Re: [IRCA] A & K Indices

[Russ Edmunds <wb2bjh@xxxxxxxxx>]

--- Jim Pogue <jpogue@xxxxxxxxxxxxxxx> wrote:
> 
> My "basic" understanding has been, the lower the A index, the better
> the BCB
> propagation is likely to be, but I'm sure there's much more to it
> than that.
> 
>

*** That's generally true, but like anything else, there are
exceptions, one of which is the fact that the A index doesn't
immediately reflect a geomagnetic distrubance. 

But it also depends on your perspective. For instance a DX'er who
devotes the majority of efforts toward foreign DX, 'auroral conditions'
( a sustained A index over 20 or a shorter period over 50 just to use
some very rough numbers for illustration here ) present an opportunity
for greater DX toward South America.

Briefly, to the extent it's possible to do it briefly, auroral activity
moves the normal area of absorption ( which normally settles in as a
somewhat narrow ring centered on the earth's magnetic North pole in
Northeastern Canada ) southward. As this occurs, stations whose signal
path from transmitter to your receiver will be subject to absorption -
often to the point that some of the large powerhouse clears to the
North, East or West may become unlistenable due to interference, or
disappear completely. Many times, this reveals stations to the south
which can only be heard during auroral conditions.

But there are also other things going on in the overall that can
depress DX even during periods of low A index. For instance year after
year conditions for Trans-Atlantic DX follow a pattern where there are
periods excellent condiitions in the Fall which often disapper during
December, January and part of February, only to return again - that
effect has become known as the "MidWinter Anlomaly", which is not yet
well understood.

And of course summer, with lengthening hours of daylight, depresses
conditions too because of what we know as 'absorption' of signals -
this is the same principle which says I won't hear KMOX-1120 in the
daytime. 







Russ Edmunds
Blue Bell, PA  ( 360' ASL )
[15 mi NNW of Philadelphia]
40:08:45N; 75:16:04W, Grid FN20ID
<wb2bjh@xxxxxxxxx>
FM: Yamaha T-80 & APS9B @15'
AM: Hammarlund HQ-150 & 4' FET air core loop

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