QST de W1AW
Propagation Forecast Bulletin 38 ARLP038
From Tad Cook, K7VVV
Seattle, WA September 22, 2000
To all radio amateurs
SB PROP ARL ARLP038
ARLP038 Propagation de K7VVV
Last week's bulletin reported that the sun was almost spotless, with one day's sunspot number the lowest since February of last year. Now this week we can report the appearance of sunspot 9169, the largest observed in nine years. The area of this spot is a dozen times the area of the earth's surface, and what should make it interesting for HF radio is that it is rotating toward the center of the sun's earth-facing hemisphere, which aims its effects right toward us. A large sunspot such as this can produce big solar flares, but so far this one, although magnetically complex, seems quiet.
This week's big one measures 2,140 millionths of the visible solar disk. The largest sunspot recorded in the twentieth century was in 1947, and it measured 6,132 millionths of the solar disk. You can see more information about big sunspots at http://www.spaceweather.com/sunspots/history.html.
Solar flux has been much higher this week than predicted. Last week's bulletin projected a solar flux around 170, but by Sunday it was above 180 and the next day over 200.
The projected solar flux for the next five days, Friday through Tuesday, is 230, 235, 235, 230 and 225. Planetary A index for those same days is expected to be around 10, 10, 10, 15 and 12. So for this weekend we not only have the autumnal equinox, which is a great time for HF propagation, but solar flux and sunspot numbers are increasing as well. This means good conditions, as long as geomagnetic activity stays low.
Geomagnetic activity was anything but low on Sunday and Monday, when the effects of several solar flares were felt. Planetary A index for both days was 40 and 45, and K indices were as high as 7. The College A index, recorded in Alaska, was 59 on September 17, and the College K index was 8 during one period. This was indicative of a severe geomagnetic storm as well as an aurora at that time.
Over the next few weeks expect the higher frequencies, especially 10 meters, to improve as we change to fall conditions.
Last week's bulletin mentioned a chart showing solar cycle progress. Check out another chart showing activity over the past few months on Jan Alvestad's Solar Activity Report at http://www.dxlc.com/solar/, or the past year of Daily Effective Sunspot Numbers at the NW Research Associates site at http://www.nwra-az.com/spawx/ssne-year.html. For an explanation of Effective Sunspot Numbers, which are based on ionospheric F-layer observations rather than solar observations, see http://www.nwra-az.com/spawx/ssne.html.
Sunspot numbers for September 14 through 20 were 109, 113, 148, 146, 154, 140 and 171 with a mean of 140.1. 10.7 cm flux was 150.8, 159.4, 174.6, 181.5, 203.8, 207.1 and 211.4, with a mean of 184.1, and estimated planetary A indices were 6, 10, 21, 40, 45, 27 and 13 with a mean of 23.1.