The Australian Space Forecast Centre issued a Geomagnetic Disturbance Warning at 0132 UTC on September 8. They wrote, “The effect of a high speed solar wind stream from a coronal hole is keeping geomagnetic activity enhanced.

Geomagnetic activity is expected to remain mostly enhanced to Active levels on 8 September with the possibility of some minor storm periods.”

If you look at ftp://ftp.swpc.noaa.gov/pub/indices/DGD.txt you can see the effect, which is high numbers indicating geomagnetic instability. What does this mean for performance on the air? For HF it can mean higher absorption rates (meaning radio waves may be absorbed more than they are reflected), but it can mean some interesting propagation modes may occur on VHF.

The Australian Space Forecast Center issued another geomagnetic disturbance warning at 0017 UTC on September 11: “A high speed solar wind stream from a coronal hole is expected to arrive late on 11 September. This may result in the geomagnetic activity rising to Active to Minor Storm levels. This rise in geomagnetic activity is expected to continue on 12 September.”


Frank Donovan, W3LPL, sent a fascinating article about a rejuvenation in solar magnetic strength toward the end of 2014, and an alternate view from the recent news stories about a long term dearth of sunspots. Instead, they propose that Cycle 25 (the next one) may be similar to Cycle 24 (the one we are now in).

Read it at http://aasnova.org/2015/09/02/witnessing-solar-rejuvenation/ .


Weak solar activity continued this week. The average daily sunspot number for September 3-9 was only 37.3, down 11 points from the previous seven days. Average daily solar flux was 85.3, compare to 97 for the previous week.

Geomagnetic activity was strong, with average planetary A index up from 19.4 to 27.1. The most active day was Wednesday, September 9 when the planetary A index was 59.

The latest solar flux prediction from USAF/NOAA has readings of 85, 90 and 100 on September 10-12, 105 on September 13-14, 110 on September 15-17, 115 on September 18-22, then 110, 105, 100 and 95 on September 23-26, 90 on September 27 to October 1, and 85 on October 2-7. Flux values then rise to 115 on October 15-19.

Predicted planetary A index is 28, 16 and 27 on September 10-12, then 16, 10 and 8 on September 13-15, 5 on September 16-17, then 8, 20 and 10 on September 18-20, 5 on September 21-23, then 15 and 10 on September 24-25, 5 on September 26-29, then 8 and 18 on September 30 through October 1, 12 on October 2-4, 8 on October 5, then 5 on October 6-7, then 8, 12 and 8 on October 8-10, and 10 on October 11-13.

Both the solar flux and planetary A index predictions are from September 9. The new forecast appears every day after 2100 UTC, but so far early on September 11 the September 10 forecast is not available. You can catch up by checking ftp://ftp.swpc.noaa.gov/pub/forecasts/45DF/ .

We do have a geomagnetic forecast from OK1MGW who predicts quiet to active conditions September 11-13, (with September 11 conditions perhaps at active to disturbed levels), quiet to unsettled September 14, quiet to active September 15-16 (with September 15 conditions possibly at active to disturbed levels), quiet to unsettled September 17-18, active to disturbed conditions September 19, then mostly quiet on September 20-21, quiet to active September 22, active to disturbed September 23-24, quiet to unsettled September 25, quiet on September 26-28, mostly quiet September 29, and quiet to unsettled September 30. He expects increases of solar wind on September 11-13, 15-16, 19, and 22-24.


Note that the mid-latitude A index for September 8 and 9 listed at the end of this bulletin are wild guesses on my part, based on tracking with the planetary A index. If you look at ftp://ftp.swpc.noaa.gov/pub/indices/DGD.txt you will see there is no data for those dates. This is probably because geomagnetic activity was strong enough to knock the Fredericksburg and College (for the high latitude number) magnetometers out of service.

For more information concerning radio propagation, see the ARRL Technical Information Service at http://arrl.org/propagation-of-rf-signals. For an explanation of the numbers used in this bulletin, see http://arrl.org/the-sun-the-earth-the-ionosphere. An archive of past propagation bulletins is at http://arrl.org/w1aw-bulletins-archive-propagation. More good information and tutorials on propagation are at http://k9la.us/.

My own archives of the NOAA/USAF daily 45 day forecast for solar flux and planetary A index are in downloadable spreadsheet format at http://bit.ly/1VOqf9B and http://bit.ly/1DcpaC5 .

Click on “Download this file” to download the archive and ignore the security warning about file format. Pop-up blockers may suppress download. I’ve had better luck with Firefox than Internet Explorer.

Monthly propagation charts between four USA regions and twelve overseas locations are at http://arrl.org/propagation.

Instructions for starting or ending email distribution of ARRL bulletins are at http://arrl.org/bulletins.

Sunspot numbers for September 3 through 9 were 27, 36, 24, 38, 47, 40, and 49, with a mean of 37.3. 10.7 cm flux was 86.5, 89.9, 85.4, 85.6, 83.7, 83.5, and 82.3, with a mean of 85.3. Estimated planetary A indices were 9, 20, 13, 14, 46, 29, and 59, with a mean of 27.1. Estimated mid-latitude A indices were 9, 18, 10, 12, 27, 24, and 31, with a mean of 18.7.