Testing sprite initiation theory using lightning measurements and modeled electromagnetic fields

Previous research has shown that the statistical measurements of charge moment changes in sprite-producing lightning are in general agreement with the predictions based on the conventional breakdown theory for sprite initiation in the mesosphere. Measurements have progressed to the point where a detailed, event-level quantitative comparison between the measurements and predictions could more rigorously test the existing theories by estimating the electric fields above the thunderstorm clouds responsible for sprite initiation. We selected for this analysis a set of sprite events from the summer of 2004 whose initiation times are well bounded. Then we measured the current moments and charge moment changes of the parent lightning discharges using the radiated electromagnetic fields recorded by our extremely low frequency/ultra low frequency lightning remote sensing systems. The measured current moments were then used as the input of a two-dimensional cylindrical full-wave finite-difference time-domain model for lightning-generated electromagnetic field simulations. We compared the simulated mesospheric electric fields to the threshold electric field for conventional breakdown to see whether, according to theory, conventional breakdown ( and thus a sprite) would be initiated by that electric field and at what altitude. By analyzing sprites that are both short and long delayed from the source lightning strokes, we compared measurements and theory across a wide range of timescales. The results show that for bright, short-delayed sprites, the measurement-inferred mesospheric electric field agrees within 20% with the threshold electric field for conventional breakdown. However, for long delayed sprite events and dimmer sprites, the measurement-inferred mesospheric electric field for sprite initiation is somewhat below the threshold for conventional breakdown.