Implications of lightning charge moment changes for sprite initiation

We report impulse lightning charge moment changes (defined as occurring in the first 2 ms after return stroke onset) in all cloud-to-ground lightning strokes detected by the National Lightning Detection Network in three storms during which above-thunderstorm sprite video was recorded. After analyzing strokes that both did and did not produce sprites and carefully accounting for lightning-sprite delay times, we found that sprite initiation on all three nights is consistent with a sharp charge moment threshold; essentially, all charge moment changes above and none below this threshold produced sprites with short delays (<5 ms) from the source lightning. On two nights this threshold was approximately 600 C km and on the other it was approximately 350 C km. This internight variability is probably due to expected variability in the nighttime mesospheric conductivity, and the thresholds themselves are consistent with predictions of conventional breakdown theory. Additionally, we found only one negative polarity lightning stroke from all three storms that exceeded this threshold, indicating that the rarity of documented sprites produced by negative strokes may be largely explained by the lack of sufficiently big negative strokes in the U. S. High Plains.