Estimation of Lightning Return Stroke Speed Using Elve Observations

Abstract:

Elves are optical emissions in the D-region ionosphere that are a result of collisional heating from the intense lightning electromagnetic pulse (EMP). The shape of elves is determined by the EMP radiation pattern at D-region altitudes, which is in turn controlled by the geometry and current propagation properties of the return stroke channel. In particular, numerical simulation of the EMP-ionosphere interaction shows a strong relationship between the elve "hole" radius and the current return stroke speed. During the summer of 2013, hundreds of elves were observed from Langmuir Laboratory, in New Mexico, with the PIPER high-speed photometer. Of these, 55 had an appropriate viewing geometry in which the elve hole is observable and is not corrupted by cloud flashes or sprites. An “emission profile”, characterizing the elve emissions versus radius and time, is fit to each of these events using a constrained least squares reconstruction, allowing easy extraction of the elve hole radius. Using these measured radii in conjunction with the numerical EMP model, we perform Bayesian inference to estimate the distribution of return stroke speeds. The MTLL engineering model is assumed for the current propagation with reasonable priors for the parameters. The results show a maximum a posteriori probability return stroke speed estimate of 0.64c for elve producing lightning.