Topband: A few words

[Robert Brown]

Friends in Radio Land -
 
Given the recent interest in skewing, a few words of explanation
would seem in order.
 
In simple terms, skewing is just a case of horizontal refraction
and results from a wave encountering a horizontal variation in
ionization.  Thus, it is like the hops on HF where signals are
continuously refracted or bent away from F-region ionization
overhead.
 
On 160 meters, the most common skewing is in connection with
auroral activity and occurs on paths heading toward the high
latitudes.  The direction of the skewing at a particular time
depends on the level of magnetic/auroral activity and whether
the path is heading toward the equatorward or poleward limit of
the auroral zone.
 
Signals will be refracted southward, away from the auroral zone,
if approach is on the equatorward side of the auroral zone.  At
high levels of magnetic activity, say with K>6, it is possible for
the geographic location of some stations to be inside the auroral
oval; in that case, signals may be refracted northward from the
poleward limit of auroral ionization.  Of course, there will be
ionospheric absorption in the process, depending on the ionization
encountered in the D- and E-regions along the path.
 
Another source of skewing is the ionization in the terminator;
while that source varies in position with season and the advance
of time, it is relatively constant in strength and most intense
when it comes to refraction and ionospheric absorption.
 
Periods of strong magnetic activity are rather infrequent but can
involve the Van Allen belts, with precipitation of very energetic
electrons at sub-auroral latitudes.  Those "hot spots" may give
rise to skewing of paths but such locations are rather difficult to
identify on magnetic instruments at ground-level.  NOAA satellites
may show those cases of electron incidence but only briefly, with
nothing in the way of details.
 
In summary, path skewing results from horizontal gradients in
electron density.  For interpretation, the problem is to locate
the gradient and identify its source.  Without any gradient or
source, reports of skewing are incomplete.
 
73,
 
Bob, NM7M