The Earth-ionosphere cavity
In first order approximation,
the Earth-atmosphere
system can be seen from
an electromagnetic point of view
as a radial shell of three layers of conductivity. The
Earth and the ionosphere in about 100-150 km height appear
as a perfect conductor with the air of negligible
conductivity in between. They form a spherical shell
of conductivty, denoted Earth-ionosphere cavity, in
which electromagnetic radiation is trapped. Lightning strikes within
the troposphere radiate energy into this system and the
waves are travelling around the Earth. In the case of constructive
interference,
Earth-ionosphere cavity resonances are excited
in the frequency range of 6-60 Hz.
