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ARRL Propagation Bulletin ARLP034 (2003)

ARLP034 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 34  ARLP034
From Tad Cook, K7RA
Seattle, WA  August 22, 2003
To all radio amateurs 

ARLP034 Propagation de K7RA

Sunspot numbers down 19%, solar flux down 7%, and the planetary A
index up 42%; Could it be any worse for HF operators? Well yes, it
could, but those percentages reflect the change in average daily
indices from last week to this week. What could be worse of course
are zero sunspots with solar flux around 70 or lower, which is what
we were seeing about seven years ago at the bottom of the solar

For an example of this, look back to Propagation Forecast Bulletin
ARLP042 dated October 11, 1996 at The sunspot number
was a flat 0 for every day of that week, and average solar flux was

What does that mean for HF propagation? Run the free W6ELprop
software available from Run one copy
for August 22 with a solar flux of 68.6, and another copy with a
flux value of 121.7. In my comparison, in each instance I ran a path
from Dallas to Germany. The signal strength on the path with the
lower values showed about the same signal levels for 40 meters, but
the path reliability rating was much lower. Looking at 20 meters,
the path seems to close about 90 minutes earlier on the one using
the lower flux value. It is fun to run these comparisons over
different paths and seasons.

Without any doubt the day most disturbed by geomagnetic storms this
week was Monday, August 18, when the planetary A index was 86 and
the planetary K index was 8 during one three-hour period, 7 during
another, and 6 during three other periods. This indicates a severe
geomagnetic storm. This kind of thing gets worse as one goes toward
either pole, and in Fairbanks, Alaska the College A index was 132.
The College K index was 8 during two periods, 7 during three
periods, and 4, 5 and 6 in the other three. This is why many Alaskan
amateur radio operators complain of long periods when they can't
seem to hear or work anyone or anything.

The Monday storm began around 0100z when the interplanetary magnetic
field tipped to the south near earth. This makes the earth
vulnerable to the effects of any solar wind or flare activity. A
solar flare erupted on the sun on August 19 at 2005z, and this
pushed a strong coronal mass ejection toward earth. The forecast
from the U.S. Air Force for planetary A index was adjusted upward on
Thursday, August 21 after the initial one at 2104z. That earlier one
predicted a planetary A index of 30 for Friday, which is quite high.

Six hours and twenty minutes later a new forecast was released which
predicts Friday's planetary A index at 50. Saturday is predicted at
30, and Sunday, Monday and Tuesday all show the same planetary A
index of 25. Predicted solar flux for Friday, August 22 is 115, and
110 is the value for Saturday through Tuesday, after which the
number is expected to head higher.

I received several emails last week asking about any correlation
between the widespread power blackout and space weather. This seems
unlikely because conditions were actually rather mild during that
time. But a solar flare is a natural thing to consider during a
massive power outage, since a big flare on March 6, 1989 brought
down an electric power grid in Canada. One who wrote to ask about
this was Tim Anderson, AG4XM of Covington, Kentucky. He sent this
article about space weather and the effects upon power grids:

David Moore of Morro Bay, California sent an article about an
11-year cycle in which the sun's magnetic poles reverse. Read all
about it at

For more information on propagation and an explanation of the
numbers used in this bulletin see the Propagation page on the ARRL
Web site at .

Sunspot numbers for August 14 through 20 were 108, 86, 92, 113, 104,
77, and 62, with a mean of 91.7. 10.7 cm flux was 129.7, 131.4,
126.9, 119.3, 115.9, 116.7, and 111.8, with a mean of 121.7.
Estimated planetary A indices were 18, 14, 11, 15, 86, 21, and 15,
with a mean of 25.7.


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