ground (+CG) lightning discharges. Sprites produced by -CGs have a much lower occurrence rate and are rarely detected. Since January 2008, a low light camera has been installed at Duke University (35.970 N, 79.094 E) and continuously record sprite events. Among more than 300 sprites captured, 4 events are produced by -CGs, which are confirmed by both the National Lightning Detection Network (NLDN) and local measurements of lightning-radiated magnetic fields. With the broad band magnetic field measurement (1 Hz - 25 kHz) and a 2-D Finite Difference Time Domain (FDTD) model, we were able to extract the lightning source current moment waveform and further infer the lightning-driven electric fields at sprite locations. Comparing with typical sprites produced by +CGs, the negative sprites initiate immediately after sprite halos and are associated with large charge moment change up to ~ 1000 C km. Thus the negative downward streamers travel and terminate at higher ambient fields comparing to the positive downward streamers existing in typical positive sprites.