Excitation by magnetospheric sources
In first order apporximation, the ionosphere can be regarded as
a low-pass filter that divides the ULF/ELF frequency range into
sources outside the ionosphere and inside the
Earth-ionosphere cavity.
Geomagnetic pulsations, or
micropulsations occur in
the ULF range
resulting from an interaction of the solar wind with
the magnetosphere (see upper figure), whereas
ELF
slow-tails result from lightning
within the Earth-ionosphere cavity.
The Earth-ionosphere cavity
resonances occur in the
transitional band between ULF and ELF frequencies
where both sources are likely to
contribute to the
wave
phenomena
observed
at the surface of the Earth.
The interference of
atmospheric
and magnetospheric sources has been addressed but
little experimental evidence has
been reported. However,
geomagnetic activity
is well known to vary with intervals of the solar
rotation period and the sunspot cycle. Therefore,
geomagnetic activity connected to the solar rotation period,
expressed by means of
sunspot numbers
may temporarily dominate over atmospheric sources,
especially during the sunspot cycle
maximum
(see lower figure).