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ARRL Propagation Bulletin ARLP038 (2003)

ARLP038 Propagation de K7RA

Propagation Forecast Bulletin 38  ARLP038
From Tad Cook, K7RA
Seattle, WA  September 19, 2003
To all radio amateurs

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

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
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

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 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,

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.


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