Satgen 468  Space Propagation Pt7  by GM4IHJ  (BID SGEN468)  98-03-15

In 1946 , Australian Radio Astronomers just returned from war torn
Europe, set up a classic experiment which monitored "Radio Stars" at a
frequency of 64 MHz. Their targets were far away beyond the limits of
the Solar system, and like the stars observed visually, these radio
objects were twinkling ie their signals were scintillating, changing
phase and amplitude in an irregular manner.

Today the Internet is full of data about ionospheric scintillation. A
good place to start looking is the literature referred to in :-
http://www.physics.uq.oz.au:8001/sp/intro.html

This scintillation can be a great nuisance to satellite users,
affecting amplitude and phase of signals anywhere between 30MHz and 3
GHz, and its effects vary at different places around the earth.

Anyone who can monitor LEO satellites transmitting around 150 MHz or
lower in frequency , as the line of sight to their station from the
satellite transits the Polar auroral zones ( north or south )can soon
become familiar with the particularly intense scintillation which
occurs as the signal transits the Polar Auroral Front. Indeed all the
polar and sub polar regions are beset by ionospheric problems which ,
while the scintillation is not completely random , can seem to be so.
Because random Solar activity, the actual intensity of the solar cycle
at that particular time, and the intensely mobile nature of the
ionospheric plasma blobs in the polar zone ionosphere all play highly
variable parts in the overall scintillation scenario. Almost no
frequencies presently used by Amateur satellites are safe. Indeed a
famous 1970s experiment which relied in great part on scintillation
being (thought to be ) absent at frequencies above 2GHz, was only a
partial success when every frequency transmitted by the participating
satellite was affect by scintillation including those above 2GHz which
it had been hoped could be used, as an unaffected reference.

Equatorial scintillation, is also a great nuisance. It is strongly
dependent on the season of the year and the location of the observing
ground station. Equatorial scintillation peaks over Peru South America
between October and April, and by contrast is quiet then over the
Central Pacific, but peaks there between April and September when the
South American ionosphere is almost completely free of scintillation.
Equatorial Scintillation is normally credited to rapidly varying
Ionospheric turbulence and it can be very serious at sunset or at
night. However it has to be noted that while the Equator can at times
feature the very highest levels of scintillation , it can. at other
times feature an almost complete absence of scintillation.

Scintillation can be experienced where ever the signal encounters
turbulent plasma.So signal coming from natural radio emitters light
years away, can suffer intense scintillation peaks if they encounter
turbulent sections of the solar wind, a feature which can reveal
evidence of turbulent plasma heading for the Earth. Still further out
in interstellar space, plasma concentrations can cause scintillation
of signals, years before they get to earth receivers.