Hello VHFers,
I have been ruminating about the mechanics of Auroral contacts. Well,
maybe not exactly ruminating, but I have been trying to think about it!
Aurora is a lot like billiards. The take off azimuth has a lot to do
with what you work. I have been actively writing down beam headings
along with my logging data for my recent AU contacts. This is an attempt
to actually determine the mechanics of each contact. Beam directivities
increase on the higher frequencies. Most six meter contacts are made
with beams that are too wide to give an accurate reading, but some 144
antennas and 222 MHz antennas can be quite precise. Back in the 1980's
I used a 144 MHz EME array on Aurora. It was very difficult as I had a
heck of a time keeping signals within the beamwidth of the array. It was
5.9 E X 9.0 H degrees. Still, it was interesting to peak signals up.
Most would peak at 275to 310 degrees if they were well off to my West.
Stations down South peaked more northerly and I found that some stations
benefited from me having some elevation cranked in. Most long distance
signals to the West were on the horizon. Any elevation made things
worse. Anyway, I am asking that you try to write down your beam
headings for each contact if at all possible. I am trying to tabulate
Auroral contacts in an effort to maximize results for everyone in the
future. Knowing the beam headings is crucial.
Every Aurora is different. The one thing that is constant is that the
scattering mechanism is at about 100 to 120 km above the Earth in the E
region. Visual Auroras are much more widespread than the actual radio
Aurora. Both are influenced by the magnetic field lines in your area.
The geomagnetic latitude is important as is the magnetic dip angle. It
all varies depending on your location. Path loss can vary by the inverse
square law up to and back from the scattering point. There is also a
large loss as the angle of the arriving wave departs from the
perpendicular to the magnetic field. I am trying to sort this all out,
but I need some beam headings from both ends of the contact to plot
contacts. The proper beam heading can vary depending on where the other
station is aimed. Headings can vary depending on the N-S thickness of
the Auroral belt and it's geographic latitude. A stronger Aurora will be
seen farther to the South.
I have been in communication with physicists at the Space Science Center
at the University of New Hampshire, which is about a half hour drive
from my house. They do lots of magnetic field experiments with rockets
and satellites, plus they have a program here in New England where they
are building a network of flux gate magnetometers in northern New
England. I will probably update my old magnetometer here and join that
network over the next year or so, They have gotten a lot more
sophisticated than I was back in 1994 when I first installed my
magnetometer. My hope is that I can get some experienced eyes to look
over the Aurora data and point me in the right direction. I figure
that radio data can be an important part of any Auroral study.
Things to consider with Auroral storms: The line of sight distance to
your radio horizon is about 1250 km. This is the farthest North that you
will see any scattering assuming a 120 km scattering height. On strong
Auroras, there may be scattering points much closer or even to your
South. (That can be bad!) The good news is that we in mid latitude areas
will almost never be totally North of the AU. The farther East or West
you turn your antenna. opens up more DX opportunities if the station on
the other end is doing likewise. The actual heading will depend on where
the Auroral scattering points are. These can be rather small areas, so
that a sharper beam can improve the S/N ratio. The downside is that
sharp beams can mean that the scattering point will appear to move!
So, If you have any azimuth data from previous sessions, please let me
know. If we get an Aurora in the near future, please note your azimuths
and send that along. I will try to match things up and try make sense of
it all.
73
Dave K1WHS
Also see:
https://directivesystems.com/wp-content/uploads/sites/2/2014/04/Magnetometer.pdf
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