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SEATTLE, WA, May 4, 2007 -- The string of zero sunspot days ended Wednesday, April 25, and -- relative to the bottom of the solar cycle -- sunspot numbers over the past few days have been quite an improvement. Average daily sunspot number jumped from the previous week by 23 points to 25.1, and average solar flux increased over 14 points to 84.8.
This modest increase in sunspot activity may noticeably improve propagation compared to weeks with no sunspots. As we understand it, popular propagation prediction programs are based on a statistical model using predicted smoothed sunspot numbers. It is probably less accurate to vary a path prediction for a particular date using daily sunspot numbers, but taking an average of several days may produce more realistic results.
If we do path predictions from California to Japan for May 4 using the average sunspot number for the past three available readings (34), then compare to a prediction using zero for the sunspot number, we can get an idea of what the average improvement would be.
Calculating for zero sunspots, W6ELprop shows us a 50 to 75 percent chance of a 17 meter opening at 2130-0600 UTC. Using 34 as a sunspot number, we see a 75 to 100 percent chance of a 17 meter opening from 2000-0630 UTC, and for the 50 to 75 percent chance, that opening extends later, to 0800 UTC.
Similarly, for 20 meters with the sunspot number at 34, the W6 to JA path shows a 25 to 50 percent chance of opening at 1230 UTC, 50 to 75 percent at 1300 UTC, 75 to 100 percent from 1330-1500 UTC, 50 to 75 percent at 1530-1600 UTC, and 25 to 50 percent at 1630 UTC.
With no sunspots from 1230 to 1330 UTC there is less than 25 percent chance, 25 to 50 percent at 1400 UTC, 50 to 75 percent at 1430 UTC, and less than 25 percent chance from 1530 to 1630 UTC. Actual on the air results would vary, of course.
Let's look at the average daily sunspot numbers and solar flux for April, compared to previous months, to spot any trends.
Monthly averages of daily sunspot numbers for April 2006 through April 2007 were 55.2, 39.6, 24.4, 22.6, 22.8, 25.2, 14.7, 31.5, 22.2, 28.2, 17.3, 9.8 and 6.9. Monthly averages of daily solar flux for the same period were 88.9, 80.9, 76.5, 75.8, 79, 77.8, 74.3, 86.3, 84.4, 83.5, 77.7, 72.2 and 72.4.
We've also recently been looking at three-month smoothed sunspot numbers, so knowing the April values, we can combine them with February and March to produce the smoothed sunspot number for March, which turns out to be 11.2 .
Here are the three-month smoothed sunspot numbers for the past 18 months:
Oct 05, 28; Nov 05, 36; Dec 05, 40.6; Jan 06, 32.4; Feb 06, 18.1; Mar 06, 27.7; Apr 06, 38.5; May 06, 39.7; Jun 06, 28.9; Jul 06, 23.3; Aug 06, 23.5; Sep 06, 21.2; Oct 06, 24.1; Nov 06, 23.1; Dec 06, 27.3; Jan 07, 22.7; Feb 07, 18.5 and Mar 07, 11.2.
Both series of sunspot numbers clearly illustrate the decline of this sunspot cycle.
Barry Roseman, W0LHK, of Stillwell, Kansas tipped us on the "CQ Sixty Meters" site, which includes a variety of information for users of the 60 meter band.
For the near term, the US Air Force Space Weather Operation predicts a planetary A index of 5 for May 4-5, 8 for May 6-7 and 5 again for May 8-18, so the expectation is for quiet geomagnetic conditions. Based on the last period of higher geomagnetic activity, April 28-29, the 27.5 day solar rotation probably is what led to a prediction of 25 for the planetary A index on May 25.
Geophysical Institute Prague predicts quiet geomagnetic conditions for May 4, quiet to unsettled on May 5, unsettled May 6-7, and quiet to unsettled again on May 8-9.
For More Information
For more information concerning propagation and an explanation of the numbers used in this bulletin see the ARRL Technical Information Service Propagation page and the article "The Sun, the Earth, the Ionosphere: What the Numbers Mean, and Propagation Predictions," by Carl Luetzelschwab, K9LA. An archive of past bulletins is on the ARRL Web site. The propagation charts that formerly appeared in QST now are available on the QST Propagation Charts page on the ARRL Web site.
Weekly Sunspot Numbers
Sunspot numbers for April 26 through May 2 were 17, 18, 20, 18, 38, 32 and 33, with a mean of 25.1. The 10.7 cm flux was 80.5, 82.7 84.9, 84.8, 87, 86.3, and 87.4, with a mean of 84.8. Estimated planetary A indices were 6, 16, 26, 23, 20, 8 and 3, with a mean of 14.6. Estimated mid-latitude A indices were 4, 10, 20, 16, 13, 5 and 1, with a mean of 9.9.
Amateur solar observer Tad Cook, K7RA, Seattle, Washington, provides this weekly report on solar conditions and propagation. This report also is available via W1AW every Friday, and an abbreviated version appears in The ARRL Letter. Readers may contact the author via e-mail.
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