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ARRL Propagation Bulletin ARLP038 (2005)

ARLP038 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 38  ARLP038
From Tad Cook, K7RA
Seattle, WA  September 9, 2005
To all radio amateurs 

ARLP038 Propagation de K7RA

Last weekend began with a high speed solar wind provoking
geomagnetic instability, hence the high A index values. Conditions
quieted down after Sunday, but then sunspot 798 returned, which we
last saw in August. The spot released several powerful X-class solar
flares over the past couple of days, and although not squarely
pointed at Earth, glancing blows could cause greater geomagnetic

Currently the interplanetary magnetic field points north, which
could offer some protection. The forecast for the planetary A index
for Friday through Monday, September 9-12 is 25, 20, 10 and 20.
Predicted solar flux for the same days is 100 for Friday, September
9, then values around 110 for the following week. A little higher
sunspot activity is welcome for the next couple of weeks, as we head
toward the Autumnal Equinox, usually a better time for HF

Randy Crews, W7TJ, asked about sunspot and solar flux numbers at the
bottom of the solar cycle, which is expected around the end of 2006,
or early 2007. Of course, there is enough daily variation that we
won't really know when the bottom was until well after it has
passed. We only know this for sure when looking at a smoothed curve
where daily numbers are averaged over many months.

Of course we will see many days, sometimes weeks, with no sunspots
at all. Solar flux drops below 70 to a background level, around 67.
A typical low period was September and October 1996, in which the
average sunspot number was 2.6, and average solar flux was 69.3.
Perhaps we'll see these low numbers again about 12-18 months from

If you would like to comment or have a tip, email the author at,

For more information concerning radio propagation and an explanation
of the numbers used in this bulletin see the ARRL Technical
Information Service propagation page at, An archive of past
bulletins is found at,

Sunspot numbers for September 1 through 7 were 24, 28, 14, 12, 12,
12 and 11 with a mean of 16.1. 10.7 cm flux was 79.2, 77.1, 74.2,
74.6, 75, 83.4, and 117, with a mean of 82.9. Estimated planetary A
indices were 21, 33, 32, 26, 14, 9 and 15 with a mean of 21.4.
Estimated mid-latitude A indices were 11, 24, 20, 18, 9, 6 and 18,
with a mean of 15.1.


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