Va. Pilipenko et al., Distortion of the ULF wave spatial structure upon transmission through theionosphere, J GEO R-S P, 105(A9), 2000, pp. 21225-21236
Distortion of the ground-based spatial structure of ULF waves is described
in detail using analytical and numerical models of the resonant Alfven wave
transmission through the "thin" ionosphere. The numerical calculations hav
e been done for geophysical conditions cor responding to the midlatitude Bo
rok Observatory and the low-latitude L'Aquila Observatory. As compared with
a resonant field-aligned oscillation above the ionosphere, the ground spat
ial structure has the following additional features which have not been des
cribed by existing models: the amplitude peak is shifted northward, the wid
th of it is changed, and amplitude and phase are modified. The effect is mo
st evident when the resonant frequency and ionospheric conductivity are hig
h. From a physical point of view the effect is caused by two reasons: (1) e
xcitation of a diffusive-like surface-type mode in the ionosphere and (2) f
inite Earth's conductivity. The possible influence of the predicted distort
ion of the ULF field on various ground-based methods for the determination
of the magnetospheric resonant frequency is discussed.