Estimation of Electric Charge in Sprites

Measurements of very low frequency radio emissions indicate that substantial electric current flows inside sprites [Cummer et al., 1998]. This charge motion, with unknown location and distribution, is related with the detail internal microphysics of sprite development that is connected to the effects sprites create in the mesosphere. However, it is hard to measure the total electric charge inside sprites due to their spatial scale, temporal scale, and inaccessibility. In this work, we estimate the amount of charges in sprites by combining measurements of lightning radiated magnetic fields, high speed sprite images (1,000 to 10,000 fps), and numerical simulations. Assuming sprite streamers propagate along the direction local electric fields, we combine background electric field computed from FDTD simulations [Hu et al., 2006, 2007] and streamer propagation direction measured from high speed images to estimate the total ambient electric field at streamer head locations. This total electric field allows us to infer the lower bound of electric charge amount in a single sprite element at its stage of maximum brightness. For 6 events caused by +CGs, this lower bound varies from 0.01 C to 0.03 C corresponding to dim and bright sprites. An upper bound can also be computed for bright sprites with detectable radio emissions with a deconvolution technique [Cummer et al., 1997]. Results from this independent approach show that the upper bound is about a factor of 5 greater than the lower bound. Our simulations also reveal that these charges are not provided or only partially provided by the conducting ionosphere. This work is a step toward further constraining the detailed physical and chemical models that can predict the impact of sprites on the mesosphere.