Sunday, December 23, 2007

Propagation Forecast Bulletin 53 ARLP053

Sunspot 978 faded this week, with Tuesday, December 18 its last day visible. The average daily sunspot number dropped nearly 13 points from last week to 24.1, and average daily solar flux was down over three points to 83.9. Geomagnetic indicators were up, with the average daily planetary A index up three points to 7.4, and the mid-latitude A index up two points to 5.4.

Sunspot 978 held no indication for the next sunspot cycle, because the magnetic polarity was the same as spots from Cycle 23, which is now ending. It was also at mid-latitude, and spots from a new cycle tend to be high latitude. But there was quite an interest this week in a high-latitude area of reverse magnetic polarity. So far this has not turned into an actual sunspot, so we wait. You can read about it at,

On Wednesday, December 19, Proplab-Pro version 3 was finally released. It claims to be "the most advanced propagation ray-tracing system in the world," and works on personal computers running Windows Xp or Vista. The introduction to the manual says it "is a state-of-the-art software package not for the feint of heart." Since the word "feint" implies a daring move, I suspect the author intended to say "faint," implying timidity in this context. With only a short time to look it over, I can say that it is quite complex. You can find information at,

Tomorrow, December 22, 2007, the Sun reaches its lowest point in the sky, marking Winter Solstice for the Northern Hemisphere, which begins at 0608 GMT. For much of North America, this happens late tonight, and this week we begin the long shift toward more daylight. Perhaps better propagation will appear this Spring. The equinox is on March 21, 2008 at 0548 GMT. See an article at, for an explanation from the U.S. Naval Observatory on why the earliest sunset in the mid-northern latitudes is around December 8, the latest sunrise is around January 5, with solstice in the middle.

We've seen no sunspots for a couple of days, and if this continues through the end of the year (10 days from now) the average sunspot number for the calendar year will be just 12.8. We will know for sure by the time the first propagation forecast bulletin of 2008 comes out, on January 4. The daily sunspot numbers averaged over each calendar year from January 1, 2001 through December 31, 2007 should be 170.3, 176.7, 109.2, 68.6, 48.9, 26.1 and 12.8. By comparison, the average daily sunspot numbers for each year, 1995-1997 (the previous solar minimum) were 28.7, 13.2 and 30.7. It seems 2007 must have been a solar minimum year, but of course dividing the data into calendar years is completely arbitrary.

We may not end the year with a blank Sun. The predicted solar flux from NOAA and the US Air Force for December 21-22 is 72, 71, and then 70 for December 23-28, then rising to 75 December 29 through January 1, then 80 for January 2-3, and 85 for January 4-11. Recently when solar flux was above 80 for ten days, sunspot numbers ranged from 24 to 43.

The predicted planetary A index for December 21-23 is 15, 10 and 8, then dropping to 5 for December 24 through January 5. Geophysical Institute Prague predicts unsettled conditions December 21-22, quiet to unsettled December 23, and quiet December 24-27.

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For more information concerning radio propagation, see the ARRL Technical Information Service at, For a detailed explanation of the numbers used in this bulletin see, An archive of past propagation bulletins is at, Monthly propagation charts between four USA regions and twelve overseas locations are at,

Sunspot numbers for December 13 through 19 were 39, 35, 39, 28, 14, 14 and 0 with a mean of 24.1. 10.7 cm flux was 93.8, 91.9, 88.9, 81.7, 79.5, 76.8, and 74.5 with a mean of 83.9. Estimated planetary A indices were 4, 2, 1, 2, 17, 18 and 8 with a mean of 7.4. Estimated mid-latitude A indices were 3, 2, 2, 1, 12, 11 and 7, with a mean of 5.4.