ARLP052 Propagation de K7RA:
December 17, 2004

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 3.

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.