ZCZC AP21
QST de W1AW
Propagation Forecast Bulletin 21 ARLP021
From Tad Cook, K7RA
Seattle, WA May 21, 2004
To all radio amateurs
SB PROP ARL ARLP021
ARLP021 Propagation de K7RA
Solar flux values and sunspot numbers were higher over the past week. Average daily sunspot numbers rose nearly 65 points to 113.6 from the week earlier. Average solar flux rose over 18 points. The sunspot number reached a peak of 148 and 147 on Sunday and Monday, May 16-17. This was due to the growth of sunspot 609, which expanded dramatically along with a host of other spots.
A medium sized sunspot group is on the sun's far side, as detected by helioseismic holography. There were some unsettled conditions last Thursday, May 13, but the rest of the week was quiet with low K and A indices.
We are currently within a weak stream of solar wind from a coronal hole, so some resulting geomagnetic activity is possible. The predicted planetary A index for May 21-24 is 15, 15, 12 and 10. Solar flux should stay around 100 over the next week.
We are moving into late spring, with the longest day of the year just one month away. As the days become longer, we'll notice worldwide openings on 15 meters are not as common as they were earlier in spring. Worldwide 40-meter openings will come later in the evening, as the hour of sunset advances.
Users of lower frequencies, which depend on darkness for long distance communication, may notice shorter duration openings through the night because there are less hours of darkness. Seasonal noise levels should begin to rise as well.
Solar numbers aren't holding up too badly, considering where we are in the current cycle. Note that the average sunspot number this week at 113.6 is higher than the same week a year ago, when it was only 83.6. However, two years ago it was 160.3 and a year prior to that it was 122.3. There are still good openings in store for us, but the frequencies used will be lower, and except for some sporadic-E skip, we won't see the exciting 10 meter propagation we did a few years back.
Sunspot numbers for May 13 through 19 were 107, 98, 117, 148, 147, 91 and 87 with a mean of 113.6. 10.7 cm flux was 100.8, 109.6, 115.3, 118.3, 111.1, 107.8 and 108.8, with a mean of 110.2. Estimated planetary A indices were 13, 8, 9, 4, 5, 4 and 6, with a mean of 7. Estimated mid-latitude A indices were 17, 4, 7, 4, 3, 4 and 6, with a mean of 6.4.