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ARRL Propagation Bulletin ARLP039 (2005)

ARLP039 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 39  ARLP039
From Tad Cook, K7RA
Seattle, WA  September 16, 2005
To all radio amateurs 

ARLP039 Propagation de K7RA

This week the Sun has given us a tremendous amount of activity in
the form of large solar flares. A geomagnetic storm is still in
progress, and the planetary A index from Saturday through Thursday,
September 10-15, was 30, 105, 66, 51, 25 and 43. These are high

The average planetary A index for this week more than doubled to
43.1. Average daily sunspot numbers more than quadrupled to 71.1.
These numbers compare the 7 days ending Wednesday, September 14 with
the previous 7 day period.

Next week is the Northern Hemisphere's Autumnal Equinox. This period
could be a good one for HF propagation if solar flares quiet down
and the sunspot count doesn't sink back toward 0. The sunspot number
rose above 100 on Sunday, September 11, the first time since August

The source of all this excitement is a single large sunspot group,
number 798. This spot was just peeking around the edge of the
visible solar disk on September 9, but by September 14-15 it was
aimed squarely at our planet. The last time around it was much
smaller and still emerging, visible until August 24. Although not
aimed at Earth on September 7, that day it produced an X17 solar
flare, the fourth largest detected over the past three decades.
Over the next week it produced 8 more flares, each causing HF radio

Of course, VHF operators probably didn't mind all the disruption.
You can check the 50 MHz Propagation Logger page at, to see what they've been up to on
6-meters, and, for a further
look back.

Steve Lyon, WB6RIB and several others sent in an article from NASA
about all this activity in the year before the solar cycle bottom,
titled "Solar Minimum Explodes." You can read it for yourself on the
web site.

Over the next few days look for declining geomagnetic numbers, but
fairly good sunspot and solar flux values. (Remember, HF operators
generally want the sunspot and the somewhat-related solar flux
numbers to remain high, with the geomagnetic A index and related K
index as low as possible). Predicted solar flux for Friday through
Monday, September 16-19 is 115, 110, 110 and 105. The predicted
planetary A index for those same days is 25, 15, 10 and 10.
Geophysical Institute Prague predicts active to minor storm
conditions for September 16, active conditions on September 17 and
20, unsettled to active conditions September 21, unsettled
conditions for September 18 and 22, and quiet to unsettled
conditions on September 19.

Ed Douglass, AA9OZ is trying for his second 5-Band DXCC award. The
first time around was as 7P8DX in Lesotho from 1986-1992, and this
time he wants to do it from Sturgeon Bay, Wisconsin, a town just
below the 45th parallel on a peninsula extending into Lake Michigan.
He aims to work 100 countries on 75 and 80 meters during the sunspot
minimum, and asked if the absence of sunspots might improve his
chances on lower frequencies. Certainly the MUF (Maximum Usable
Frequency) is lower with fewer sunspots, so openings on 20-15-10
meters are much less common. But I seemed to recall Carl
Luetzelschwab, K9LA having something to say about the absence of
solar activity producing better conditions overall on the low

Carl referred me to an interesting article he wrote for the
September/October 2005 issue of "The DX Magazine" titled "Getting
Ready for Solar Minimum." Among other issues, the article discusses
what happens with 160 and 80 meter propagation passing through the
auroral zone. This includes West Coast North America to Europe,
Midwest to Europe and Japan, and East Coast to Japan paths. During
the solar minimum, there is less chance of ionospheric disturbance
in northern latitudes. Carl writes, "In general, a quiet high
latitude ionosphere provides the best propagation on the lower bands
for paths near or going through the auroral zone."

Although this excellent article is not online, you can find similar
material by entering a query for +K9LA +"auroral zone" in a search
engine such as Google. One link returned was for another of our
bulletins, ARLP008 from 2004, which talked about this same issue,
and mentions that Carl wrote about this in the March 2004 issue of
WorldRadio. The bulletin is located at, You can find out
more about "The DX Magazine" at You can also
look at current conditions in the auroral zone north of North
America at,

Terry Oldham, KH6MT wrote asking about 10 meters, an often difficult
band at the bottom of the cycle. He lives in North Central Florida,
between Jacksonville and Tampa, and wants to know when 10 might be
open to El Paso, Texas next. He mentioned that the window used to
run from September through April, but last year he saw no window at

I told Terry about W6ELprop (the free propagation program for the
PC, from and mentioned he could plug
in numbers for that path to see what pops up. I tried it, and over
the approximately 1500 miles at this time of year, a sustained
sunspot number a little higher than we've seen recently would help.
Still, if you plug in the average sunspot number for the past week
(71.1), according to this program propagation is quite possible.
With about 10 more points, or perhaps going over to the 12 meter
band, the odds look quite good.

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 8 through 14 were 36, 59, 59, 101, 62,
95 and 86 with a mean of 71.1. 10.7 cm flux was 94.1, 99, 116,
109.7, 118, 114, and 116.6, with a mean of 109.6. Estimated
planetary A indices were 8, 17, 30, 105, 66, 51 and 25 with a mean
of 43.1. Estimated mid-latitude A indices were 5, 12, 15, 53, 32, 26
and 13, with a mean of 22.3.


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