MHD-Wave Exposure on the Ionosphere - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 341

12
MHD-Wave Exposure on the Ionosphere
Another point of interest is the influence of an MHD-wave on ionospheric
plasma. It is apparent that the electric field of an MHD-wave causes the
movement of the background electron component and of ionospheric plasma
inhomogeneities. The field-aligned currents in an Alfven wave are directly re-
lated to the electron transfer along the external field. The FMS-wave magnetic
pressure results in perturbations in plasma density. All these effects can be de-
tected by ground-based and satellite methods using radio waves to investigate
the ionosphere. Electron movement and density change cause displacement in
the frequency and a turn of the polarization plane of the radio signal radiated
either from the ground or a satellite. Both these effects are detected using
Doppler and Faraday sounding.
12.1 The Doppler Effect Provoked by an MHD-Wave
In Doppler sounding the transmitted pulse filled by a wave with a predominant
frequency, reflects from the ionosphere and returns to the ground. In the sim-
plest situation of vertical sounding, the transmitter and receiver are located
in the same place. Here the frequencies of the upgoing and downgoing waves
are compared and subtracted from each other. The small difference between
them versus time is called a 'dopplergram'. Variation in the dopplergram are
defined either by displacement of the reflected layer and by variations of the
refraction index by the way of propagation of the radio wave to the reflection
point and back.
The coordinated observations of short-period fluctuations in the frequency
of stable UF-transmitters and ULF-variations ([6], [7], [8]) have demonstrated
that almost any magnetic perturbations give a response in the radio wave
Doppler oscillations. Rishbet and Garriott [16] and Jacobs and Watanabe [9]
suggested a theory to explain radio frequency oscillations by reflection point
displacement as a result of electron drift in the electric field of the Alfven
wave.