QST de W1AW
Propagation Forecast Bulletin 38 ARLP038
From Tad Cook, K7RA
Seattle, WA September 19, 2003
To all radio amateurs
SB PROP ARL ARLP038
ARLP038 Propagation de K7RA
Last week's bulletin called for stable geomagnetic conditions over the weekend, which we got. Planetary A indices, a measure of geomagnetic stability for the day, were 11, 11, 7 and 6 for last Friday through Monday, September 12-15. There was a strong solar wind, but a north-pointing interplanetary magnetic field (IMF) kept any destabilizing effects to a minimum. The IMF continued to point north through Sunday, September 14, but then pointed south. This led to the geomagnetic storm and high planetary A index of 37 and 61 on Tuesday and Wednesday, September 16 and 17.
The IMF continues to point south, and currently we are entering a stronger solar wind stream. The predicted planetary A index for Friday, September 19 through Monday, September 22 is 35, 25, 20 and 15. For a review of interplanetary magnetic fields, check http://spaceweather.com/glossary/imf.html.
Solar flux this week was down and average daily sunspot numbers were up slightly. The sun has appeared nearly blank this week, with any sunspots toward the edge of the disk, not pointing radiation at Earth as spots in the center do. See the solar disk for September 16 to observe a nearly blank sun at http://science.nasa.gov/spaceweather/images2003/16sep03/midi512_blank.gif. You can substitute the date in the URL to see what the sun was like on other days.
Solar flux was lowest in the past couple of weeks at 94.4 on Friday, September 12. Recent daily sunspot numbers were lowest on September 10 at 42. As the solar cycle declines over the next couple of years, we will eventually see long periods with sunspot counts of zero. The last really long period where this was observed was at or near the bottom of the last sunspot cycle. For 38 days, from September 13 until October 20, 1996 there were no visible sunspots. The daily sunspot number was zero for that entire time. During that period, the daily solar flux was below 70 nearly the entire time. The lowest was 66.4 on October 11, 1996.
Now this week and the week prior we've observed nine consecutive days when the daily solar flux was below 100. There is nothing particularly significant about 100, but we humans notice nice even numbers like this, kind of like waiting and watching for that car odometer to turn over from 99,999 to 100,000 miles. But this nine-day period seems significant, because the last time we had this many days in a row with a solar flux value below 100 was back in 1998, from May 19-31, with 13 continuous days. This was way over on the other side of the peak of cycle 23.
Recently this bulletin looked at a prediction for the minimum of the current sunspot cycle. Read about prediction methods used to determine long-term trends in solar cycles at http://science.msfc.nasa.gov/ssl/pad/solar/predict.htm.
Note that the fall equinox is in a few days, and this is a good time for worldwide DX on the HF bands, even with the low solar activity. Solar flux is currently rising as we progress toward the equinox around September 23, next Tuesday. The current solar flux forecast for the short term shows flux values of 110 for September 19-21, and 115 for September 22-23.
Carl Luetzelschwab, K9LA, advises that the Space Environment Center (SEC) in Boulder, Colorado, recently introduced a new operational product to assess the impact of geomagnetic field activity on the F region. It's called the STORM Time Empirical Ionospheric Model. It provides--in real-time--an F region critical frequency (foF2) scaling factor due to geomagnetic field activity that can be applied to the quiet time foF2 value. The scaling factors are expressed as percentages above or below the quiet time values, and thus can be applied to the MUF output of your favorite propagation software.
The model uses the previous 33 hours of geomagnetic field activity as its driver, indicating that the F region doesn't necessarily respond immediately to elevated K indices. Check out http://sec.noaa.gov/storm for the current plot, historical plots of significant geomagnetic storms, and a discussion of how the model was developed and validated.
For more information on propagation and an explanation of the numbers used in this bulletin see the Propagation page on the ARRL Web site, http://www.arrl.org/tis/info/propagation.html.
Sunspot numbers for September 11 through 17 were 55, 58, 57, 58, 68, 89 and 83, with a mean of 66.9. 10.7 cm flux was 96.7, 94.4, 96.1, 94.7, 97.3, 99.3 and 105.9, with a mean of 97.8. Estimated planetary A indices were 15, 11, 11, 7, 6, 37 and 61, with a mean of 21.1.