Early/fast VLF events produced by the quiescent heating of the lower ionosphere by thunderstorms

• Published in: Journal of geophysical research. Atmospheres 2017-06, Vol.122 (12), p.6217-6230
• Main Authors: Kabirzadeh, R., Marshall, R. A., Inan, U. S.
• Format: Article
• Language: English
• Subjects: Altitude; Atmosphere; Atmospheric models; Detection; Disturbances; early/fast; Earth; Electric discharges; Electric field; Electric fields; Electrostatic properties; Fields; Geomagnetic field; Geomagnetism; Geophysics; Heating; Interactions; Investigations; Ionosphere; Ionospheric heating; Lightning; Lightning discharges; Lightning flashes; Lower ionosphere; Magnetic field; Magnetic fields; Modelling; Night; Night-time; Nighttime; Perturbations; Profiles; propagation; Radio; Radio waves; Three dimensional models; thunderstorm; Thunderstorms; VLF; Wave propagation; Weather
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1002/2017JD026528

Large and easily distinguishable perturbations of the VLF transmitter signals due to interactions with thundercloud‐driven ionospheric modifications have been observed and studied for about three decades. These events are called “early/fast VLF” or “early VLF” events due to their immediate detection (∼20 ms) after the causative lightning flash on the ground and the fast rise time of the perturbed signal. Despite many years of study, the physical mechanisms responsible for these perturbations are still under investigation. Modifications of the sustained heating level of the ionosphere due to a lightning flash has been previously proposed as the causative mechanism of early/fast VLF events. The perturbations predicted by this mechanism, however, have been much smaller than experimental observations of 0.2–1 dB or higher. In this study, by using an improved 3‐D thundercloud electrostatic upward coupling model which uses a realistic geomagnetic field, we find that the sustained heating model can predict perturbations that are consistent with reported experimental observations. Modifications in the quiescent heating of the lower ionosphere by thundercloud fields by individual lightning flashes may thus account for some observations of early/fast VLF events. Plain Language Summary Intense lightning discharges cause changes to the lower ionosphere, the charged layer of the atmosphere beginning at 85 km altitude. These changes have been attributed to a number of physical mechanisms in the past two decades. In this work, we investigate the possibility that the ionosphere is held in a continuous state of heating by the presence of thunderstorms; the sudden disturbances that we measure are then caused by changes in the electrical configuration of the thunderstorm, resulting in changes to the state of heating. We investigate this proposed mechanism through modeling of thunderstorm electric fields and through modeling of propagation of the radio waves used to measure these disturbances. We find that the proposed mechanism is consistent with the data. This result was previously not possible because prior models did not consider the tilt of Earth's magnetic field, which has a significant effect on the ionospheric heating. Key Points Early/fast VLF perturbations may be caused by sustained heating of the ionosphere by thunderstorm electrostatic fields These events are likely only detectable for low magnetic field dip angles or at low latitudes These events are more promine