Subionospheric VLF perturbations associated with lightning discharges

Abstract

The use of very-low frequency (VLF) transmissions propagating inside the waveguide formed by the Earth and the lower ionosphere is a well developed technique for probing conditions within the waveguide. This paper seeks to review the current understanding of lightning discharge associated processes that lead to changes in the characteristics of the waveguide and thus variations in the received phase and/or amplitude of VLF transmissions. Particular emphasis is placed upon events which appear to be produced directly by lightning discharge processes. These include VLF sprites and early Trimpi for which significant research efforts have been made over the last 10 years. The properties and interpretation of these events are discussed in detail. QE-field produced early Trimpi are due to relatively large disturbed regions (horizontal extent ∼100 km ) in the lower ionosphere. VLF sprites are produced by red sprite discharges, leading to relatively small, dense, ionospheric modifications made up of fine-scale horizontal structures (∼300 m) that stretch over wide altitude ranges (⩽50– 80 km ). Differences in the ionospheric modifications electron temperatures and ionisation structure produces the differing delay times and scattering patterns in the experimentally observed VLF perturbations. Lightning-EMP produced Elves lead to large (∼500 km) , relatively smoothly varying ionospheric disturbance at high altitudes (∼85 km) . Such a modification should create sudden step-like changes in received VLF amplitude and phase that lack a clear relaxation signature and occur along narrow forward-scattered directions. The VLF techniques described in this article are part of the larger field of research into high-altitude processes connected with thunderstorms. Lightning EMP produces a large (∼500 km) , relatively smoothly varying ionospheric disturbance at high altitudes (∼85 km) .