The use of VLF transmitter signals in sprite research
The part of the electromagnetic spectrum termed as VLF (Very Low Frequency) spans the frequency region 3-30 kHz and has free-space wavelength of 100-10 km. Ground-based observations of this frequency spectrum are dominated by “sferics”, the strong impulses radiated by lightning discharges (seen as vertical lines in the figure). In the region above 10 kHz man-made signals from communication and navigation transmitters can be observed (as horizontal lines) and local electrical noise is also present below ~5 kHz.
VLF waves propagate in the so-called Earth-ionosphere waveguide, reflecting from the conducting surface of the Earth and the lower part of the ionosphere (the so-called D-region) at ~70-90 km heights. They have a low attenuation rate and can therefore be received at thousands of km-s from their source. They can also penetrate sea water. These properties led to the construction of VLF transmitters all over the globe, which are used for communication and navigation purposes. The signals of such man-made transmitters can be recorded using VLF receivers and can also be used for scientific purposes.
As VLF transmitter signals propagate in the Earth-ionosphere waveguide, they are affected by the changes of atmospheric electrical conductivity on the boundaries. The propagation conditions along the Earth surface can be regarded constant at these timescales, but the variations in the conductivity of the ionospheric D-region result in changes in the observed amplitude and phase of the VLF transmitter signals.
There exist different mechanisms directly related to lightning discharges which influence the lower-ionospheric conductivity, giving us the opportunity to study them through the perturbations they produce on man-made VLF transmitter signals. Such mechanisms include heating and ionization by quasi-electrostatic fields above thunderstorms, ionization from red sprites and from lightning-driven electromagnetic pulses (which also produce elves).
The perturbations related to these phenomena (called early-Trimpis) begin very shortly after the lightning discharge (< 100ms), which means that they are produced by a direct effect of the lightning discharge on the ionosphere. The recovery time of the signal back to the pre-event levels gives information on the relaxation of the conductivity, which in turn gives hints on the altitude of the perturbed ionospheric region.
The location of the lightning discharge leading to an early Trimpi event relative to the great circle path (GCP, the shortest line between two points on the surface of the globe) connecting the transmitter and the receiver gives information on the nature of the disturbed ionospheric region. For instance, large diffuse regions of altered conductivity (~100 km across, typically produced by QE heating) will scatter the incident VLF waves in the forward direction, which make them observable only if they are located close to the GCP. On the other hand, the highly structured ionization related to red sprites will produce wide angle scattering, thus the parent lightning discharges can be located further away from the GCP and they can still produce detectable perturbations.
During EuroSprite2005 VLF data will be recorded by receivers located at Nancay in France, in Hungary, on the Island of Crete, in the Negev desert in Israel and in Turkey, which will provide a good coverage of the sprite-watch area. These receivers are operated continuously during nighttime, typically from 17 to 05 UT.
For more details on the sprite research at the University of Crete use the link at the bottom of this page.