VLF AND LF SIGNATURES OF MESOSPHERIC LOWER IONOSPHERIC RESPONSE TO LIGHTNING DISCHARGES

New evidence is presented of disturbances of the electrical conductivi ty of the nighttime mesosphere and the lower ionosphere in association with lightning discharges. In addition to extensive documentation of the characteristics of a class of, events heretofore referred to as ea rly/fast VLF events [Inan et al., 1993], our data reveal a new feature of these event;;, consisting of a postonset peak that typically lasts for 1-2 s. We also report the observation of short-duration VLF or LF perturbations, in which the amplitude of the subionospheric signal ex hibits a sudden change within 20 ms of the causative lightning dischar ge, and recovers back to its original level in < 3 s. These short-dura tion events have characteristics similar to the previously observed ra pid onset, rapid decay VLF signatures [Dowden et al., 1994]. Both the typical and rapidly recovering events are observed primarily when the causative lightning discharge is within +/-50 km of the VLF or LF grea t circle propagation path, indicating that the scattering from the loc alized disturbance is highly collimated in the forward direction. The latter in turn implies that for the parameters in hand, the transverse extent of the disturbance must be at least similar to 100-150 km. The measured VLF signatures are compared with the predictions df a three- dimensional model of subionospheric VLF propagation and scattering in the presence of localized ionospheric disturbances produced by electro magnetic impulses and quasi-electrostatic (QE) fields produced by Ligh tning discharges. The rapidly recovering or short-duration events are consistent with the heating of the ambient electrons by quasi-static e lectric fields, in cases when heating is not intense enough to exceed the attachment or ionization thresholds. When no significant electron density changes occur, the conductivity changes due to heating alone l ast only as long as the QE fields, typically less than a few seconds. When heating is intense enough so that attachment or ionization thresh olds are exceeded, reductions or enhancements in electron density can respectively occur, in which case the medium would relax back to the a mbient conditions with the time scales of the local D region chemistry ,typically 10-100 s.