Observations of Elves and Radio Wave Perturbations by Intense Lightning

Electromagnetic pulses (EMPs) and quasi-static electric fields (QE) from powerful lightning heat and ionize the lower ionosphere. The EMP disturbance may appear as an elve at similar to 80-95 km altitude, and the QE field as a halo or a sprite at similar to 60-80 km altitude. Both are thought to perturb crossing radio signals because of changes to the electrical conductivity of the regions. Here we present an analysis of 63 elves and corresponding radio signal perturbations from an almost stationary thunderstorm system that allows us to untangle some of the dependencies of perturbations on the lightning characteristics. The amplitude perturbations of a VLF-transmitter signal are characterized as either long-recovery, early events (LOREs) or as early events. We find that LOREs are related to lightning with high peak currents and bright elves, and that their sign (amplitude increase or decrease) depends on the relative locations of the transmitter, disturbance and receiver. Based on a subset of strokes, lightning with elves has on average similar to 3 times the impulse charge-moment-change and power in broadband as lightning of similar peak currents without elves. The early events occur without observed elves, sprites or halos. They recover in similar to 10-100 s and are observed for both polarities of cloud-to-ground lightning and for intracloud flashes. It is proposed that these observations may relate to regions of reduced conductivity caused by an electron attachment/detachment process at lower heights, or by electron enhancements associated with TLEs that are too dim to be detected by the camera. Plain Language Summary Powerful lightning can create local disturbances to the atmosphere at around 70-100 km altitude. They appear as optical phenomena known as halos, sprites and elves and as changes in phase and amplitude of radio signals that pass through the disturbed region. The characteristics of the lightning strokes leading to the various types of perturbations are not fully understood. In this work, we analyze 63 elves and corresponding amplitude changes in radio signals from an almost stationary thunderstorm system that allows us to untangle some of the dependencies of perturbations on the lightning characteristics. We find that lightning that produce elves has three times the power and impulse charge moment change than lightning of similar peak current without elves. Also we find that elves are associated with the longest types of perturbations (similar to 10 min duration) in the 3-30 kHz radio signals, whereas the shorter types of perturbations (similar to 1 min duration) appear to be without optical emission. We propose that these observations may relate to regions of reduced conductivity caused by electron attachment/detachment processes at lower heights, or by electron enhancements associated with TLEs that are too dim to be detected by the camera.