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ARRL Propagation Bulletin ARLP024 (2007)

ARLP024 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 24  ARLP024
From Tad Cook, K7RA
Seattle, WA  June 8, 2007
To all radio amateurs 

ARLP024 Propagation de K7RA

After five days of no sunspots from May 24-28, spots returned on May
29, and have increased since in number and size.  There are
currently several sunspots visible, and the sunspot number for the
past five days (Sunday through Thursday) was 58, 58, 63, 47 and 59.
Coupled with quiet and stable geomagnetic indicators, this is good
for HF propagation.  Our reporting week for this bulletin (the
numbers reported at the end) runs from Thursday through Wednesday,
and the average daily sunspot number for May 31 to June 6 rose
nearly 43 points to 46.1 when compared to the prior seven days.
Average daily solar flux rose nearly 15 points to 83.7.

Last week's Propagation Forecast Bulletin ARLP023 mentioned ARRL
Field Day, now just two weeks away.  The bulletin reported (in some
uncorrected versions) that the event is always held on the last full
weekend in June, but Don Jackson, AE5K of Marion County, Arkansas
pointed out that the rules specify the fourth full weekend.  Both
fourth and last are on the same weekend this year (June 23-24),
which is usually the case, but not always.  In 2002, the fourth full
weekend was June 22-23, but the last full weekend was June 29-30.
This occurs whenever June 1 is a Saturday, as it will be again in
2013 and 2024.

Last week the latest projection looked like no sunspots around Field
Day with a declining geomagnetic disturbance, but this week the
forecast looks a little better.  Including the Friday before (the
event doesn't begin until Saturday) the projected solar flux last
week for June 22-24 was 65 for all three days, with a planetary A
index of 20, 12 and 5.  This week's prediction for those dates shows
the same A index, but a solar flux 10 points higher, at 75 for all
three days.

A check of recent sunspot numbers alongside solar flux values on the
same dates at,, shows
no sunspots when the solar flux was down around 65, but at 75 the
sunspot number can be in the 40s.

For the next few days expect continued quiet geomagnetic activity,
with the same moderate (for the low point of the sunspot cycle)
sunspot count.

Alex Mendelsohn, AI2Q in Kennebunk, Maine sent a link to an article
about a solar burst last December 6, which caused problems for GPS
receivers.  You can read it at,  See how
we reported the same flare at,  Our bulletin shows
that the solar flux observatory in British Columbia also had
problems from that event.

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 May 31 through June 6 were 11, 41, 45, 58, 58,
63 and 47 with a mean of 46.1. 10.7 cm flux was 74.6, 79.4, 83.2,
87, 85.7, 88.8, and 87.1, with a mean of 83.7. Estimated planetary A
indices were 4, 6, 7, 10, 8, 3 and 2 with a mean of 5.7. Estimated
mid-latitude A indices were 2, 5, 5, 7, 8, 2 and 2, with a mean of


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