MHD-Wave Generation by HF-Heating - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 361

13
MHD-Wave Generation by HF-Heating
13.1 Introduction
Since the wavelength of the ULF-signal in the ground is an of the order to hun-
dreds kilometers, therefore to produce a signal in the ULF-range, one needs
a tremendous size of radiated antenna. For example, in order to produce the
large-scale signal at 1 Hz and amplitude of about 0 . 05 nT we need a ground
loop with a radius of about 100 km with a current of 1 kA ([8], [13]). An im-
portant restriction of this method of generation, besides the enormous size of
the radiating system and the large currents, is associated with the conducting
ground, which may reduce the indicated value to 10 − 4 nT.
The main point of the man-made ULF-generation is the attempt to lift up
a radiation system above the ground, transferring it to either the ionospheric
or magnetospheric heights.
In this chapter we examine an intensity of conceivable artificial magnetic
disturbances which could be produced by a powerful HF-wave. One can choose
the frequency of a strong HF-wave so that its pump frequency ω will be close
to one of the resonance ionospheric frequencies. The powerful HF-wave is
incident onto the ionosphere results in a rise of the electron temperature and
in fluctuations of the ionized component ([11], [12]). In doing so the electron
temperature T e can increase in 20-40 times. It leads to a change in the collision
frequency of electron with neutrals. This HF-wave is modulated in amplitude.
The modulation frequency Ω can be varied within very wide limits, depending
on the range of the geomagnetic variations of interest. As a result, geomagnetic
variations occur with the frequency determined by the modulation frequency
of the strong HF-wave. The conductivity changes due to an increase of the
electron collision frequency ν e with T e increase.
There is also another possibility of the variation of the conductivities: in-
crease in T e with a consequent displacement of the ionization-recombination
balance and changing of the electron concentration N e . An anomalous re-
gion in the background ionospheric current results in a magnetic disturbance.