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| ISSMI'98 | ISSMI'98 |
ON THE ROLE OF LARGE-SCALE PLASMA IRREGULARITIES AT THE FINAL STAGE OF ANOMALOUS ABSOPTION DEVELOPING UNDER HF HEATING OF THE IONOSPHERE N.A.
Zabotin, G.A. Zhbankov |
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Poster paper full text
Present paper suggests one possible physical mechanism of the reflected signal amplitude increasing at the final, third stage of anomalous attenuation developing what is observed in high-power (more than 20 MW) HF ionospheric heating experiments [1]. It is shown that quantitative and qualitative characteristics of this phenomenon can be explained by interaction of radio waves with large-scale (~10-50 km) irregularities in the heating region.
Main radio wave propagation peculiarity in
presence of large-scale irregularities consists in complicated
ray structure of the signal. At a relatively low level of
electron density disturbance (when 
is of order of 1%) the multi-ray
propagation takes place. The angular spectrum of radio wave
arriving to the observation point becomes discrete in this
conditions. Hence, the signal on the Earth's surface is a result
of the interference of several different rays (see Fig. 1).
Fig. 1. Plot qualitatively explaining the mechanism of multy-ray reflections from a plasma layer with large-scale irregularities.
With the aid of numerical statistical modeling
based on multiple ray tracing it is shown that in presence of
large-scale irregularities the reflected signal amplitude mean
value is increased significantly. Likeness of this effect with
the phenomenon observed in experiment is illustrated by two
plots, showing experimental data [1] on anomalous absorption
temporal development (Fig. 2, time 
>1 c) and calculated dependence of the
reflected signal mean amplitude on the irregularity level 
(Fig. 4). Quick
growth of the large-scale irregularities may be due to the
nonuniform heating caused by natural plasma irregularity in scale
lengths 10-50 km.
Fig. 2. Typical view of the pump
wave aplitude 
dependence on time for a single heating period, according to
[Berezin I.V., Boiko G.N., Volkov B.M. et al. Izvestiya VUZov.
Radiofizika, 1987 V.30, p.702 (in russian).].
The calculations were made for the common model of a plane plasma layer with superimposed wave-like large-scale irregularities:
Solving the 'extended geometric optics equation
system' (Eq. 2) we can get the parameters of all 'echoes' for the
given values of 
, 
and disturbance phase
. We keep only
those echoes which ranges differ not more than by 50 mks from the
range of the strongest one. Then the interference task is solved.
The full field amplitude 
is determined assuming the echo phases are random
and uniformly distributed over the interval 
. The resulting value
of 
is obtained
by averaging over the echo phases and the disturbance phase . It is taken into
account that number and parameters of the echoes are different
for different values of .
Fig. 3. Dependence of the mean
amplitude of the reflected from the ionosphere signal on the
large-scale (10-50 km) electron density irregularity level. The
amplitude 
of the
signal reflected from a regular (without irregularities) layer
has been taken for a unit. The irregularity level 
is in percent (0.01).
In the Figure 3 the dependences of on for five different
values of (10,
20, 30, 40, 50 km) are presented. The is expressed in units of the undisturbed
( =0) reflection
amplitude and is
in percent (0.01). Here are the main features of the presented
dependences:
is always above 1; the dependence of on passes three
stages. The first: starting from =0, = 1, slow, more or less regular
increase up to ~ 2-3. The second: sharp increase up to
several tens, sometimes with several pikes and sharp
return to the "stationary" value ~ 4-5. The
third: absence of further growth, small oscillations near
the "stationary" value ~ 4-5. when increasing. Interpretation of these results is quite simple.
When the reflection level is disturbed, the number of 'echoes' is increased and more energy is returned to the sounder location. The effect 'focusing prevailing over defocusing' also plays its (but less significant) role. All this explains the feature 1.
Stage 1: Number of echoes is increased, but the
most probable local curvature radius 
of the reflection surface is much larger
than the distance from the surface to sounder 
. (The relation 
for the
'near-zenith' reflections exists, where 
is the layer thickness).
Stage 2: According to the latter relation, the
most probable value of is decreased and becomes of order of . The focusing in
literal sense of the word takes place frequently and gives large
contribution to .
Stage 3: The most probable value of becomes much smaller
than . The number
of reflection points achieves of saturation.
At last, the feature 3 is a consequence of the
relation if = is being substituted
into it.
Thus one can conclude that the considered phenomenon is not directly connected to the mechanisms of radio wave anomalous attenuation, such as multiple scattering on kilometer irregularities or transformation into plasma waves on small-scale irregularities (the latter one influences ordinary waves only). This circumstance must be taken into account for correct determining of the anomalous attenuation magnitude.
References
Fig. 4. Typical statistical distribution of the total field amplitude of multi-ray signal.