Satgen 575  The Auroral Oval  by GM4IHJ  (BID SGEN575)  2000-04-01

For most radio amateurs, the polar aurora is something which provides
rough difficult copy of 2m DX once or twice a year. But for HF DXers
and satellite users , the aurora can be an ever present problem. Even
for stations so far removed from the magnetic pole that they rarely
get 2m auroral DX.

Consider the following. RS13 29.458 Mhz beacon, is proceeding north at
1134z 28 March 2000. It is overhead 20N 111W ( ie some way south of
the tip of Baja California), and it is putting a T8 nearly completely
clear tone signal into Scotland via sub horizon propagation. But by
1140z the signal is broad band noisy T2 60 Hz wide although totally
remote from polar climes , overhead southwest Wyoming at 41N 109W.

Unfortunately the great circle path to Scotland has just begun to
graze the edge of the auroral oval as it passes over Ungava Bay in N
Quebec, at about 60N 70W. Then as the satellite goes further north the
signal disappears completely at 1142:30z 49N 108W , because the great
circle to Scotland has now penetrated the auroral oval , only to
reappear at 1144:30z 52N 105W ,with a T2 rough wide band signal now
that the satellite is overhead the inside of the auroral oval.

By 1148z 68N 99W, with the satellite still inside the auroral oval the
signal is coming directly to Scotland and is now 100Hz wide. Worse
still, it is subject to very rapid doppler shifts at roughly 30 second
intervals. Producing an FFT audio spectrum display resembling the
sloping slightly overlapping tiles on a roof. ( Note northern aurora
seems to produce tiles sloping down in frequency , while the southern
aurora australasis seems to produce tiles dopplering up in frequency
as seen from Scotland).

Then whilst still inside the auroral oval at 1153:30 82N 40W, the
satellite finally comes above the Scottish horizon, quickly reverting
to a near perfect clear T8 signal. Changing to T9 as the satellite
finally exits the auroral oval as it overflies Svalbard at 80N 345W.

If you look at a Mercator projection map of the world. Wyoming appears
to be slightly south of west from Scotland . But on the great circle
map it bears 306 degrees true from Scotland , which accounts for the
auroral activity on the path even in quiet Kp2 auroral conditions . So
there is a clear requirement for a computer print out of a great
circle map showing the hour by hour position of the aurora borealis
oval, if your sub horizon DX takes you into areas where the great
circle path looks like going anywhere near this feature. Scots folks
look up at jet airliners heading north each day . Maybe a few wonder
why everyone is going to Iceland. Few realise that, that is the great
circle route for planes or radio to Western Canada and USA.

Second Thoughts. 1. The aurora is not always quiet Kp2 but when active
it is possible to extrapolate the rough location from a Kp2 plot. 
2. With sub horizon from RS13 over the antarctic Icecap a daily
occurance in Scotland at this time. It would be nice to have a Kp2
hourly plot of the aurora australasis. If such a thing exists.