QST de W1AW
Propagation Forecast Bulletin 29 ARLP029
From Tad Cook, K7VVV
Seattle, WA July 12, 2002
To all radio amateurs
SB PROP ARL ARLP029 ARLP029 Propagation de K7VVV
Conditions are still a bit in the doldrums, with solar flux and sunspot numbers fairly flat. This week average daily sunspot numbers were up over 24 points and average daily solar flux was down nearly 12 points, when compared to the previous week. Of course this seems low compared to the great conditions last fall, but the numbers are slightly higher than they were for the same week last year.
In the July 13 Propagation Forecast Bulletin last year covering July 5-11, 2001, average sunspot numbers were only 98.1 and average solar flux was 124.6. One thing that skewed the numbers a bit this time was on the last day of last week's reporting period the solar flux jumped unusually high, and the next day, which was the first day of the current reporting period (July 5), it was now the sunspot number that was high.
The current outlook is for a slowly rising solar flux of 135 for July 12-13, 140 for July 14-15, and 145 for July 16-17. There isn't any forecast for higher numbers above this narrow range for the near future. The prediction for the planetary A index for Friday through Monday is 8, 15, 10 and 8. There is a new large sunspot coming around the sun's northeast limb, and another large spot on the sun's far side, as detected by helioseismic holography imaging. It has been a while since this imaging scheme was mentioned in this bulletin, and you can read more about it at http://spaceweather.com/glossary/farside.html.
A good source for an explanation of shortwave propagation and some of the numbers used in this bulletin (by K9LA) is at http://www.arrl.org/tis/info/k9la-prop.html. This is part of the ARRL Technical Service propagation page at http://www.arrl.org/tis/info/propagation.html.
This week our earth has reached its aphelion, the farthest distance from the sun in its annual orbit. Earth's orbit has an eccentricity of 1.7 percent, and this week we are nearly 95 million miles from our closest star. When we are at the closest point, or perihelion, we are almost 92 million miles from the sun. There was an article on http://www.cnn.com saying that the aphelion will be this Saturday, but according to the U.S. Naval Observatory at http://aa.usno.navy.mil/data/docs/EarthSeasons.html it was on July 6. The perihelion for 2002 was January 2, and in 2003 and 2004 it will be January 4.
In this bulletin we report the observed solar flux from the Dominion Radio Astrophysical Observatory in Penticton, British Columbia, about 160 miles east of Vancouver. They have a nice web site at http://www.drao.nrc.ca.
Energy is measured at 2.8 GHz with a parabolic dish pointed at the sky. Of course this value would vary somewhat with the distance from the observation point to the sun. So to account for this difference and give a measurement that might be more indicative of the level of energy from the sun at that frequency, there is an adjusted value of solar flux. On July 6 at the aphelion the observed solar flux at 2000z was 133.5, but the adjusted flux value was 138. At the perihelion the observed 2000z flux was 231.1, but the adjusted value was adjusted down to 223.5. April 2 or 3 must be about half way in between, and the observed and adjusted values on those days are quite close. You can see a very large table of the solar flux values measured thrice daily at http://www.drao.nrc.ca/icarus/www/current.txt.
Sunspot numbers for July 4 through 10 were 175, 149, 123, 121, 125, 129 and 118, with a mean of 134.3. 10.7 cm flux was 146.3, 138.8, 133.5, 136.9, 130.9, 136.3, and 128.8, with a mean of 135.9. Estimated planetary A indices were 7, 13, 23, 11, 10, 16, and 11, with a mean of 13.