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ARRL Propagation Bulletin ARLP013 (2010)

ARLP013 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 13  ARLP013
From Tad Cook, K7RA
Seattle, WA  April 2, 2010
To all radio amateurs 

ARLP013 Propagation de K7RA

Even though this bulletin may arrive a little early (due to the Good
Friday holiday on April 2) jokesters are still too late for sneaking
any first of April pranks into this bulletin.  No matter when you
read it, this bulletin is dated April 2 for good reason.  Your
bulletin editor serves as a solid wall against nonsense.  Do not
think of us as humorless, for your editor has enjoyed perpetrating
many elaborate pranks, japes, and stunts, but never on April 1
(because that might give it away) and never in this bulletin
(standing on principle).  Also, we prefer to be perpetrators rather
than recipients.

The solar report in Thursday's ARRL Letter attempted to correct some
slightly faulty data regarding sunspot group numbers and their dates
of emergence, but the result was more wrong than what it was
attempting to correct.  All of it is my fault.  Now we see the
problems, which included not looking back far enough to find the
dates that regions emerged, and forgetting that the date of our
source is the day after the date the data was gathered.  The
summaries are available for all to check at

Sunspot group 1056 emerged earlier than reported, back on March 17,
and was gone on March 24.  The area it covered (all areas are in
units of one-millionth of the solar hemisphere) on March 17-23 was
20, 50, 10, 30, 70, 50 and 5.

On March 23 new sunspot group 1057 appeared, and through March 31
the area it covered was 5, 240, 400, 320, 380, 410, 290, 260 and
250.  It is still with us.

On March 25 a small group, 1058 appeared, and only lasted two days.
The area it covered was just 1 and 5.  On March 27, group 1058
became a "plage without spots".  What is a plage?  In this context,
not a beach at a seaside resort, but a bright region in the solar
chromosphere, in this case minus any spots.

March 27, group 1059 emerged, and through March 31 the size was 120,
100, 150, 130, and 70.  1059 never became as large as 1057.

Average daily sunspot numbers for the week were up over 4 points to
28.9, and average daily solar flux changed from 84.2 to 84.9.  The
renewed sunspot activity seems constant and steady.  We have new
3-month moving averages of daily sunspot numbers through the end of
March, which is centered on February, and the number has increased a
few points from last time.  The 3-month moving average of daily
sunspot numbers centered on July 2009 through February 2010 was 4,
4, 4.6, 7.1, 10.2, 15.2, 22.4 and 25.7.  The rate of increase has
slackened, but it steadily moves higher.

Last week we reported that the trailing 50-day moving average of
daily sunspot numbers for the prior week was 27.34, 28.18 for the
end of last week.  This week, ending on March 31, the trailing 50
day average is 26.8.

No report yet from Geophysical Institute Prague, and the April 1
NOAA prediction is not available either, but the March 31 prediction
had solar flux at 80 through April 2, then dropping to 75 (is that
an expectation for zero sunspots?) for April 3-5, then back to 80 on
April 6-10.

The report also predicted quiet planetary A index of 5 through April
5, then values of 7, 7, 8, 5 and 5 for April 6-10.

Sorry we are up against an early deadline here.  Check next week for
more propagation information, and perhaps a bit less solar data.
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

Instructions for starting or ending email distribution of this
bulletin are at

Sunspot numbers for March 25 through 31 were 25, 27, 30, 33, 32, 32,
and 23 with a mean of 28.9. 10.7 cm flux was 87.6, 85.9, 88.1, 86,
83.2, 82.8 and 81 with a mean of 84.9. Estimated planetary A indices
were 5, 4, 3, 7, 4, 5 and 4 with a mean of 4.6. Estimated
mid-latitude A indices were 4, 2, 1, 4, 5, 4 and 5 with a mean of


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