Inhomogeneous Ionosphere - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 288

equatorial features near the terminator, contrary to the case when only the
incident FMS-wave is under consideration.
9.3 Experimental Verification of the
MHD-Wave Polarization
Experimental data based on the observations of pulsations at low latitudes,
especially in the region of the equatorial electrojet and near the terminator,
provide a unique possibility to classify oscillations not only according to mor-
phological criteria, but also primarily, according to physical features and mech-
anisms of their excitation. The advanced ideas and numerical results show
essential differences between the mechanism of ionospheric transformation of
Alfven polarized MHD-waves and that of FMS-polarized waves.
The isotropic homogeneous ionosphere shields the Earth from the magne-
tosphere almost completely for low-frequency Alfven waves. A deviation from
isotropy, i.e. when Hall conductivity exists or when the ionosphere is inhomo-
geneous, makes the ionosphere becomes transparent and allows the magnetic
field to be seen below the ionosphere from the ground. Since the Pedersen and
Hall conductivities have approximately the same order of magnitude, an in-
crease will take place in the transmission coecient of the Alfven wave in the
terminator zone which separates the high-conductive sunlit ionosphere from
the low-conductive night ionosphere. The initial azimuthal magnetic compo-
nent of the incident wave makes a π/ 2 turn under the ionosphere due to Hall
currents.
In an homogeneous ionosphere moving from the dayside to nightside, the
angle does not change by more than a few degrees. On the other hand, at the
terminator itself, the horizontal magnetic vector may rotate by more than 10
degrees.
Another anomaly occurs in the region of equatorial electrojets. This region
is extremely sensitive to whether the Alfven wave mode is even or odd. The
equator is a special area, even in the event, that waves originate in remote
magnetospheric regions and penetrate from high to middle and low latitudes.
This is primarily due to the large Cowling conductivity, and also owing to
the change of the Hall conductivity sign as a result of the proximity of the
ionospheres of the Northern and Southern Hemispheres. The current systems
generated by the symmetric and antisymmetric modes of the Alfven wave
display an essential difference on the equator.
When field-aligned currents have the same direction in conjugate hemi-
spheres (the symmetric mode), a ground magnetic variations anomaly appears
on the equator, about a quarter of the variation below the source field-aligned
currents. A well-defined seasonal effect of the terminator also takes place, as it
follows from Figures 9.4-9.5. Long-period pulsations due to the reconnection of
the geomagnetic field and the IMF [8] may be considered as an example of such