Solar activity increased last week, making one of those occasional upticks on the way down from the peak of the solar cycle.

Average daily sunspot numbers increased from 43.3 to 74.7, and average daily solar flux increased by ten points to 91.9. Average planetary A index went from 9.4 to 11.3 and average mid-latitude A index went from 11.6 to 10.7.

At 0415 UTC on May 2 the Australian Space Forecast Centre issued a geomagnetic disturbance warning. At the end of the day, the planetary A index was 31, indicating a geomagnetic storm.

The latest forecast has solar flux at 90 on May 6, 85 on May 7-12, 90 on May 13, 95 on May 14-15, 90 on May 16, 82 on May 17-22 and 85 on May 23, 90 on May 24, 95 on May 25-28, then 90, 95 and 105 on May 29-31, 110 on June 1-2, 105 on June 3, 100 on June 4-6, and 95 on June 7-11. Solar flux is forecast to drop below 90 on June 12 and beyond.

Predicted planetary A index is 8 on May 6, 12 and 18 on May 7-8, 25 on May 9-10, then 20 on May 11, 8 on May 12-13, 12 on May 14 and 5 on May 15-18, and 12, 15, 12, 8, 5 and 10 on May 19-24, and 5 on May 25-27. The next active days are June 6-8, when the planetary A index is expected to be 18, 25 and 20.


The OK1HH geomagnetic activity forecast for the period May 6-June 1, 2016.

Geomagnetic field will be:
Quiet on May 7-8, 16-17, 26-28, June 1
Mostly Quiet on May 9, 14, 18, 22, 29, 31
Quiet to Unsettled on May 6, 13, 21, 23-25
Quiet to Active on May 12, 15, 19-20
Active to Disturbed on May 10-11, 30

Increases in the solar wind from coronal holes are expected on May 7, 12-14, (17,) 22-26, 31, and June 1.

Parenthesis means lower probability of activity enhancement.

David Moore sent this link http://huff.to/236Xxl8 to an article titled “Spectacular Solar Flare Captured in New NASA Video.” The flare occurred on April 17. Thar day, the high latitude College A index (measured at Fairbanks, Alaska) was 33. But a few days earlier the index was higher, at 45 and 47 on April 13-14.

As the season shifts toward summer, you may notice some changes in HF propagation. For instance, on 15 meters from California to Japan back at the vernal equinox you would see good strong signals, with openings beginning around 2100 UTC and fading out around 0430 UTC. The path should be open 75-100 percent of the time from 2130-0300 UTC. But by the middle of this month you might see signals several dB lower and the path not as robust, with the probability of openings being more toward the 50-75 percent range. Openings would begin around 2000 UTC and signals should start getting weaker after 0300 UTC.

On 40 meters over the same path at the vernal equinox (around March 23) we would see the band start to open after 0630 UTC with strong signals through the night until around 1530 UTC. But by the middle of May, the period that this path is open is much shorter, starting around 0830 UTC and fading out after 1400 UTC.

Looking the other direction, from California to Atlanta, Georgia during the spring, the strongest 40 meter signals over that path would be between 0230-1200 UTC. But by the middle of May that opening would shift to 0300-1030 UTC.

You can play with these numbers yourself, varying seasons, locations and frequencies by using a propagation prediction program. A free one, W6ELprop is available from K9LA. His web site is http://k9la.us/ and he has lots of useful information on radio propagation. The link to propagation programs is at http://k9la.us/html/tutorials.html .  An alternate source is at http://brucerichards.com/army/w6elprop.htm .


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/.

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 April 28 through May 4 were 84, 82, 82, 76, 75, 57, and 67, with a mean of 74.7. 10.7 cm flux was 94.9, 92.4, 94, 92.2, 89.8, 90, and 90.1, with a mean of 91.9. Estimated planetary A indices were 5, 4, 7, 16, 31, 11, and 5, with a mean of 11.3. Estimated mid-latitude A indices were 6, 3, 9, 16, 26, 11, and 4, with a mean of 10.7.