ZCZC AP39
QST de W1AW
Propagation Forecast Bulletin 39 ARLP039
From Tad Cook, K7RA
Seattle, WA September 26, 2003
To all radio amateurs
SB PROP ARL ARLP039
ARLP039 Propagation de K7RA
At last the sun is showing one large sunspot. Sunspot 464 is expanding rapidly and now is part of an extended dark area about 13 Earth diameters wide. A helioseismic holography image shows a large sunspot currently on the side of the sun that faces away from Earth, often referred to as "the other side." Mentioning the name of this method for sensing activity on the sun's far side always generates questions asking, "what's that?" So check the glossary on the SpaceWeather.com Web site for an explanation.
The emergence of this spot has raised the sunspot count and solar flux higher than predicted a week ago, with solar flux about 20 points greater. Solar flux for the past few days has edged above 130, and the prediction for Friday through Monday, September 26-29, is for flux values of 135, 130, 130 and 125.
Frantisek K. Janda, OK1HH, of the Czech Propagation Interest Group reports that except for October 13-17, the upcoming weeks will have quieter geomagnetic conditions than have prevailed over the past five months.
This is the weekend of the CQ World Wide RTTY DX Contest, and the interplanetary magnetic field currently points south. This means that Earth is susceptible to blasts of energy from the sun. A solar wind is flowing from a coronal hole on the sun, so the predicted planetary A index for Friday through Monday, September 26-29 is 15, 30, 20 and 15. Saturday might be a bit rough for the contest, but let's hope for a change. The sunspot count this week turned out to be higher than we thought last week, another example of how things are changing.
On September 23, wee transitioned from summer to fall in the Northern Hemisphere and from winter to spring in the Southern Hemisphere. Around the equinox is a great time for worldwide DX, because all parts of Earth are bathed approximately equally by the sun's energy. When it is summer in one hemisphere, the maximum usable frequency (MUF) is lower during the day, so 10 through 15 meters are affected. At that same time, the other half of the earth has winter, and the ionosphere over that region is exposed to a lot less solar radiation. At the equinox, winter or spring, the MUF is higher, and the whole world is affected the same.
For more information on propagation and an explanation of the numbers used in this bulletin see the Propagation page on the ARRL Web site.
Sunspot numbers for September 18 through 24 were 92, 71, 72, 64, 91, 133 and 121, with a mean of 92. The 10.7-cm flux was 109.2, 111.1, 111.9, 119.9, 122.6, 124.9 and 133.5, with a mean of 119. Estimated planetary A indices were 40, 32, 25, 21, 18, 17 and 33, with a mean of 26.6.