MHD-Wave Exposure on the Ionosphere - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 356

Typically,

2 π
300 km − 1 ,

10 3 km , 10 4 nT . k ≈ H ≈ 0 ≈ 2 ×

It follows from (12.56) that the recorded fluctuations

δN T
N T ≈

10 − 5 10 − 4 5 × − 3 ×

can be caused by geomagnetic pulsations with amplitudes on the ground of

b ( g )
x

≈ 5 − 30 nT .

Such pulsation amplitudes are not frequent, but quite realistic at middle lat-
itudes.
The formulae presented above are of an evaluating character. For more
precise quantitative conclusions, one can conduct numerical calculations ac-
cording to the following scheme. Firstly, we calculate the vertical profile of
electron concentration, the integral conductivities and the altitude of the thin
ionosphere for a given set of geophysical parameters in accordance with the
IRI-model [3]. Using the values obtained, we calculate the vertical distribu-
tion of the MHD-wave field with given transverse scales. And lastly, we find
the TEC-perturbation N T caused by the MHD-wave by integrating (12.48)
numerically. Figure 12.3 demonstrates the results of the computations for the
dayside and nightsight ionospheres.
The presented amplitude (Figures 12.3a, c) and phase (Figures 12.3b, d)
relations between the b ( g x -component of the ground magnetic variation and
TEC N T perturbation correspond to the middle-latitude ionosphere. It was
assumed that the pulsation period T = 40 s, the azimuthal wavenumber k y =
2 π/ 5000 km − 1 . The value δN T /N T presented in the plots is normalized on the
value of b ( g x = 1 nT.
The sharp rise at large k x (Fig. 12.3a) is connected with the exponential
decreasing of the ground wave magnetic field at the horizontal scales compa-
rable to the ionosphere altitude. At horizontal scales 3

10 2 km typical for ×

middle-latitude Pc 3 , 4 pulsations, the relative fluctuations

δN T
N T ≈

10 − 4 b ( g )
x

(nT) . (12.57)

Quasi-periodic fluctuations of TEC with amplitudes of δN T /N T ≈ 5 ×

10 − 5 10 − 4 were discovered experimentally ([7], [11]). One can believe

that geomagnetic pulsations of the Pc 3 , 4 type with amplitudes from fractions
to several nT at middle latitudes can indeed cause the observed periodic TEC
variations.
The dependence δN T /N T on the wavenumber in the nighttime ionosphere
(Figures 12.3c,d) is qualitatively of similar nature. It follows from Fig. 12.3c

− 3 ×