Topband: So far, so good?

[Robert Brown]

Friends in Radio Land,

The red glow reported by KN4LF is a high-altitude, type A aurora and has
its origin in an excited state of atomic oxygen that has a 110 second
lifetime.  With collisions infrequent at high altitudes, the order of
1,000 km, O atoms may persist in that state and then radiate 6,300 A
photons in returning to the ground state of atomic oxygen.

An amusing example of such observations is to be found in the historical
chapter of "Geomagnetism" by Chapman and Bartels.  It seems the Romans saw
such a glow in the sky and thinking it was from the Huns pillaging Roman
cities, they saddled up and roared off to the north in their chariots.
Come the light of day, they found no Huns.

Other examples of low-latitude auroral sitings are at Mexico City (March, 
1989) and Athens, Greece (February, 1958).  I'm sure there are others in
the historical record.

As for "shattering" of the ionosphere, the question comes down to just how
deep the impact of solar wind affected geomagnetic field lines.  The
"shattering" mentioned by KN4LF has to do with depletion of F-layer
electrons when the field lines they reside on suffer serious distortion by
the impact of the solar wind, taking the field lines back into the
magneto-tail.  There is essentially a dividing line between dipole-like
field lines at the lower latitudes and those field lines that go into the
magneto-tail.  Above that line, solar protons have easy access to the
polar cap but the line may move equatorward with the onset of a
geomagnetic storm.

So during events like this, the polar cap will be bombarded by the
low-energy solar protons and propagation paths which cross the polar cap
will be adversely affected while paths across lower latitudes seem to
be relatively immune to the disturbance.

Typically, there is further disruption of propagation when the solar
plasma hits the outer field lines, driving the magnetopause inward and
increasing the size of the polar cap.  The things to look for are the
the times of magnetopause crossings noted by satellites at geosynchronous
altitudes and the sudden commencement of the magnetic storm.  With all the
solar/terrestrial activity in the past week or so, it may take some time
to sort out these effects.

I can conclude by saying there were no auroral sightings here in
Anacortes, WA (48.5N/122.6W) at 06, 08 and 10 UTC on July 16.  The sky was
clear but there was a full moon in the sky.  What a disappointment.

73,

Bob, NM7M



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