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ARRL Propagation Bulletin ARLP040 (2008)

ARLP040 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 40  ARLP040
From Tad Cook, K7RA
Seattle, WA  September 26, 2008
To all radio amateurs 

ARLP040 Propagation de K7RA

This week saw the appearance of another sunspot, this one a new
Cycle 24 spot, larger and lasting a little longer than last week's
Cycle 23 sunspot.  It appeared for two days, September 22-23, with
sunspot numbers of 18 and 16.

Geomagnetic activity is very quiet, and on September 25 the K index
was 0 for a good part of the day.  This was true for the
mid-latitude index, planetary, and the college K index in Alaska.
The three corresponding A indices for the day were 4, 1 and 1
respectively.  This is especially nice for the lower part of the HF
spectrum, such as our 160 and 80 meter bands, and something we don't
see during periods of higher sunspot activity.

Last week's Propagation Forecast Bulletin ARLP039 mentioned an
announcement coming on Tuesday this week from NASA.  It concerned
data from the Ulysses spacecraft indicating solar wind pressure is
declining.  The speed of solar wind hasn't changed much, but the
density and temperature are lower.  The Ulysses craft orbits the Sun
over a six year period, during which it flies over both the Sun's
south and north poles.  The average solar wind pressure measured
from February 1992 to February 1998 declined over the following
decade, and the pressure during the February 2002 to February 2008
pass was about 20 percent lower.

In addition, the solar magnetic field dropped more than 30 percent
over the same period.  Because the data has been collected over such
a short time, there isn't much historical context for these

What does this indicate for radio propagation?  Unknown, but perhaps
this shows overall slowing of solar activity.

Last week's bulletin said the predicted planetary A index for
September 30 through October 2 was 8, 30 and 8.  This prediction has
moderated somewhat, and currently the expected values are 8, 20 and

Geophysical Institute Prague predicts quiet conditions September
26-29, unsettled September 30, active October 1, and unsettled
conditions on October 2.

Scott Bidstrup, WA7UZO who lives in Costa Rica, sent an interesting
article about noctilucent clouds as a possible medium for UHF
propagation.  According to the article in Science Daily (see, the
clouds contain ice coated with sodium and iron from micro-meteors,
and sit at about 53 miles (85 km) altitude, mostly between 50-70
degrees latitude, and sometimes as far south (or north, in the
southern hemisphere) as 40 degrees latitude or less.

The clouds are highly reflective of radar signals, and instead of
diffraction as we see in ionospheric propagation, ripples in the
clouds seem to reflect in unison, reinforcing each other.

Noctilucent clouds are sometimes visible at night, because their
altitude is so high that they reflect sunlight into areas of
darkness.  They are also known as polar mesospheric clouds, and
appear most often at twilight during the summer.

Go to to see
images of noctilucent clouds.

John Becker, K9MM of Prospect Heights, Illinois wrote about some
unusual 30 meter propagation.  On September 23 at 2006z on 30 meters
he worked H40MY in Solomon Islands with solid copy.  John used an
inverted vee, and thinks perhaps propagation was via long path,
because the short path was entirely in daylight.  Short path
propagation would be best between 0600-1400z.

Cesar, PY2YP sent in a reference to his web site, which describes an
interesting method of propagation prediction based on cracking large
pileups.  Instead of just calculating the signal strength over time
between you and your target, it also factors in signal strength to
the target station from various pileup generators, or sources of
QRM.  It determines when you are likely to be louder toward the
DXpedition than other regions.  Check it out on Cesar's web site at, and click on "QSO Window Tutorial" on the
upper left.

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, see the ARRL
Technical Information Service web page at,  For a detailed
explanation of the numbers used in this bulletin see,  An archive of past
propagation bulletins is at

Monthly propagation charts between four USA regions and twelve
overseas locations are at

Instructions for starting or ending email distribution of this
bulletin are at

Sunspot numbers for September 18 through 24 were 0, 0, 0, 0, 18, 16,
and 0 with a mean of 4.9.  10.7 cm flux was 67.2, 67.9, 67.8, 67.9,
69.1, 69.4, and 68.4 with a mean of 68.2.  Estimated planetary A
indices were 9, 5, 3, 2, 4, 3 and 4 with a mean of 4.3.  Estimated
mid-latitude A indices were 5, 3, 1, 1, 2, 2 and 1 with a mean of


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