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ARRL Propagation Bulletin ARLP039 (2010)

ARLP039 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 39  ARLP039
From Tad Cook, K7RA
Seattle, WA  October 1, 2010
To all radio amateurs 

ARLP039 Propagation de K7RA

HF radio conditions were good over this past week, with average
daily sunspot numbers up more than five points to 45.3, and average
solar flux rising 1.5 points to 84.4.  On Wednesday, September 29
solar flux was 90.7.  The last time flux values were nearly this
high was August 7 at 90.5, and it last topped this value at 91.8 on
March 13, 2010.

A coronal wind on September 24 provided the moderate geomagnetic
activity for the week, with the planetary A index rising to 13.

With the end of September comes the opportunity to examine some
average sunspot numbers based on calendar months.  These numbers
look good.  Every month since early 2007 we have taken a 3-month
moving average of sunspot numbers.  The hope in the beginning was to
help us spot the bottom of the sunspot cycle without resorting to
smoothed sunspot numbers, which average a year of sunspot data.  But
three months of data allows a lot of smoothing of short term

Now at the end of September we know the 3-month average centered on
August, taking all the data from July, August and September.  It is
a simple arithmetic average in which we add all the sunspot numbers
together, then divide the sum by the number of days in those 3
months the data was taken from.

The three-month moving averages of daily sunspot numbers for October
2008 through August 2010 were 4.5, 4.4, 3.6, 2.2, 2, 1.5, 2, 4.2,
5.2, 4, 4, 4.6, 7.1, 10.2, 15.2, 22.4, 25.7, 22.3, 18.5, 16.2, 20.4,
23.2 and 28.9.  This takes into account all of the daily sunspot
numbers from September 1, 2008 through September 30, 2010.  It is
clear that there was a minimum centered around April through July
2008, and that for the past few months numbers have risen steadily.
The monthly averages for August and September 2010 were 28.2 and

For this week NOAA and USAF predict quiet geomagnetic conditions
with a planetary A index of 5 on October 1-2, 7 for October 3-5, and
5 again on October 6-10.  Geophysical Institute Prague sees quiet
conditions October 1-3, unsettled October 4-5, and quiet October

Dave Fisher, KA2CYN of New City (Rockland County) New York says he
had a very active weekend in the CQ World Wide RTTY Contest last
weekend, September 25-26.  With a 4-band roof-mounted rotatable
dipole and low power on 40-10 meters he made 513 contacts overall,
with 6 contacts into South America on 10 meters, and 161 QSOs into
North America on 40 meters.  20 meters was the best band into
Europe, with 101 QSOs on that band.

Andy Gudas, N7TP of Amargosa Valley, Nevada wrote: "Since we have
now entered the 'Solar Cycle That Couldn't,' is there any way to
compare where we think we are in the new sunspot cycle with an
average cycle, or maybe a really poor cycle?  I think we're
something like two years into the new cycle and not much is

Check for a graphic
comparison of Cycles 21-23, with the beginning of Cycle 24 plotted.
The time axis is marked in months since the start of the cycle,
which could be somewhat subjective.  Note that smoothed sunspot
numbers are displayed here, and since that takes a year of data to
compute, the latest number always lags the current date by about six
months.  The latest update of this chart as of October 1 was
September 4.

You can see another comparison at, and note that Cycle 24 is
just a little squiggly line to the right of Cycle 23.  This page
says the current forecast is for the current cycle to peak in 2013
with a smoothed sunspot number of 50-70.  If that turns out to be
true, it will be a much lower cycle than previous ones.

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 web page at, For an explanation of the
numbers used in this bulletin see An archive of past
propagation bulletins is at  Find more good
information and tutorials on propagation at

Monthly propagation charts between four USA regions and twelve
overseas locations are at

Instructions for starting or ending email distribution of ARRL
bulletins are at

Sunspot numbers for September 23 through 29 were 34, 34, 40, 57, 52,
49, and 51 with a mean of 45.3. 10.7 cm flux was 84.3, 82.6, 82.9,
83.9, 83, 83.2 and 90.7 with a mean of 84.4. Estimated planetary A
indices were 5, 13, 7, 6, 6, 7 and 5 with a mean of 7. Estimated
mid-latitude A indices were 6, 9, 5, 8, 5, 7 and 2 with a mean of 6.


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