ZCZC AP18
QST de W1AW
Propagation Forecast Bulletin 18 ARLP018
From Tad Cook, K7RA
Seattle, WA May 2, 2003
To all radio amateurs
SB PROP ARL ARLP018
ARLP018 Propagation de K7RA
Even as the current solar cycle slowly declines, there still will be periods of rising activity, and this week was one of those times. As this bulletin is being written May 1, a large sunspot-- number 349--is aimed squarely at Earth. The SpaceWeather Web site shows the spot centered in the solar disk such that Earth has maximum exposure. Earth is currently in a solar wind stream, as it has been for weeks, and the appearance of sunspot 349 as well as other new spots resulted in a rising sunspot number. After periods in the single digits, the daily sunspot number reached 224 on April 29. It hasn't been this high since March 9.
Average daily sunspot number for this week rose nearly 85 points to 185.1. Average daily solar flux was up nearly 29 points. Average daily sunspot number from this week in 2002 was 152.1, 143.6 in 2001, 144.4 in 2000, 74 in 1999 and 39.7 in 1998. Just sampling the numbers from Solar Update reports between 1998 and this year makes it appear that we are currently enjoying the peak of the cycle. Averaging the sunspot numbers for the full month of April may give us a better feel for our current place in the cycle. Average April sunspot numbers for 1998-2003 are 73.6, 92.9, 193.4, 163.6, 194.9 and 114.3. Still, what matters most to HF operators is the sunspots over the recent few days, so enjoy conditions when and if geomagnetic conditions stabilize to a K index of three or less.
The past week saw active geomagnetic conditions, and the activity only declined to an unsettled level on April 26-27 when the planetary A index was 15. The most active days were April 30 to May 1, with a planetary A index of 40 and planetary K indices as high as six over both days. This indicates a strong geomagnetic storm.
Over the next week geomagnetic activity should settle a bit, providing improved HF conditions. The predicted planetary A index for today through Monday, May 2-5, is 25, 15, 10 and 15. Predicted solar flux for those same days is 145, 140, 135 and 130.
Maximum usable frequencies (MUFs) over most paths should be somewhat lower during May than in April, and with May being further from the spring equinox, 10, 12 and 15-meter openings should be shorter as well. To compare, run W6ELprop (available free from the W6ELprop Web site) twice, so you can ALT-TAB to switch between them. Try different paths using the same sunspot number, but one program for April 15 and the other for May 15. Switching back and forth highlights the differences and makes it easier to compare. For instance, a path from Salt Lake City to Costa Rica with a sunspot number of 111 shows MUFs between 1830-2230 UTC as 24.4-25.9 MHz for April 15, but 20.4-20.6 MHz for May 15. This means that, given the same sunspot count, most of the time 17, 15 and 12 meters would be open on April 15, while only 17 meters would be open for that path for May 15.
For more information about propagation and an explanation of the numbers used in this bulletin see the Propagation page on the ARRL Web site.
Sunspot numbers for April 24 through 30 were 171, 173, 193, 200, 175, 224 and 160, with a mean of 185.1. The 10.7-cm flux was 128.3, 143.6, 143.7, 154.1, 152.2, 155.1 and 153.5, with a mean of 147.2. Estimated planetary A indices were 24, 32, 15, 15, 20, 20 and 40, with a mean of 23.7.
