Satgen 469 Space Propagation Pt8  by GM4IHJ  (BID SGEN469) 1998-03-21

Propagation phenomena such as Sporadic E, Scintillation, and weather
factors , which affect satellite links, are present at almost all
receiving stations. Aurora, the subject of this present paper is by
contrast prevalent over polar and sub polar areas, but completely
absent over equatorial regions. Indeed the effects of aurora on
satellite communications are very dependent on latitude. The higher
your station latitude , the greater the incidence of auroral
disturbance of your HF and VHF satellite signals. Today in March 1998
we are just begining to climb out of the recent solar activity cycle
minimum. Solar flux levels are rising , and concurrent with this we
are seeing an increase in both the number and the strength of auroral
events. With the short duration aurora of March 10th being the
strongest event for some time.

There are 3 basic ways that aurora can effect space propagation :-
a.  By subjecting HF and VHF satellite links to a broadening of signal
bandwidth due to the irregular motion of the auroral ionosphere. In
these conditions signals are often received from satellites far below
ones station horizon, via forward scatter off the auroral front.
b. In addition to doppler widening and roughening of the signal note,
aurora also introduces multipath effects and major doppler shifts,
such that a single original signal can be received as two signals .
One signal clear tone , the other perhaps separated by 2 KHz in
frequency (on a VHF signal) ,having a rough T1 or T2 auroral tone.
c. Often present at the begining and end of an auroral event, is a
phenomenon refered to as "Auroral Es" , whereby relatively stable
dense patches in the ionosphere's E region refract or forward scatter
signals to and from space , particularly when the satellite is
poleward beyond the aurora and the ground station is on the other side
of the aurora nearer the equator.

So here in Scotland at 58N maglat, there are opportunities at the
start and finish of small auroras to the north, for Auroral Es
propagation. With forward scatter propagation during the aurora.
By contrast in big powerful auroral storms, the polar auroral front is
often overhead Scotland or someway to the south. So the propagation
opportunities are present for an extended period of time ( often 1200
to 2000 local. With both, auroral Es being present continuously via
parts of the aurora far from Scotland, and, looking southwards through
the aurora to any passing satellites. Lest this be considered the best
of all worlds, it should be noted however that these big auroras give
almost no opportunities for forward scatter propagation to and from
satellites when ones station in located poleward of the auroral front.
Please note , that the position of the north magnetic pole in America
south and west of the rotation pole , results in auroral effects 
being experienced much further south in North America than they are in
Europe. Equally noteworthy the very different situation in the
Southern Hemisphere whereby the South Magpole is a very long way from
the south rotation pole, results in auroral effects being very station
longitude dependent in that hemisphere.