ULF-Sounding of Magnetosphere and Earth - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 327

200

0

60
latitude [deg]

Fig. 11.4. Latitudinal dependency of the FLR-period

where δ i is the half-width of the FLR-region in the ionosphere, h is the E -
layer height, x is the distance along the meridian, x r ( f ) is the location of the
FLR. Coecient C 1 is defined by the curvature of the FLR-line. It vanishes in
the 'box' model with a straight field (see Chapter 6). Such a model provides
reasonable accuracy with experimental data at x r ≤

500 km.
Let a half-width of an FLR-area on the ionospheric level be δ i =20km.
Then a characteristic spatial scale of the resonance structure on the ground
is about 100-200 km. It is evident that there are geoelectrical cross-sections
for which the effective skin depth d g in the Pc 3 , 4 range and a horizontal field
scale are comparable. Therefore phase and amplitude features of the initial
waves noticeably influence the results of the magnetotelluric soundings of such
geological profiles.
Let Φ, Λ be the geomagnetic coordinates of an observation point. Assume
that the observed field at some frequency f results from FLR-oscillations of
the corresponding FLR-shell located at distance

180
π
from the observer. Here Φ r ( f ) is the latitude of the FLR-shell with resonance
frequency f r = f (see Fig.11.4). Latitude-dependence of FLR-frequency is
shown in Fig.11.4. The ground model is a four-layered geoelectrical cross-
section with ρ 1 = 30 Ohm

x r ( f ) = 6370 · ( Φ r ( f ) − Φ ) ·

10 3 Ohm · m ,h 1 =3km; ρ 2 =3 × · m ,h 2 =

10 2 Ohm

50 km; ρ 3 =3 m, h 3 = 50 km and lower half-space with ρ 4 =0 .

Ionospheric conductivities are Σ P = 13 Ohm − 1 ,Σ H =14 − 1 Ohm − 1 .
The height of the 'thin' ionosphere is h = 105 km. The calculations are
carried out for wavenumber k y =0 . Period T = 100 s is the FLR for the
geomagnetic latitude Φ =60 ◦ . The dip is I =74 ◦ . There were computed

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