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ARRL Propagation Bulletin ARLP036 (2006)

ARLP036 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 36  ARLP036
From Tad Cook, K7RA
Seattle, WA  September 1, 2006
To all radio amateurs 

ARLP036 Propagation de K7RA

The second magnetically reversed sunspot appeared this week, and
this time it remained longer.  This is the second sunspot from the
new solar cycle 24.  Eventually there will be more of the new
reversed sunspots than the old ones from cycle 23, and that would be
one way to mark the beginning of the next sunspot cycle.

August has passed, so let's look at some monthly averages of sunspot
numbers and solar flux.

The average daily sunspot numbers for the months August 2005 through
August 2006 were 65.6, 39.2, 13, 32.2, 62.6, 26.7, 5.3, 21.3, 55.2,
39.6, 24.4, 22.6 and 22.8.  Average daily solar flux for the same
months was 92.4 , 91.9, 76.6, 86.3, 90.8, 83.4, 76.5, 75.5, 88.9,
80.9, 76.5, 75.8 and 79.

We can use these averages to spot trends, but the sunspot numbers
jump around quite a bit.  To get a smoother view of the sunspot
numbers, you can do a moving average, for instance, over 3 months.
This would mean averaging any month's average daily sunspot numbers
with the month before and the month after data.  To the data above,
we can add sunspot numbers for the months April through July 2005.
That allows us to look at a 3-month moving average centered on May
2005 through July 2006.  We cannot yet figure a 3-month moving
average centered on August 2006, because we don't know what the
sunspot numbers for September will be.

So the 3-month moving average from May 2005 through July 2006 is
55.6, 64.6, 64.7, 57.8, 39.3, 28.1, 35.9, 40.5, 31.5, 17.8, 27.3,
38.7, 39.7, 28.9 and 23.3

That is pretty smooth, but we see numbers decline, then rise, and
then decline again.  An even smoother set of numbers would be
7-month moving averages from July 2005 through May 2006.  Those
numbers are 50.5, 49.1, 48.7 , 44, 34.9, 28.6, 30.9, 34.7, 33.6,
27.9 and 27.3.  Although the numbers rise for January and February
2006, the decline of the cycle is more obvious.

Despite the decline in the sunspot cycle, we will see improved HF
propagation over long distances in September.  The Autumnal Equinox
later this month is a time when sunlight is equally distributed over
the northern and southern hemispheres.  Expect rising sunspot
numbers and solar flux over the next few days, with mostly quiet
geomagnetic conditions.  The predicted planetary A index for
September 1-7 is predicted to be 10, 10, 20, 12, 10, 5 and 5.

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 .

Sunspot numbers for August 24 through 30 were 22, 23, 21, 26, 48, 30
and 19 with a mean of 27. 10.7 cm flux was 78.2, 77.2, 75.7, 78.6,
76.4, 73, and 74.2, with a mean of 76.2.  Estimated planetary A
indices were 5, 2, 3, 18, 11, 9 and 6 with a mean of 7.7.  Estimated
mid-latitude A indices were 4, 0, 2, 12, 9, 9 and 5, with a mean of


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