ULF-Waves on the Ground and in Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 48

• One is located between the reflection point A and the magnetopause.

• The wave is decayed behind it and gets into the BC region between the

plasmapause and the internal reflection point C .

• Penetrating to the other side of the turning point C , the wave again decays

exponentially as it gets the equatorial ionosphere.

FMS-wave transformation into the Alfven wave is intensified on the FLR-
frequencies. If the spectrum of the initial source contains a wide spectrum of
FMS-oscillations, each of them finds its own resonant field-line. In this case,
it is possible that global oscillations with lattitude dependent frequencies will
appear. If initially, the FMS-source radiates in a narrow frequency band, then
the wave is amplified only within a single resonance shell. In this case, oscilla-
tions with the same frequency should be observed. Intensity of such oscillations
decreases with latitude and increases sharply within a narrow region.

Alfven Resonances

We distinguish two types of resonance interaction. One of them, the resonance
of FMS-waves with standing oscillations, was already mentioned. The other is
the resonance with traveling Alfven waves, when the FMS-wave phase velocity
is equal to the local Alfven velocity on a certain field-line. If the phase dis-
placement between the waves remains unchanged, the Alfven wave amplitude
is increased by resonance. This type of resonance is noticeably manifested if
the waves are propagating together for a suciently long time. This is possible
only if the Alfven wavelength is small in comparison with the length of the
field-line.
Some peculiarities of this type of resonance are:

1. Reflecting walls are not necessary for the wave coupling. Resonance in-
tensification occurs on traveling waves. This means in particular that the
Alfven wave moving in only one direction, say towards the Northern Hemi-
sphere, is intensified. The excited traveling Alfven wave reaches the North-
ern Ionosphere, is reflected, and goes back to the conjugate ionosphere
without resonance amplification. Oscillations with noticeably differing am-
plitudes must then be observed at the two ends of the field-line.

The latitude dependent amplitudes on the ground are determined for both
resonances by 4 factors:

1. general attenuation of the FMS-wave as it penetrates deep into the mag-
netosphere;
2. its spatial spectrum;
3. the effectiveness of FMS-wave transformation and
4. ionospheric attenuation.

With the decrease in geocentric distance, the Alfven velocity grows on
average. As FMS-waves propagate towards Earth, they are sorted by their