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ARRL Propagation Bulletin ARLP051 (2003)

ARLP051 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 51  ARLP051
From Tad Cook, K7RA
Seattle, WA  December 12, 2003
To all radio amateurs 

ARLP051 Propagation de K7RA

Declining sunspot numbers and high geomagnetic activity made rough
conditions this week. Average daily sunspot numbers for the week
dropped 57% from the previous week, and average daily solar flux was
down by 34%. The average daily planetary A index more than tripled
to 28.7.

Currently earth is inside a high-speed solar wind. The
interplanetary magnetic field points north, and geomagnetic
conditions would be even more active if it pointed south. The wind
is from a large coronal hole, and the stream began affecting earth
on December 8. There aren't any sunspots currently facing the earth,
and sunspot 517 is leaving the visible solar disk, while 520 and 521
are coming around the opposite limb.

The active region that caused all the wild space weather in October
and November is around sunspot 488, which is now crossing the sun's
far side. Its return to the earth side should cause a rise in the
sunspot and solar flux numbers. Solar flux is expected to stay below
100 until next Tuesday, December 16, and then rise suddenly from
December 18-19, next Thursday and Friday.

Unfortunately, conditions should be rough for the ARRL 10 Meter
Contest this weekend. Predicted solar flux values for Friday through
Monday, December 12-15, are 85, 90, 90 and 95. Predicted planetary A
index numbers for the same period are 40, 35, 25 and 20.

10-meter paths really need a high MUF value to sustain them, and the
low sunspot numbers we're seeing now don't help. You can get an idea
how conditions might differ from past years by trying some historic
numbers on the W6ELprop program, mentioned frequently in past

For this weekend, a path from California to Cleveland (W6 to W8 in
the program's atlas) using a flux of 90 and K index of 4 would yield
a rather narrow opening, probably only reliable from 1800-1900z. A
path from Dallas to Brazil (W5 to PY in the atlas) shows a poor
probability for a path (although Seattle to PY looks much better).

On the ARRL website you can sample values from past bulletins at Looking back at 2002 to bulletin
number 53, the average solar flux for Friday through Sunday of the
10-meter contest weekend was 185, and the K index must have been
quite low, because planetary A indices were 7, 11 and 9.

Using the flux value of 185 and a K index of 2, the California to
Cleveland path looked fantastic from 1530 through 2330z, and the
other paths looked much better as well. What a difference a year can
make in a changing solar cycle. As mentioned in past bulletins, you
can download the W6ELprop software free at

Sunspot numbers for December 4 through 10 were 115, 88, 87, 53, 49,
23 and 46 with a mean of 65.9. 10.7 cm flux was 115.8, 111.7, 108.9,
92, 93.7, 92.2 and 89.2, with a mean of 100.5. Estimated planetary A
indices were 9, 43, 22, 15, 39, 31 and 42, with a mean of 28.7.


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