[B-R-I] Re: Pattern Changes, etc
Kaimbridge M. GoldChild
Sun Feb 19 16:22:18 EST 2006
One of our esteemed practitioners of law, Dr. A. Joseph Ross, JD, wrote,
> How is it that AM DX reception is highest at sunspot minimum? I
> would have thought it would be the other way around.
Because there are three different ionospheric "layers" (in descending
--F: The upper layer; the stereotypical "skip" layer that
is mainly responsible for mediumwave ("AM") and
shortwave skip. Actually, the F-layer splits into
two parts during the day--"F_1" and "F_2"--then
recombines at night.
--E: The middle layer; this layer plays a more limited
role in "regular" skip, but is more well known
(and appreciated) for its seasonal, sporadic skip
inducements (thus, the term "sporadic E-skip") in
the VHF regions (including TV Ch.s 2-6, FM and,
rarely, into Ch.s 7-13), peeking in mid-spring
to mid-summer (with a mini-season around The
--D: The lower layer; this is the "wet blanket" of skip
that, when charged, causes absorption of skip in
the lower frequencies (including MW). This layer
is normally just present/active during local
daytime--which is why you normally can't hear
MW skip during the day, except sometimes during the
shortest days of winter, when the sun is weakest.
During the peak of the solar cycle, the sun is sending out more solar
and geomagnetic radiation that, yes, does charge the F-layer up (causing
more MW and SW skip), but also charges up the D-layer (particularly
during geomagnetic storms), which can result in "daytime dead", whiteout
conditions on the MW band, for up to days at a time (though, sometimes,
especially early in a whiteout, there may be a hole in the overcharged
D-layer, resulting in dead conditions out to 1000-1500 mi.s, yet more
distant stations--usually south, as enhanced D-layer charging usually
drains down from north to south--may come poking through).
Thus the sunspot minimum--while not charging the F-layer--promotes a
thinner, less affecting D-layer, hence better MW conditions, overall.
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