[B-R-I] Re: Pattern Changes, etc

[Kaimbridge M. GoldChild]

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 
order):

    --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
         Holidays).

    --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.

      ~Kaimbridge~

-----
      Wikipedia—Contributor Home Page:

                  http://en.wikipedia.org/wiki/User:Kaimbridge

        *****  Void Where Permitted; Limit 0 Per Customer.  *****