Satgen264  Auroral Event Timing  by GM4IHJ                    16 April 94
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Last week sgen263 reported Arctic path trans auroral propagation to and
from HF satellites up to 20 degrees below the UK horizon. This report
produced a number of readers questions, amongst which , the most common,
asked " What is the timing sequence of radio aurora ( ie both via back
scatter and trans auroral paths ) " ? This sgen tries to answer this
complex subject.
The Sun, either via a Coronal Mass Emission CME ( the actual source of
Flares and the rest of the menagerie of explosive effects ), or, via a
less intense Coronal Hole CH, projects Sun stuff ionised Hydrogen =
electrons and protons, into the Solar system. CMEs take about 36 hours to
reach the Earth. CH material takes about twice as long. Both are deflected
by the Earths magnetic field and according to which ever of two very
different theories you chose, finally result in precipitation of Sun
stuff, downwards into the Polar ionospheres . Usually, but not always ,
the bigger the magnetic disturbance created, the further the disturbance
spreads away from the magnetic poles.
A very big K8 storm can send material over the UK north eastern horizon
from perhaps 1100 ut onwards in winter or 1130 onwards in summer. The more
typical small storms K4 , do not get as far away from the magnetic poles .
So we have to wait until perhaps 1500 ut in winter or 1620 in summer
before Earth spin brings the Auroral plasma over our north eastern
horizon. PLEASE NOTE THESE TIMES ARE FOR RADIO PROPAGATED OFF OR THROUGH
THE AURORA .THIS IS NOT VISUAL AURORA. Big storms cease usually before
1900 ut in winter, 1830 in summer . Small storms may last till much later
ie 2200 in winter and 2140ut in summer. There may then be a second phase
of radio propagating aurora starting after another hour or so.
The crucial point to note is the difference between the Winter and summer
timetables. In theory the Earth's magnetic field would align the aurora
around the magnetic pole. But Solar wind pressure flattens the circle and
also pushes it so that the centre of the resulting oval is anti sunward of
the magnetic pole with the oval's minor axis along the extension of a line
drawn from the Sun through the magnetic pole
Complicating the above is the fact that Earth axial tilt causes the
position of the Sun to move nearly 47 degrees north between northern
winter and summer. So the actual UT time of day when the Sun, your station
and the magnetic pole are on the same line ( = magnetic noon or midnight )
is very different from UTC midday or midnight , and this difference varies
with the season of the year as reported above.
We are presently in that part of the Solar Sunspot cycle when Coronal
holes produce lots of moderate auroras. GM4IHJ uses GB3LER 50.065 MHz for
auroral warning, with GB3RMK and GB3BUX 50.061 and 50.000 respectively
being useful to define the extent of auroral coverage. Recent Coronal hole
auroras have been starting up around 1437 ut. They sometimes sputter for
about half an hour before they get strong and steady . At which point
consistent 28/50/144 back scatter DX and 29 MHz satellite trans auroral
sub horizon contacts may appear. Good 28/50/144 stations may also
experience short range ( eg UK to Northern Sweden ) terrestrial T9
contacts via trans auroral propagation . EG OH9TEN 28.2525 is sometimes
heard at GM4IHJ T9. Good Hunting  73 de GM4IHJ @ GB7SAN