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Fig. 5.1 A schematic daytime height profiles of the Alfvén speed for (1) maximum and (2) mini-
mum of sunspot activity. Taken from Greifinger and Greifinger ( 1968 )
(Greifinger and Greifinger 1968 ). It is clear from these figures that there is a
minimum in the Alfvén speed at an altitude of about 300-400 km both at the daytime
and nighttime conditions and that this minimum is located near the F2ionization
peak. In the exosphere, the region above the minimum, the altitude profile of the
Alfvén speed shown in Fig. 5.1 is qualitatively similar to that shown in Fig. 5.2 .In
the region below the minimum, the variation of the Alfvén speed with altitude is
dependent on ionospheric conditions.
In the first place an enhancement of the Alfvén speed results from the ionospheric
plasma density fall off with height. The strong variation in the Alfvén speed in the
topside ionosphere and an increase of the plasma conductivity in the gyrotropic E-
layer that is in the bottom of the ionosphere, results in a strong variation of the
Alfvén and FMS wave reflection indices from below and from above that gives rise
to the formation of resonance cavity. The lower boundary of this resonator cavity is
the conducting E-layer with enhanced Pedersen and Hall conductivities, that is, the
altitude range 100-130 km, where nearly all plasma parameters undergo a strong
impact. The semi-transparent IAR upper boundary is located at 600-1;200 km
altitudes from the Earth's surface. The physical reason for the upper boundary
occurrence is due to an exponential decrease of the background plasma density at
this altitude range, which in turn results in an increase of the Alfvén speed by ( 1.60 ),
thereby producing a partial reflection of Alfvén waves from the steep gradient of the
wave velocity/plasma density.
This resonance cavity can serve as the ionospheric waveguide for the FMS
mode, which can propagate along the waveguide walls around the Earth for a long
distance. As the ionospheric inhomogeneities are ignored, the spherical form of
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