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ARRL Propagation Bulletin ARLP019 (2005)

ARLP019 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 19  ARLP019
From Tad Cook, K7RA
Seattle, WA  May 13, 2005
To all radio amateurs 

ARLP019 Propagation de K7RA

Suddenly the sun is peppered with spots. While it's true that the
sunspot cycle is declining, that represents an overall trend, and
there is plenty of variation until we later examine a moving average
that smoothes out the bumps.

Average daily sunspot numbers this week rose nearly 22 points to
82.7, and the average daily solar flux was up 10 points to 110.7.
On Wednesday, May 11 when the solar flux reached 125.3, that was the
highest reported reading since January 19, 2005. Also on May 11 the
daily sunspot number was 117, the highest value observed since
November 4, 2004, although Valentine's Day, February 14, was almost
as high at 115.

On Mother's Day, Sunday, May 8, a big blast of solar wind sparked a
geomagnetic storm, and the planetary A index went all the way to 64,
Alaska's College A index went to 80, and the mid-latitude A index
was 38. This provoked some nice aurora displays over the weekend.
Then sunspot 758 began to expand rapidly, and the sunspot count for
Monday through Wednesday, May 9-11 was 106, 106 and 117.

At 1040z on May 11 a coronal mass ejection emerged from sunspot 758.
There is a good chance of aurora for Friday, May 13 as a result.
The predicted planetary A index for Friday through Monday, May 13-16
is 25, 10, 15 and 10. Predicted solar flux for those days is 120,
115, 110 and 105. So the sunspot count has been relatively high, and
during days when the A index is low, such as 10 or below,
propagation should be good. If you listen to WWV at 18 minutes after
the hour (or call the recording at 303-497-3235) and hear that the K
index is lower than 3, that is also a very good sign for HF

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, An archive of past
bulletins is found at,

Sunspot numbers for May 5 through 11 were 50, 66, 55, 79, 106, 106
and 117 with a mean of 82.7. 10.7 cm flux was 109.1, 110.4, 99.9,
101.3, 110, 119.2 and 125.3, with a mean of 110.7. Estimated
planetary A indices were 6, 4, 10, 64, 11, 10 and 11 with a mean of
16.6. Estimated mid-latitude A indices were 3, 3, 10, 38, 10, 6 and
7, with a mean of 11.


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