Magnetospheric MHD Resonances and ULF Pulsations - Ultra and Extremely Low Frequency Electromagnetic Fields - page 226

Fig. 6.5 A schematic plot of the diurnal and latitudinal variations of the amplitude and sense of
rotation of the horizontal component for 5 mHz pulsations as observed by Samson et al. ( 1971 ).
The amplitude of the pulsations reaches a peak value at auroral zone, with latitude changing in
local time as shown with line of maximal intensity. The horizontal polarization switches sense
across this line. Taken from Glassmeier ( 1995 )

When propagating from the magnetopause into the magnetosphere the FMS wave
decays in amplitude until the resonance point will occur, as shown in Fig. 6.4 .The
switch in polarization at this point is consistent with waves propagating eastwards

k y <0 in the afternoon and westwards k y >0 in the morning. On account of
the fact that the radial derivative of the azimuthal electric component (dE y =dx
in the MHD-box model) changes sign across the resonant point, a four-quadrant
pattern arises due to the FLR phenomenon much as observed by Samson et al.
( 1971 ) (Fig. 6.5 ).

6.2.6 Effect of the Ionosphere on Ground-Based Observation

As has already been stated, the amplitude of the FMS-waves falls off exponentially
as they propagate towards the ionosphere and their amplitude becomes smaller than
that of Alfvén waves. This means that the polarization of the MHD waves incident
to the ionosphere can be considered to be basically corresponding to that of Alfvén
mode.
The ionospheric plasma may greatly affect the ground-based observation of the
ULF pulsations. The dominant effect is the attenuation of MHD waves and rotation