Sunspot activity rose again this week, with the average daily sunspot number up nearly 18 points to 83, but the geomagnetic indices -- both planetary and mid-latitude -- were each down slightly. The high sunspot number for the week was 121, recorded on Wednesday, August 31. The last time the sunspot number was higher was on July 31 and August 1, at 128 and 130. Sunspot numbers for August 25-31 were 97, 76, 63, 73, 66, 89 and 121, with a mean of 83.6. The 10.7 cm flux was 104.2, 104.6, 103.8, 101, 101.2, 101.3 and 109, with a mean of 103.6. The estimated planetary A indices were 4, 3, 5, 6, 9, 4 and 2, with a mean of 4.7. The estimated mid-latitude A indices were 4, 3, 3, 4, 7, 2 and 1, with a mean of 3.4. If you check the Daily Sun image, there are quite a number of spots currently on the Sun.

The predicted solar flux is 110 on September 2-5, 105 on September 6, 100 on September 7-8, 95, 90, 92, and 95 on September 9-12, 100 on September 13-15, and then peaking at 105 on September 18-22. The solar flux values for the next few days are slightly higher than the prediction in yesterday’s ARRL Letter. The predicted planetary A index is 5 on September 2, 8 on September 3-4, 5 on September 5-10, then 8, 12, and 8 on September 11-13, and back down to 5 on September 14-17. As always, the Czech Republic has a slightly different view. The Geomagnetic Department of Geophysical Institute Prague says September 2 should be quiet, September 3 unsettled, September 4 quiet to unsettled and September 5-8 quiet again.

Looking back at the three-month moving average of sunspot numbers, we now have the data for June through August, giving us the three-month average centered on July. At 63, it is only slightly above the average centered on June, which was 61.5. The three-month averages for this year -- centered on January through July -- were 35.3, 55.7, 72.3, 74.4, 65.9, 61.5 and 63. For the 2010 three-month averages, the high was 35.6, centered on October, and the low was 16.4, centered on May.

Utah State University has a Space Weather Center website where they post frequency availability related to communications with the National Hurricane Center in Miami. You can see current and projected NVIS (Near Vertical Incidence Skywave) coverage across the Eastern Seaboard, as well as worldwide propagation maps for 75, 40 and 20 meters centered on Miami.

Thanks to Thomas Otterbein, DG8FBV, of Babenhausen Germany, for noticing that in the last bulletin we referred to the NOAA Preliminary Report and Forecast as a good source for smoothed sunspot numbers. That is true as a general statement, and you can see the past and predicted progress of the current sunspot cycle. But Thomas points out that this is not the smoothed sunspot number you should use with VOACAP propagation prediction software. He writes: “For VOACAP, VOAAREA users and for users of prediction programs such as GUI which uses VOACAP, the predicted SSN should be taken from here. You might also want to check out this website, too.

We are only a few weeks away from the autumnal equinox in the Northern Hemisphere, September 23, 2011, and the transition occurs at 0904.  We are transitioning out of summer HF propagation to the more favorable fall conditions. To get an idea of how much better fall propagation is, try using W6ELprop or a similar propagation program with some dates, such as July 15 compared to September 23. You can have two of these running at the same time, making it easy to switch back and forth for comparison. I did one from the center of the contiguous 48 United States to Germany, and used a sunspot number of 120 (slightly optimistic) for both dates. The propagation is radically different between those two dates, and so much better for the fall date. Give it a try.

Frank Donovan, W3LPL, of Glenwood, Maryland, pointed out that a new monthly prediction for the solar cycle was released by NASA in early August. But it isn’t really a new forecast, as the only thing that changed was the date from July 1 to August 2, which was why we didn’t mention it in the bulletin. But on September 1, a new forecast was released with a slightly revised prediction for the sunspot cycle maximum. Instead of the smoothed sunspot number peaking at 69 in June-July 2013, it is now predicted to peak at 70 in May 2013. These forecasts are not archived, and the same URL is used every month, so a personal record must be kept to note changes.

Frank also sent this note on September 1: “Trans-polar HF propagation has significantly picked up in the last week on 20, 17 and 15 meters. This is the usual seasonal change, as well as solar flux consistently above 100 and K indexes mostly 2 and below.”

All times listed are UTC, unless otherwise noted.

Amateur solar observer Tad Cook, K7RA, of 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 each Thursday in The ARRL Letter. You can find a guide to articles and programs concerning propagation here. Check here and here for a detailed explanation of the numbers used in this bulletin. An archive of past propagation bulletins can be found here. You can find monthly propagation charts between four USA regions and 12 overseas locations here. Readers may contact the author via e-mail.