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ARRL Propagation Bulletin ARLP017 (2005)

ARLP017 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 17  ARLP017
From Tad Cook, K7RA
Seattle, WA  April 29, 2005
To all radio amateurs 

ARLP017 Propagation de K7RA

Solar flux and sunspot numbers continue their lull, although again
this week their relative positions see-sawed. For last week's
bulletin of April 22 (ARLP016), we reported average daily sunspot
numbers up a little, and average daily solar flux down a bit. This
change was noted when the numbers were compared to the data in the
April 15 bulletin (ARLP015). This week the daily sunspot number on
average dropped 25 points to 25.9, and the average daily solar flux
increased a little over two points to 84. Nothing significant about
any of this, which is common behavior as the solar cycle continues
to decline.

This week on Sunday, April 24 the earth-facing sun was spotless, and
the sunspot number was 0. But the next day a new sunspot appeared,
number 756, and it is a big one. The spot is growing quickly, and
the resulting sunspot numbers for April 26-28 were 20, 45 and 71.
Solar flux is heading toward 100. Predicted solar flux values for
this weekend, April 29-May 1 are 103 on all days. Flux values should
rise above 105 by Monday, May 2.

Geomagnetic conditions have been nice and quiet, but this could
change over the next week. The predicted planetary A index for April
29 through May 3 is 8, 8, 20, 40 and 15. A planetary A index of 40
indicates a major geomagnetic storm, which is expected from the
reappearance of a recurring coronal hole and associated high
velocity wind stream.

The predicted smoothed sunspot number drops in May from 22 to 19.
It will continue to decline, with the predicted smoothed sunspot
number at 10 for the end of 2005 and 5 for the end of 2006, the
predicted bottom of the solar cycle. In 2007 solar activity should
rebound, and around the end of that year the smoothed sunspot number
should be back to where it is now, 18 for November and 21 for
December 2007. In 2008, based on the behavior of past solar cycles,
sunspots should make a strong return. That's only three years from

Robert Wilson, AL7KK wrote to say he worked for Voice of America and
planned medium wave AM broadcasts that were around the 160 meter
wavelength. He said that in 1989 he was using some propagation
software that worked fine on 160 meters outside of the auroral zone.
I've asked him to give us more detail, as the propagation programs
I've used don't work below 3 MHz. I hope to have more information

David Moore sent us a fascinating article from about
the structure of coronal holes and the associated solar wind. The
article talks about observations published in the April 22 edition
of Science. You can read the SpaceRef article at .

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 April 21 through 27 were 22, 34, 35, 0, 25, 20
and 45 with a mean of 25.9. 10.7 cm flux was 77.1, 77.2, 79.3, 82.3,
86, 90.9 and 95.3, with a mean of 84. Estimated planetary A indices
were 4, 9, 6, 10, 11, 5 and 4 with a mean of 7. Estimated
mid-latitude A indices were 2, 7, 5, 5, 9, 2 and 1, with a mean of


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