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ARRL Propagation Bulletin ARLP051 (2006)

ARLP051 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 51  ARLP051
From Tad Cook, K7RA
Seattle, WA  December 8, 2006
To all radio amateurs 

ARLP051 Propagation de K7RA

The past couple of days have seen robust solar activity, with flares
and strong solar wind. On Wednesday and Thursday, December 6 and 7,
the planetary A index rose to 28, then 25. On December 5 a large X9
class solar flare emerged from the sun's eastern side, but it wasn't
earth directed. This was from a large sunspot 930, which drove the
sunspot number to 59 on the same day as the solar flare, the same
level as five days earlier. Wednesday, December 6 saw a smaller X6
flare, and currently early Friday morning we are seeing a strong
solar wind, with the interplanetary magnetic field pointing south,
making us vulnerable. There is a chance of more flares, which might
be bad news for the ARRL 10 Meter Contest this weekend.

Flare activity caused a 10.7 cm solar flux reading at Penticton,
British Columbia to jump off the scale. The noon reading showed a
solar flux of 573.4, and had to be adjusted downward to 103 for the

Currently a forecast from NOAA and the U.S. Air Force calls for a
planetary A index on December 8-12 of 30, 50, 40, 20 and 10.  The
predicted A index of 50 and 40 for December 9-10 does not bode well
for the weekend contest. Even if the geomagnetic activity is this
strong, there still should be some north-south trans-equatorial
propagation on 10 meters. But here we see an average sunspot number
of 52 for this week, 35 points higher than last week, and of course
the downside is greater chance for solar flares.

There is a new prediction for the sunspot cycle minimum. Until
recently, the minimum was predicted to have a smoothed sunspot
number of 6 centered on March and April 2007. The new forecast moves
the minimum out a little further and not as low, with a smoothed
sunspot number of 7 for May 2007. You can see the difference in the
table on page 11 of the SEC Preliminary Report and Forecast for this
week, compared with the issue from four weeks ago.  The old forecast
is at, on page 11,
and the new one at, ,
also on page 11. This later and higher minimum makes sense, because
we haven't seen weeks in a row of 0 sunspot days.

Last week's Propagation Forecast Bulletin ARLP050 mentioned a theory
about cooling in the upper ionosphere causing greater density in
lower layers. It turns out that data from different locations shows
different results, and the net effect doesn't look strong or
conclusive. If you do a web search for terms such as "long term
ionosphere change" you'll find abstracts for several scientific
papers on the subject. The most interesting thing I ran across
(thanks to K9LA) was the map on page 12 of the PDF at, The hmF2 referred to in this document is
the height of the densest portion of the F2 layer, with positive
trends at some ionosonde stations and negative at others. The trend
for change in height of the F2 layer is also very slight. It comes
out to about two and a half miles per decade, or
.41 km per year, for ionosonde stations that show any effect.

Jon Jones, N0JK commented that recently in the CQ Worldwide CW
Contest even with solar activity so low, HC8N on 10 meters worked
1,838 stations in 32 zones and 113 countries. Also, Dave Deatrick,
WA8OLD way up in Northern Michigan next to Sault Ste Marie, Ontario,
remarks that he's had great luck recently on 40 meters with a
shortened dipole at 28 feet and 100 watts. He worked 5A7A, CN2R and
several European stations. He is surprised at what he can work on 40
meters with a simple antenna and low power.

If you would like to make a comment or have a tip for our readers,
email the author at,

For more information concerning radio propagation, see the ARRL
Technical Information Service at, For a detailed
explanation of the numbers used in this bulletin see, An archive of past
propagation bulletins is at, .
Monthly propagation charts between four USA regions and twelve
overseas locations are at,

Sunspot numbers for November 30 through December 6 were 59, 58, 55,
46, 43, 59 and 44 with a mean of 52. 10.7 cm flux was 84.1, 84.2,
87.3, 86.5, 92.8, 102.4, and 103, with a mean of 91.5. Estimated
planetary A indices were 28, 4, 2, 3, 1, 2 and 28 with a mean of
9.7. Estimated mid-latitude A indices were 15, 3, 1, 2, 1, 2 and 15,
with a mean of 5.6.


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