Lightning discharges are considered to be the primary
natural source of Schumann resonance excitation; lightning channels (e.g clouds)
behave like huge antennas that radiate electromagnetic energy at frequencies below
100 hz.These signals are very weak at large distances from the lightning
source, but the Earth–ionosphere
waveguide behaves like a resonator at ELF frequencies and amplifies the
spectral signals from lightning at the resonance frequency.
Waveguide:
Waves in open space propagate in all directions, as
spherical waves. In this way they lose their power proportionally to the square
of the distance; that is, at a distance R from the source, the power is the
source power divided by R2. The waveguide confines the wave to propagation in
one dimension, so that (under ideal conditions) the wave loses no power while
propagating.Conductors used in waveguides have small skin depth and hence large
surface resistance.
Waves are confined inside the waveguide due to total
reflection from the waveguide wall, so that the propagation inside the
waveguide can be described approximately as a "zigzag" between the
walls. This description is exact for electromagnetic waves in a hollow metal
tube with a rectangular or circular cross-section.
The real Earth–ionosphere
waveguide is not a perfect electromagnetic resonant cavity.
Cavity resonators:
A cavity resonator is a hollow conductor blocked at both
ends and along which an electromagnetic wave can be supported. It can be viewed
as a waveguide short-circuited at both ends .
The cavity's interior surfaces reflect a wave of a specific
frequency. When a wave that is resonant with the cavity enters, it bounces back
and forth within the cavity, with low loss (due to standing waves which are formed
due to interaction of waves with different velocities). As more wave energy
enters the cavity, it combines with and reinforces the standing wave,
increasing its intensity.
Losses due to finite
ionosphere electrical conductivity lower the propagation speed of electromagnetic
signals in the cavity, resulting in a resonance frequency that is lower than
would be expected in an ideal case, and the observed peaks are wide (rather sharp).In
addition, there are a number of horizontal asymmetries – day-night difference in the height of the
ionosphere, latitudinal changes in the Earth magnetic field, sudden ionospheric
disturbances, polar cap absorption, variation in the Earth radius of +/- 11 km
from equator to geographic poles, etc. that produce other effects in the
Schumann resonance power specification.
Schumann resonances are recorded at many separate research
stations around the world. The sensors used to measure Schumann resonances
typically consist of two horizontal magnetic inductive coils for measuring the
north-south and east-west components of the magnetic field, and a vertical
electric dipole antenna for measuring the vertical component of the electric
field. A typical passband of the instruments is 3–100 Hz. The Schumann resonance electric field
amplitude (~300 microvolts per meter) is much smaller than the static
fair-weather electric field (~150 V/m) in the atmosphere. Similarly, the
amplitude of the Schumann resonance magnetic field (~1 picotesla) is many
orders of magnitude smaller than the Earth magnetic field (~30–50 microteslas. Specialized receivers and antennas
are needed to detect and record Schumann resonances.