Merry Christmas! This is the first time we have released a bulletin on Christmas Day. The last time Christmas fell on a Friday -- our normal day for publishing this bulletin -- was back in 2009, and we released the bulletin on Christmas Eve. Note that the solar indices for that week were no better than for this past week, 6 years later.

This will be the last propagation bulletin for 2015; our next bulletin will be released next Friday, January 1, 2016.

Over the past week, average daily sunspot numbers rose just 1.6 points to 49.6 compared to the previous 7 days, December 10-16. Average daily solar flux rose from 102.2 to 122.3 for the December 17-23 reporting period.

A big geomagnetic storm on Sunday, December 20, drove our geomagnetic averages way up this week. On that day the mid-latitude A index (recorded in Virginia) reached 33, the planetary A index (recorded at a number of Northern Hemisphere sites) was 66, and the college A index (recorded near Fairbanks, Alaska) was 89.

The day before, at 2323 UTC on December 19, the Australian Space Forecast Centre issued a geomagnetic warning:

“Geomagnetic disturbance in progress following a CME impact after 1500 UTC December 19. Expect Active geomagnetic conditions December 20.

“Increased geomagnetic activity expected due to coronal mass ejection from 20-21 December 2015.”

Then on December 23, Spaceweather.com sent out this bulletin:

“A new sunspot (AR2473) is growing rapidly in the sun's Southern Hemisphere, more than quadrupling in size in the past 24 hours. Crackling with M-class solar flares, the sunspot has already caused several minor shortwave radio blackouts, mainly south of our planet's equator. More flares and radio blackouts are in the offing as the growing sunspot turns toward Earth. Visit http://spaceweather.com for more information.”

Over this reporting week (December 17-23) average planetary A index rose from 9.9 to 21.7 and average mid-latitude A index rose from 6.6 to 12.6.

Predicted solar flux for the near term is 130 on December 25, 125 on December 26-31, then 115, 110 and 105 on January 1-3, 110 on January 4-6, 115 on January 7-9, 120 on January 10-13, 118 on January 14-15, 115 on January 16, 120 on January 17-18, then 115, 105 and 100 on January 19-21, 98 on January 22-23, 95 on January 24-25, 98 on January 26-27, and 100 on January 28-29.

Predicted planetary A index is 12, 10, 8 and 6 on December 25-28, 5 on December 29-31, then 15, 20, 18 and 10 on January 1-4, then 8, 20, 18 and 12 on January 5-8, then 10, 20, 18, 10 and 8 on January 9-13, and 5 on January 14-16.

OK1MGW from the Czech Propagation Interest Group sends us his geomagnetic prediction this week, and it says to expect the geomagnetic field to be quiet to active December 25-28, quiet to unsettled December 29, mostly quiet December 30-31, active to disturbed January 1-2, quiet to active January 3, quiet to unsettled January 4, quiet to active January 5, active to disturbed January 6, quiet to active January 7, quiet to unsettled January 8-9, active to disturbed January 10, quiet to active January 11, quiet to unsettled January 12-14, mostly quiet January 15-17, quiet on January 18-19, and quiet to unsettled January 20.

OK1MGW expects increased solar wind on December 25-28, January 1-3, January 5-6, and January 10-11.

The winter solstice was 3 days ago, on Tuesday, December 22. Now the hours of daylight will become longer for the next 6months, until the summer solstice in the Northern Hemisphere, on June 20, 2016. Spring begins at the equinox, March 19-20.

As the days get longer, HF conditions will generally improve in the Northern Hemisphere. We can test some paths with a simple free program, W6ELprop.

Testing from Dallas, Texas, to Germany, a path of 8,222 km (5,109 miles) on the shortest day of 2015 we can see that 15 meters has the best possibility of propagation from 1500-1630 UTC with an A rating (75-100 percent chance of communication) at 23 dB above 0.5 mV at the receiving end) and a B rating (50-74 percent probability) 1430-1700 UTC.

At the end of January 2016, the opening runs from 1430-1730 UTC for the B rating, (A rating 1500-1700 UTC), with signals about 2 dB lower.

At the vernal equinox, on March 20, 2016, the 15 meter opening stretches from 1400-2100 UTC. All of these tests were done with a solar flux of 127, to look at seasonal variation only.

Over the same path on 17 meters on December 22, 2015, it is open from 1400-1730 UTC, on January 31 1400-1830 UTC and on March 20, 2016, 1300-2200 UTC.

But for lower frequencies, such as 75 meters, conditions are better during long periods of darkness, such as late December. On December 22, over the same path 75 meters looks best 2330-0830 UTC, but on January 31 the best conditions are from 0030-0730 UTC and on March 20 conditions look best at 0200-0530 UTC.

Sunspot numbers for December 17 through 23 were 52, 49, 44, 33, 38, 68, and 63, with a mean of 49.6. The 10.7 cm flux was 117.8, 117.1, 119, 116.6, 121.7, 130.1, and 133.9, with a mean of 122.3. Estimated planetary A indices were 7, 5, 12, 66, 38, 13, and 11, with a mean of 21.7. Estimated mid-latitude A indices were 6, 4, 8, 33, 22, 8, and 7, with a mean of 12.6. 

For more information concerning radio propagation, see the ARRL Technical Information Service. For an explanation of the numbers used in this bulletin, see “What the Numbers Mean, and Propagation Predictions -- a brief introduction to propagation and the major factors affecting it,” by Carl Luetzelschwab, K9LA.

An archive of past propagation bulletins is on the ARRL website. More information and tutorials on propagation are on K9LA's website. 

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 the download.

Monthly propagation charts between four US regions and 12overseas locations are on the ARRL website.

Instructions for starting or ending email distribution of ARRL bulletins are at http://arrl.org/bulletins. --Thanks to Tad Cook, K7RA