Tom,
Here is the abstract of Muldrew's article; needless to say, that actual
article is complicated and more than I want to wrestle with, never having
heard a LDE. But the section of the article on the history of LDE
reports in fascinating.
Enjoy,
73,
Bob, NM7M
------------------------------------------------------------------------
Here is the abstract of the paper
"Generation of Long-Delayed Echoes"
D.B. Muldrew, Communications Research Center,
Dept. of Communications, Ottawa, Canada K2H 8S2
from the JGR, Vol. 84, NO. A9, p. 5199-5215, 1979
Text:
Long-delay echoes (LDE), defined as echoes received from a fraction
of a second to several seconds after a radio signal is transmitted,
have been observed off and on for about 50 years. A variety of
explanations have been proposed in the past but none is completely
satisfactory. The following models are presently proposed for LDE:
(1) Radio waves of frequency less than about 4 MHz can become trapped
in magnetic field-aligned ionization ducts with L values less than
about 4. These waves after being trapped can propagate to the opposite
hemisphere of the earth where they become reflected in the topside of
the ionosphere. They can then return along the duct, leave it and
propagate to the receiver. Delays of up to 0.4 s result and they
probably account for most of the LDE at frequencies below 4 MHz with
estimated delays of 1-2 s.
(2) The signals from two separated transmitters T1 and T2, transmitting
a CW or quasi-CW signal, interact nonlinearly in the ionosphere or
magnetosphere. If the wave vector and frequency of the forced
oscillation at the difference frequency of the two signals satisfies
the dispersion relation for electrostatic waves, such a wave would
exist and begin to propagate. This wave could grow in amplitude due
to wave-particle interaction. At a later time, it could interact
with the CW signal from T2 and if the wave vector and frequency of the
forced oscillation at the difference frequency (frequency of T1)
satisfy the dispersion relationship for electromagnetic waves, such
a wave would exist and could propagate to T1, or some other receiving
station tuned to the frequency of T1. Reasonable ionospheric or
magnetospheric plasma parameters lead to delays of up to about 6 s
with this model.
(3) A large percentage of LDE have been reported with delays of tens
of seconds. These delays could be explained if the model in (2) is
applied to a magnetospheric ionization duct. Electrostatic waves
could propagate for about 1,000 km or more over the magnetic equator
in such a duct and delays of about 40 sec are possible. Dispersion
for finite frequency bandwidth would probably not be so large in
cases (1) and (2) as to make a voice unrecognizable. Dispersion in
model (3) for delays greater than about 10 s would normally be too
severe for voice modulation, but occasionally compensating effects
might occur for which voice would be recognizable.
Note: The models in the abstract were separated for clarity; nothing
else was changed.
Also, L-values are the distance in earth-radii (Re) where field
lines cross the geomagnetic equatorial plane.
Bob, NM7M
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