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ARRL Propagation Bulletin ARLP014 (2005)

ARLP014 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 14  ARLP014
From Tad Cook, K7RA
Seattle, WA  April 8, 2005
To all radio amateurs 

ARLP014 Propagation de K7RA

Sunspots, solar flux and geomagnetic numbers all averaged out to a
slight rise this past week over the previous period. Average daily
sunspot numbers rose over five points to 39.4, and average daily
solar flux was up over two points to 82.5.

The most active geomagnetic day was Tuesday, April 5, when the
planetary A index was 48 and the mid-latitude A index was 30. This
was triggered by a solar wind stream and a south-pointing
interplanetary magnetic field. The field pointing north, as it was
on April 3 offers some protection, but when it points south the
earth is vulnerable to the effects of flares and coronal holes.

Sunspot numbers and solar flux are expected to decline very slowly
for the rest of April. Sunday, April 10 looks like a day for
possibly unsettled to active geomagnetic conditions. Spring is a
great time of year for propagation, but we wish there were more
sunspots. Comparing the average daily sunspot number for the past
week with monthly averages for April in years past, this week was
34% below 2004, 66% below 2003, and 80% below 2002.

Tim Young, WB7UVH of Washougal, Washington wrote, "Some time ago I
used a graphing program that imported data from your reports to
graph flux values. Is that program still available?"

Yes, it is available, and the data file has just been updated. Tim
is remembering the WA4TTK Solar Data Plotting Utility, which has
worked in conjunction with this bulletin for nearly a dozen years.
It is freeware written generously by Scott Craig, and you can find
it on Scott's web site at Just click
on the link, "Download Freeware Software I Have Written." 

Scott updated the GRAPH.dat data file through April 6, 2005, and you
can find a link for this below the software download link. Scott has
programs working with all Windows versions beginning with 3.1, and
even one that works with MS-DOS. The program can update by
automatically grabbing the sunspot and solar flux data from a text
file version of this bulletin, or it can download the data from ARRL
servers over the net.

Andy Gudas, N7TP of Nevada, sent in this question. "On the packet
spotting nets they report SFI, A, K, and R. What is R?"

R is the sunspot number. It is calculated by multiplying the number
of visible sunspot groups by ten, then adding the number of
individual sunspots. So if four sunspot groups are observed, and
there are 29 individual sunspots visible, the sunspot number would
be 69. That is the simple explanation. What really happens is
observations are made from multiple sites, and then there is a K
factor, sort of a fudge factor, that the number is multiplied by.
That K factor is different for each observatory, and is used to make
the resulting numbers approximately equivalent. All of the results
are averaged to yield the daily sunspot number.

By the way, SFI is the Solar Flux Index, which is a measurement of
10.7 cm (about 2.8 GHz) energy with an antenna pointed at the sun.
We get our solar flux numbers from a Canadian observatory in
Penticton, British Columbia.

You can find the definition for R and many other terms in the
Glossary of Solar-Terrestrial Terms, hosted by the High Altitude
Observatory at the National Center for Atmospheric Research. Look it
up at,

The story in last week's Propagation Forecast Bulletin ARLP013 about
the Novice license and the red sweater provoked a huge email
response. Thanks to everyone who wrote.

If you would like to comment or have a tip, email the author at,

For more information concerning radio propagation and an explanation
of the numbers used in this bulletin see the ARRL Technical
Information Service propagation page at,

Sunspot numbers for March 31 through April 6 were 22, 26, 30, 38,
54, 56 and 50 with a mean of 39.4. 10.7 cm flux was 76.7, 78.3,
80.2, 81.1, 84.8, 88.3 and 88, with a mean of 82.5. Estimated
planetary A indices were 9, 8, 4, 6, 17, 48 and 11 with a mean of
14.7. Estimated mid-latitude A indices were 5, 6, 1, 4, 11, 30 and
7, with a mean of 9.1.


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