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ARRL Propagation Bulletin ARLP052 (2004)

ARLP052 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 52  ARLP052
From Tad Cook, K7RA
Seattle, WA  December 17, 2004
To all radio amateurs 

ARLP052 Propagation de K7RA

The 7-day averages for solar flux and sunspot numbers declined this
week, and the averages for geomagnetic A index rose. The average
daily sunspot number declined over 19 points to 26.9, and average
solar flux was down over 8 points to 88.7. Sunspot counts have been
quite low, and will continue their retreat for about 2 more years.

If you look on page 12 of the latest edition of NOAA SEC's weekly
Preliminary Report and Forecast of Solar Geophysical Data (for this
week see, , and the
index at, you'll observe
the predicted smoothed sunspot number for this month is 30, December
2005 is 10, and the lowest value is 5, projected for December 2006
and January 2007.

Following the December three years from now, in 2008, the monthly
sunspot count for cycle 24 should increase rapidly. But in between
will be some long spells with no sunspots at all. It is interesting
to note that the longest period of 0 sunspots in recent memory was a
little over 7 weeks during September and October of 1996, and the
next low period we are talking about predicted for the end of 2006
is about 10 years after that.

Average sunspot cycles are about 11.1 years long, but have been as
short as 7 and as long as 17 years. The average rise from minimum to
maximum takes 4.8 years, and the average decline is around 6.2
years, so cycles rise faster than they decline.

The most active day in terms of geomagnetic indices over the past
week was Monday, December 12, when a robust solar wind stream drove
the mid-latitude A index to 24, the planetary A index to 36, and
Alaska's college A index to 48. The quiet period this week was 2-3
days later on December 14-15 when the mid-latitude A index was 4 and

Over last weekend during the ARRL 10-Meter Contest, Sunday had worse
conditions than Saturday. There were periods when I couldn't hear
anything from Seattle on 10-meters except stations in South America.
This was the north-south trans-equatorial propagation commonly
observed during periods of high geomagnetic activity. It isn't a
case of propagation being enhanced over the north-south path, but
this might be the only propagation mode available.

Our earth is now passing through a solar wind stream from a coronal
hole. For Friday, December 17 the predicted planetary A index is 20,
followed by 15, 8 and 5 for Saturday through Monday. We may expect
slightly higher solar flux (which is somewhat related to sunspot
counts) with this weekend's flux value around 90. This is expected
to rise to around 105 by December 23.

A note from the Geomagnetic Dept of the Geophysical Institute of
Prague says December 19 and 20 should be quiet, December 21 quiet to
unsettled, and December 18 and 23 unsettled.

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

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

Sunspot numbers for December 9 through 15 were 39, 39, 16, 26, 22,
18 and 28 with a mean of 26.9. 10.7 cm flux was 87.4, 84.8, 89.8,
90.5, 89.7, 89.3 and 89.3, with a mean of 88.7. Estimated planetary
A indices were 8, 10, 15, 36, 11, 7 and 6 with a mean of 13.3.
Estimated mid-latitude A indices were 7, 8, 11, 24, 8, 4 and 3, with
a mean of 9.3.


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