ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 47

Usually, the transfer of wave energy from an FMS- to an Alfven wave is
not effective. But the coupling between these modes comes into particular
prominence within the resonance region ([10], [11], [90], [113]).
The FMS-wave encountering inhomogeneities on its way causes a guided
Alfven wave, which, propagating along a field-line, can reach one of the con-
jugate ionospheres. Here the wave penetrates through the ionosphere to the
ground, is partially reflected and returns to the conjugate ionosphere. The
Alfven waves are then confined between Northern and Southern hemispheres.
If the double traveltime between the ends of the field line coincides with the
period of the exciting external source or one of its harmonics, then the wave
is resonantly amplified. This resonance is called a 'field line resonance' (FLR)
[24].
Thus, there are two channels for an FMS-perturbation to reach the Earth.
The decayed FMS-wave is reradiated into an Alfven wave guided along a
field-line. Then the perturbation can be amplified by the resonance. If the
FMS-wave is excited by a source located on the magnetospheric boundary, it
can reach the ground directly but only in a very narrow target angle of about
1 ◦ because of strong refraction [93].

A Source on the Magnetospheric Boundary

A sketch of propagation of an FMS-wave caused by a local source from the
magnetospheric boundary to the ground is shown in Fig. 3.1. There are three
characteristic regions:

FMS Source

Magnetopause

A

Plasmapause

B

C

Ionosphere

Atmosphere

Earth

Fig. 3.1. A sketch of an FMS-propagation from a local source on the magnetospheric
boundary to the Earth