Broadband very low frequency measurement of D region ionospheric perturbations caused by lightning electromagnetic pulses

Prompt early/fast perturbations on narrowband sub-ionospherically propagating very low frequency (VLF) signals are the primary evidence for the direct coupling of energy released by lightning discharge to the lower ionosphere. Different mechanisms have been advanced to explain the fast ionospheric perturbations, such as heating and ionization from the lightning electromagnetic pulse (EMP) associated with elves or from quasielectrostatic fields associated with sprites and halos. By comparing the broadband VLF spectra (3-25 kHz) of lightning discharges that shortly followed high peak current lightning discharges with the spectra of lightning discharges that did not, we detect D region perturbations caused by these intense lightning strokes over the U.S. East Coast and the U.S. High Plains. The electron density changes are measured by analyzing the broadband VLF propagation changes, and the perturbed electron density profiles from both regions are found to be consistent with those theoretically predicted for strong lightning EMP. In one case, a D region perturbation was detected following a lightning stroke that produced an isolated elve recorded by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrument on the FORMOSAT-2 satellite, confirming the EMP origins of these ionospheric perturbations. For this case, we measure electron density enhancements of 460 cm^-3 averaged over a 220-km radius and 10-km-high perturbation region, in good agreement with the 210 cm^-3 measured optically by Mende et al. (2005) for a different elve event. The characteristics of the lightning responsible for these ionospheric perturbations are investigated by comparing high peak current lighting strokes that do and do not generate detectable ionospheric perturbations.