Estimation of electric charge in sprites from optical and radio observations

Abstract

[1] Measurements of very low frequency radio emissions indicate that substantial electric current flows inside some sprites. This charge motion, with presently unknown location and distribution, is related to the detailed internal microphysics of sprite development that is in turn connected to the impact sprites have on the mesosphere. Assuming that sprite streamers propagate along the direction of local electric fields, we combine lightning-driven background electric fields computed from electromagnetic measurements and simulations with streamer timing and propagation direction measured from high-speed images to estimate the total ambient electric field at the locations of upward streamers. From this total field, we are able to establish a lower bound on the electric charge in six observed sprites driven by positive lightning. We find that individual bright sprite cores contain significant negative space charge between −0.01 C and −0.03 C. Accounting for the uncertainty caused by using different vertical charge distribution profiles, these values could be as much as 50% higher. This charge, when spread over the observed sizes of several hundred meters in diameter and approximately 10 km in length, results in a local electric field close to the breakdown field and that the relatively long duration (several milliseconds) optical emissions from this region are driven predominantly by instantaneous local electric fields, in agreement with published simulation results. Given the significant negative charge, we interpret the sprite core region as the partial and perhaps dominant sink of the negative charge created by the downward positive polarity streamers. This further suggests that when downward streamers supply more charge than can be absorbed by the sprite core, slightly delayed upward negative streamers initiate from the sprite core to disperse this charge. This is consistent with observations that show that the subsequent upward streamers are not always present, especially in smaller sprites. Assuming that the sprite core balances all of the positive charge in the downward streamers, we find that the average streamer contains a typical charge of at least 2–4 × 10−3 dC. Observed sprite current in one bright sprite suggests a positive charge in each distinct downward streamer of only 2.5 times larger than this lower bound, indicating that our estimate of the positive charge in downward sprite streamers and the negative charge in the sprite core is physically reasonable.