Negative sprites produced by consecutive impulse charge transfers in negative strokes

On June 5, 2013, the SpriteCam network recorded a relatively weak sprite over a thunderstorm in Oklahoma. The broadband magnetic fields recorded at multiple stations consistently indicate that this event was produced by a negative cloud-to-ground (CG) event that probably contained two ground strokes separated by about 3 ms. Although neither stroke caused substantial impulse charge moment change that is sufficient to produce a negative sprite, the contributions of the two strokes are combined so that. The similar sprite-associated sferic waveform is also observed for a negative sprite recorded by ISUAL from space on August 15, 2012. In this case, the negative sprite appeared to be unusually bright due to the electric field enhancement cased by the second stroke. Therefore, the intrinsic complexity in the parent stroke is probably responsible for the observed variance in the morphology of negative sprites.