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ARRL Propagation Bulletin ARLP040 (2006)

ARLP040 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 40  ARLP040
From Tad Cook, K7RA
Seattle, WA  September 29, 2006
To all radio amateurs 

ARLP040 Propagation de K7RA

Average daily sunspot numbers were down again this week, but only by
less than one point, to 12.7. Average daily solar flux was down five
points from the previous week to 72. Geomagnetic indices were down,
also by a small amount.

The Air Force projects that today, September 29, geomagnetic
conditions will be quiet, with a planetary A index of 8, rising to
15, then 20 on Sunday, and dropping back to 8 on Monday, October 2.
Currently the interplanetary magnetic field is pointed south, which
signals that Earth is vulnerable to solar flares or solar wind from
coronal holes. Sunspots 913 and 914 are rotating into view, and
sunspot number and solar flux should rise slightly by Monday.

Geophysical Institute Prague projects unsettled conditions for
September 29, quiet conditions on September 30, unsettled to active
conditions on October 1, unsettled October 2, quiet to unsettled for
October 3, then quiet conditions on October 4-5.

Bill Huntimer, KI0CW in Dell Rapids, South Dakota wrote to ask,
"Would you explain what sunspots do to radio interference?"

Sunspots can be great for HF radio propagation. The more sunspots we
see, the greater the density of charged particles in the ionosphere,
which reflect or refract radio waves. Higher densities mean higher
frequencies can propagate worldwide, instead of just passing through
the ionosphere and off into space. When there are sunspots, there is
also the possibility of other solar activity, such as solar flares,
and holes in the Sun's corona, which can spew out a solar wind of
charged particles. These particles, as well as x-rays from flares,
may disturb the Earth's magnetic field, and cause charged particles
in the ionosphere to re-combine, canceling the charge. Lower density
of charged particles means that the Maximum Usable Frequency is
lower. In addition, polar cap absorption occurs, so propagation over
the poles becomes poor.

Sometimes in the media, we may hear of interference to broadcast TV
or FM radio signals related to sunspots. The interference can be
from distant stations, when listeners are accustomed to hearing only
local stations. The interference is actually due to improved

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

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

Sunspot numbers for September 21 through 27 were 11, 17, 13, 13, 11,
13 and 11 with a mean of 12.7. 10.7 cm flux was 71.4, 72.3, 70.4,
69.8, 70.2, 70.7, and 72, with a mean of 71. Estimated planetary A
indices were 4, 3, 9, 23, 6, 6 and 4 with a mean of 7.9. Estimated
mid-latitude A indices were 2, 1, 7, 15, 6, 5 and 2, with a mean of


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