ZCZC AP17
QST de W1AW
Propagation Forecast Bulletin 17 ARLP017
From Tad Cook, K7RA
Seattle, WA April 25, 2003
To all radio amateurs
SB PROP ARL ARLP017
ARLP017 Propagation de K7RA
Thanks so much to Carl Luetzelschwab, K9LA for writing last week's bulletin while your regular reporter was away.
Sunspot numbers and solar flux rose over the past week. Average daily sunspot numbers were up over 44 points to 100.4, and solar flux was nearly 17 points higher to 118.6. Solar wind and the resulting geomagnetic instability that has been prevalent for the past few weeks continued, and is expected to last at least through this weekend.
The current outlook has the Friday through Monday planetary A index for April 25-28 at 20, 25, 25 and 20. Current projections forecast quiet conditions (where the A index is 10 or lower) for May 3-5, but higher just before that period and for nearly two weeks following. This is because of coronal holes rotating into view from April 29 through May 2 and again May 6-8. A large coronal hole should return around May 12.
The reason for the unsettled to active geomagnetic forecast for this weekend is because of an ongoing solar wind, and an active sunspot that released two solar flares on Wednesday. This was not squarely directed at earth, but could cause some mischief today. The most active days recently were April 16-17, when planetary A indices were 31 and 30 and the K index went as high as 6. Over those same two days the higher latitude college A index (measured in Alaska) was 54 and 50, and the K index went to 7, indicating a severe geomagnetic storm.
Predicted solar flux is 128 and 130 for Friday and Saturday, April 25-26, and then around 135 through the next week. When the sun shows some spots and geomagnetic indices are quiet, this is when HF conditions are most promising. But lately with the decline in the solar cycle, there haven't been many sunspots but there have been enough flares and coronal holes to keep conditions unstable. Current projections show this cycle hitting minimum around the end of 2006 and the beginning of 2007. A few years after that we should be back to where we are now (in terms of sunspots and solar flux) and on the rise again.
For more information about propagation and an explanation of the numbers used in this bulletin see the Propagation page on the ARRL Web site at http://www.arrl.org/tis/info/propagation.html.
Sunspot numbers for April 17 through 23 were 37, 51, 69, 93, 154, 147, and 152, with a mean of 100.4. 10.7 cm flux was 101, 107.8, 112.1, 118.5, 125.8, 132.4, and 132.8 with a mean of 118.6. Estimated planetary A indices were 30, 20, 18, 16, 21, 22, and 18, with a mean of 20.7.