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ARRL Propagation Bulletin ARLP020 (2006)

ARLP020 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 20  ARLP020
From Tad Cook, K7RA
Seattle, WA  May 19, 2006
To all radio amateurs 

ARLP020 Propagation de K7RA

Our Sun is in another period of spotless days. Since Monday, May 15,
no spots are visible, through Thursday so far. We don't have access
to an actual sunspot prediction model for the next week, but the
U.S. Air Force does issue a daily 45 day outlook for solar flux and
planetary A index. The forecast shows a steady solar flux of 75
until May 26, when it jumps 10 points to 85.

The solar flux on these four spotless days ranged from 71.5 to 73.5,
and rises slightly over the period. With a prediction of solar flux
at 75 over the next week, that suggests low sunspot numbers or
perhaps even more days with no sunspots. Currently sunspot 884 (a
small one) is just beginning to peek around our Sun's eastern limb.
We should see some daily sunspot number until it either fades away
or completes a transit of the Earth-facing side of our sun.

As the sunspot cycle continues its descent, we will see long
stretches with no visible spots. Nearly a decade ago, 38 consecutive
days, from September 13, 1996 through October 20, 1996 had no

You can look at projected smoothed sunspot numbers for the next year
on page 11 at This
prediction hasn't changed for several years. It shows a smoothed
sunspot number of 11 for May 2006, so according to this chart, in
September 2007 we should see more sunspots than we do now. Next
month we can observe that in July 2007 we should have more sunspots
than June 2006.

Earth has been within a solar wind stream from a coronal hole. As a
result, late on May 18 both the planetary and mid-latitude K index
reached 4, and the high-latitude college A index went to 5. For May
19 the planetary A index is predicted to be 20. Conditions quiet
down in the following days, and the A index is not predicted to rise
again to that level until early June.

Geophysical Institute Prague predicts unsettled to active conditions
for May 19, unsettled on May 20-21, quiet to unsettled on May 22,
quiet on May 23-24, and quiet to unsettled on May 25. With no
sunspots, MUF for most long-range propagation paths is lower, so 10,
12 and 15 meters are not yielding results as they were when there
was at least some sunspot activity.

Ever wonder exactly where these geomagnetic observatories used for
the various A and K indexes are located? Here is a table of recent
observations: . Jim
Henderson, KF7E of Queen Creek, Arizona (southeast of Phoenix) sent
a page from USGS devoted to magnetic observatories: Click on the link for
Newport, and you can see photos of the observatory buildings, this
one near the Idaho border in Eastern Washington. Note the arcane
latitude/longitude numbers. To convert those to the normal
coordinates we are used to, subtract the co-latitude number from 90,
and the longitude from 360. You can then enter these numbers (40.27
N, 117.12 W) in an online map, such as Google Maps at Note if you zoom in far enough you will see
that the buildings are just off Geophysical Observatory Road, near
the town of Newport, Washington.

On the Daily Geomagnetic Data page (the table mentioned in the
previous paragraph), you'll see that our middle-latitude K and A
index come from Fredericksburg. Click on the FRD link on the USGS
page mentioned above, and you can do the same location calculations,
which yield 38.2 N, 77.37 W. The map shows this is near another
Observatory Road, this time near the western perimeter of Fort A.P.
Hill Military Reservation (not named on this map), southeast of
Fredericksburg, Virginia. There is another link from the USGS page
(CMO), which shows the home of the college A and K index in
Fairbanks, Alaska, and another for the Boulder observatory, source
of the numbers heard on WWV. Recent indices from Boulder are at

Don Eiler, WA4PLD of Knoxville, Tennessee sent in a couple of links
with information on the IMF (Interplanetary Magnetic Field). The
first is one this bulletin has mentioned before: The second,, is new to me.

If you would like to make a comment or have a tip for our readers,
email the author at,

For more information concerning radio propagation and an explanation
of the numbers used in this bulletin, see the ARRL Technical
Information Service propagation page at, An archive of past
propagation bulletins is at,

Sunspot numbers for May 11 through 17 were 36, 38, 24, 11, 0, 0 and
0 with a mean of 15.6. 10.7 cm flux was 76.4, 75.7, 73.5, 72, 71.7,
71.5, and 72, with a mean of 73.3. Estimated planetary A indices
were 18, 16, 11, 8, 4, 2 and 5 with a mean of 9.1. Estimated
mid-latitude A indices were 21, 10, 9, 6, 3, 0 and 4, with a mean of


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