Bruce K1FZ said: "I have noticed that there appears to be a link between
the start of an aurora and long path 160 meter openings."
Others have noted this interesting effect. The only physical tie between
the two that I'm aware of is that a spike in the K index can initially
improve the electron density valley that is above the E region peak in the
nighttime ionosphere, resulting in a better chance of ducting. This comes
from a paper by Zhang and Kamide in JGR in 2004 (they just reported their
results, but didn't tie it to amateur radio propagation - heck, they might
not even be aware of amateur radio!).
I'm not saying this is exactly what happens, just that it *may* explain the
link between the start of an aurora and long path 160 meter openings.
As for the echoes that started this thread and aurora, one respondent
wondered about the velocity of propagation of the RF slowing down due to a
back scatter mechanism. Ionosondes report virtual height (not true height)
because they assume the speed of light for the vertical incident RF. But as
the RF goes up in the ionosphere, the group velocity decreases as the index
of refraction approaches zero. If these echoes are of Earthly origin, then
the appropriate plasma frequency could decrease the group velocity and
contribute to the echo mechanism. Just speculation.
Having said all this, right now I think Andy G4PIQ's comments are most
appropriate for the echoes - unless there is some data and physical
mechanism that says otherwise. Of course this is the problem with many of
these unusual occurrences - we don't have enough relevant data with respect
to the ionosphere or whatever - just observations of an interesting
happening.
Carl K9LA
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