Radio atmospheric signal
Sferics received from about 2,000 kilometres' distance or greater have their frequencies slightly offset in time, producing tweeks.
When the electromagnetic energy from a sferic escapes the Earth-ionosphere waveguide and enters the magnetosphere, it becomes dispersed by the near-Earth plasma, forming a whistler signal.
A lightning channel with all its branches and its electric currents behaves like a huge antenna system from which electromagnetic waves of all frequencies are radiated.
Beyond a distance where luminosity is visible and thunder can be heard (typically about 10 km), these electromagnetic impulses are the only sources of direct information about thunderstorm activity on the ground.
[2][3] The longwave electromagnetic propagation of sferics takes place within the Earth-ionosphere waveguide between the Earth's surface and the ionospheric D- and E- layers.
Maximum spectral energy is generated near frequencies of f ≈ 1⁄τ = 10 kHz ,[6] or at wavelengths of λ = c⁄f ≈ 30 km (where c is the speed of light).
[1] Its "pulse" time typically varies between about 10–150 ms , its electric current is of the order of J ≈ 100 A , corresponding to the numbers of Q ≈ 1–20 C , f ≈ 7–100 Hz and λ ≈ 3–40 Mm .
The electric field strength of the impulse increases to a maximum value within a few microseconds and then declines like a damped oscillator.
[8][9] The orientation of the field strength increase depends on whether it is a negative or a positive discharge The visible part of a lightning channel has a typical length of about 5 km.
The channel of a R stroke can be considered as a thin isolated wire of length L and diameter d in which negative electric charge has been stored.
In the case of the ionospheric D-layer, it depends, in addition, on time of day, season, latitude, and the geomagnetic field in a complicated manner.
Resonant waves of this zeroth mode can be excited in the Earth–ionosphere waveguide cavity, mainly by the continuing current components of lightning flowing between two return strokes.
[13][14] About 100 lightning strokes per second are generated all over the world excited by thunderstorms located mainly in the continental areas at low and middle latitudes.
[14] One can apply the dispersive property of the Earth–ionosphere waveguide by measuring the group velocity of a sferic signal at different frequencies together with its direction of arrival.
The group time delay difference of neighbouring frequencies in the lower VLF band is directly proportional to the distance of the source.
[17] For the regional range (< 1,000 km), the usual way is magnetic direction finding as well as time of arrival measurements of a sferic signal observed simultaneously at several stations.
