QST de W1AW
Propagation Forecast Bulletin 38 ARLP038
From Tad Cook, K7VVV
Seattle, WA September 17, 1999
To all radio amateurs
SB PROP ARL ARLP038
ARLP038 Propagation de K7VVV
Solar flux and sunspot numbers were up this week. Average solar flux rose about ten points and average sunspot numbers were up almost 33 points when compared to the week previous. Next week is important because Thursday is the autumnal equinox, which is generally a good time for HF propagation. Solar flux is expected to rise over the week, peaking around 220 on Thursday, when the hours of darkness are equal in the southern and northern hemispheres. After September 26 flux values should drop below 200, down to 140 by the end of the month, and fall to a minimum around 105 on October 5.
For this weekend, look for the Friday through Sunday solar flux to be 160, 165 and 170, and planetary A index to drop from 25 to 20 to 12. The A index has been high this week, which has not been good for HF conditions. The worst day was Monday, when the A index registered a very stormy 38. The K index was consistently 4 and 5 throughout the day. Wednesday and Thursday were also bad, with an A index of 32 and planetary K index as high as 7. On Thursday the College A index in Alaska was 63. Alaskan HF operators were probably able to hear very little, although VHF fans outside of the higher latitudes likely had fun.
All of these disruptions were caused by a series of solar flares and coronal holes, throwing proton energy that disrupts the ionosphere and bends the earth's magnetic field. Geomagnetic conditions are not expected to really settle down until after October 1, when the A index should stay below 10. Sunspot region 8674 is rotating into view, and could produce more flares and coronal holes.
Sunspot numbers for September 9 through 15 were 113, 107, 145, 126, 183, 158 and 155 with a mean of 141. 10.7 cm flux was 106.5, 122, 122.5, 140.8, 154.6, 156.4 and 154.9, with a mean of 136.8, and estimated planetary A indices were 8, 15, 10, 26, 38, 23 and 32, with a mean of 21.7.
Here is a path projection for this weekend from the geographical center of the contiguous 48 United States, somewhere in Kansas.