MHD-Wave Exposure on the Ionosphere - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 342

Let V ∗ be the velocity of the reflecting layer, f r be the frequency on
which radio sounding is performed, f ( t ) be the frequency of the received signal
reflected from the ionosphere. Then

∆f r = f ( t ) − f r (12.1)

is the observed frequency deviation. If the radio wave is incident onto the
ionosphere normally, then the relation between these two values is determined
as

2 f r V ∗
c

∆f r = − . (12.2)

This theory was used to interpret observations in most of the subsequent
works. Watermann [20] showed, however, that it is impossible to explain all
experimental results in the framework of this concept. A revision of the earlier
conceptions was carried out in ([13], [14], [17], [18]). Numerical calculations
of the Alfven wave transformation and of the corresponding perturbations in
ionospheric plasma demonstrated the need for a thorough modification of the
views expounded in ([9], [16]).
Figure 12.1 demonstrates an example of variations in H -and D -compon-
ents of Pi 2 oscillations (panels ( a ) and ( b ) and of simultaneous Doppler dis-
placements in the ∆f ( t ) signal on frequency f r =4 . 94 MHz, (panel ( c )).
The measurements were performed at a middle-latitude observatory (49 ◦ 40 N ,
36 ◦ 50 E ) [2].
The task of detecting MHD-wave effects in dopplergrams requires the uti-
lization of fine enough methods of signal extraction at a high noise level. The
basic idea of [2] consisted in ∆f ( t ) filtration by an optimal filter constructed
by Pi 2 pulsations. The filtered Doppler displacements (panel ( d )) and the
magnetic oscillations are depicted in Fig. 12.1 ( H -component in the ( e ) panel
and D -component in the ( f ) panel). The filtered magnetograms frames have
been extracted using the Doppler trains ( d ) as a basis for the digital adapta-
tion filter for the total initial horizontal magnetic components. The amazing
similarity can be seen of the simultaneous Pi 2 pulsations on the ground and
Doppler displacements of radio frequency.

The Doppler Velocity

The total change of the phase in the wave transmitted from the ground surface
to the reflection point and back is [5]

z 0

Φ = 4 πf r
c

π
2 ,

µdz − (12.3)

0

where µ =Re n is the real part of the refractive index n ; the radio wave
reflection point z 0 is located at altitude z at which µ = 0. Displacement of the