Ionospheric Effects of Lightning

Particularly strong forms of lightning can produce fanstastic optical displays at 60-100 km altitude. Sprites, sprite halos, elves, and gigantic jets (see Figure 1) are collectively known as Transient Luminious Events (TLEs), and they exemplify the direct coupling of lightning energy to the overlying ionosphere. While these effects are often studied using optical detectors, such as high-speed cameras and photometric arrays, our research group uses the VLF remote sensing method. TLEs are strongly associated with conductivity changes in the lower ionosphere, and these conductivity changes affect VLF wave propagation in the Earth-ionosphere waveguide.

Figure 1. Top left, clockwise: (1) sprite halo and sprite [Marshall et al., 2006]; (2) column sprite [Stenbaek-Nielsen et al., 2008]; (3) gigantic jet [Pasko et al., 2002]; (4) elve, as seen from space (the bright 'bloom' below the elve is the lightning flash) [Boeck et al., 1992].

VLF transmitters in the 20-30 kHz range are operated by navies worldwide for submarine communications, and we use these transmissions as signals-of-opportunity to detect ionospheric conductivity changes associated with TLEs. Disturbances in the lower ionosphere scatter VLF radio waves, producing changes in the received amplitude and phase of subionospherically propagating VLF signals (see Figure 2). Certain types of perturbations occur within 20 msecs of a causative lightning flash, indicative of direct ionospheric coupling associated with TLEs. Other types of events exhibit delays on the order of seconds, and may be associated with lightning-induced electron precipitation from the Earth's radiation belts.

Figure 2. A diagram of VLF scattering produced by an ionospheric disturbance. An example of an "early" VLF event is shown at the top right.


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