Friends in Radioland -

Having seen a large number of postings on "strange conditions" on
160 meters, I think it's worthwhile to indicate what might be
happening, especially since correlations with K and A indices are
not found.

First, the free electrons on which propagation depends are in
thermal equilibrium with their surroundings - other electrons,
positive ions and neutral atoms and molecules.  In the absence of
any disturbance that "heats" the region, say a blast of solar wind
heating the electrons or Joule heating of the neutral gas from
strong ionospheric currents at auroral latitudes, propagation of
signals from a transmitting antenna proceeds normally.

So signls advance by low-angle E-hops, high-angle F-hops and E-F
ducting in the electron density valley above the E-region.  What
is heard will be signals from the surviving mode(s).  Low-angle
E-hops are the most numerous and suffer high losses from ground
reflections and by being confined to the D-region.  High-angle
F-hops are the elliptically polarized O-waves in magneto-ionic
theory and propagate with far less loss than X-waves and E-hops.

But when the F-hops are no longer detectable, ducted signals may
still be heard, providing they emerge from the duct.  That will
happen around local sunrise, where critical frequency contours are
being lowered; that lowering tilts electron density surfaces
downward and brings signals out of the duct at higher angles than
they had earlier in their propagation.

But ducted signals may also "leak" out of a duct before reaching
the sunrise end; that happens when a disturbance reaches the level
of the duct, upsetting the orderly condition of ionospheric
surfaces established in the dark of night.  Such disturbances
may be of atmospheric origin or geomagnetic origin at higher
latitudes.  Atmospheric disturbances might have some "indicator"
in local weather records but have not been identified to date.

Even with these uncertainties in propagation, it should be noted
electrons gyrating in the geomagnetic field RAISE the effective
frequency on 160 meters from 1.8 MHz to 3.0-3.4 MHz, thereby
LOWERING ionospheric absorption on paths by a factor of 2.8-3.6.
So, in spite of its complications, not all aspects of magneto-
ionic theory make DXing difficult.

73,

Bob, NM7M