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ARRL Propagation Bulletin ARLP041 (2009)

ARLP041 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 41  ARLP041
From Tad Cook, K7RA
Seattle, WA  October 9, 2009
To all radio amateurs 

ARLP041 Propagation de K7RA

Sweet memories of that 11-day run of sunspots ending on the second
day of this month will eventually fade. So far the number of
spotless days is seven--or perhaps eight by later today--with the
last sunspot seen on October 1.

A peek at for the latest STEREO images
shows two active areas in our Sun's southern hemisphere beyond our
direct view, but it is hard to tell if these will emerge as sunspots
when they appear over the eastern limb.  A rough guess has the first
one possibly emerging around October 10, and the second around a
week later.  Currently the solar flux forecast shows flux values
lower then 70 through October 15, then around 72 during the two-week
period from October 16-30.

The second spot is seen on the look-ahead image, and is passing into
the invisible area, now slightly more than 60 degrees wide in
latitude, or about one-sixth of the solar surface.

The same NOAA/USAF forecast predicts the same very quiet geomagnetic
conditions we've seen for some time, with a planetary A index around
5.  Two very small increases are predicted for October 11-12, with
an A index of 7, and October 24-25, with the A index at 8.

Average sunspot numbers for the week dropped 15.3 points below the
September 24-30 period, and average daily solar flux for the week
declined 2.2 points to 70.5.

Regarding the invisible area on the other side of the Sun, this is
becoming smaller as the "Ahead and Behind" STEREO platforms slowly
converge.  You can figure out the size of the dark area in terms of
degrees by going to the "Where is STEREO Today?" page at  The last figure at
the bottom of the page is "Separation Angle A with B."  Subtract
this value from 180 to get the size of the dark area.  Currently
early Friday morning it shows 119.702, which corresponds with a dark
area (beyond the sight of the A and B platforms) of 60.298 degrees.

At you can see how wide
that invisible area will be at any date and time in the future.  On
October 1 at 1800z the blank area was 61.7 degrees wide, on November
1 it will be 56.3 degrees, and December 1, 51.6 degrees.  The
non-visible area will be exactly 60 degrees wide around 0021-0028z
on October 11.

In 2010 that invisible area will shrink from 47.7 degrees on January
1 to 9.3 degrees on December 1.  The entire Sun will be visible
beginning on February 6, 2011 around 1302-2047z.  After that, the
blank spot in STEREO's vision will be Earth-facing, and as it grows
the information can be filled in from observation here on Earth.

Larry Banks, W1DYJ of Woburn, Massachusetts sent in an article from
New Scientist magazine titled, "Phantom Storms: How our weather
leaks into space."  It has many interesting details about our
ionosphere and stratospheric warming.  Read it on the web at,

Giles Berry, KE3CR of New Castle, Delaware commented about last
week's reports of VHF propagation along fog banks.  He recalls that
in the early 1950s when television first came to the state of Maine,
TV reception seemed to improve with fog.  Giles notes that on a
foggy night he could receive television broadcasts from Boston
reasonably well. He was on the mid coast of Maine.

Patrick Hamel, W5THT of Long Beach, Mississippi notes, "Here on the
Gulf of Mexico coast we have always known that when we get fog, we
can talk from Florida to Texas 'under the inversion' on 2 meters.
It also works on 6 meters, and I regularly heard other stations
making contact on 2 meters and then jumping to the higher bands
during fog conditions."

Gus Malmberg, SM0EGK says, "When I read WA2AMW's comments about the
extraordinary conditions on 2m, I recalled what happened early in
1963, when one could listen to all of northern Europe and see many
band I TV stations for a few days. As a youngster I didn't
understand the mechanism, but have since realized that it must have
been an extreme temperature inversion. In 1963 we had a solar

He continues, "I worked for fifteen years as a radio and television
transmitter engineer for the Swedish Telecommunication
Administration, but have never again experienced something like

Alan Vigeant, KI6HPO of San Marcos, California wrote, "I'd like to
inform you of past and most recent conditions here in NE San Diego
County. Since early April, I've been having daily chats with my good
friends at the Santa Barbara Amateur Radio Club, which is situated
about 165 miles NW of my QTH."

Alan continues, "I am at 2,200 feet of altitude, on the SW rim of
what I like to call the Palomar Ridge, which is about 5,600 feet
above sea level.  Between the two rims is what I call the 'Palomar
Trench.' My QTH is about 12 miles SW of the Palomar Observatory.
Using William Hepburn's 'Worldwide Tropospheric Ducting Forecasts
web site (see, which has
proven most helpful to me, I can pretty much set my watch as to when
the duct will be forming. My QTH is about 40 miles east of
Oceanside, California. My signal follows the ducting which occurs in
the trench and then goes up the coast of Southern California to
Santa Cruz Island, where their 220 MHz repeater is."

He goes on to say. "The past week proved to be a most curious one
for me. While I was speaking to the 7:00 AM SBARC Morning Net, I
noticed that the S-meter on my 220 rig (I utilize the club's 223.920
repeater) began to fluctuate rapidly."

He ends with, "Looking out my garage side door, I could watch the
cloud cover come in from the Pacific; as I looked on, I could
actually see that as the cloud cover rose, the meter would decline
to an S-3. As the Sun grew higher in the sky and the cloud bank
began to recede into the Pacific, the meter would register an S-9 to
almost 40 over. At times, during the summer evenings, I could look
out along the Palomar trench and look to the horizon to see if prop
was going to be good or not so good for the evening."

Thanks, Alan!  Very interesting report.

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 October 1 through 7 were 11, 0, 0, 0, 0, 0, and
0 with a mean of 1.6.  10.7 cm flux was 72, 71.6, 71.6, 71.1, 69.9,
68.9, and 68.5 with a mean of 70.5.  Estimated planetary A indices
were 2, 2, 1, 4, 2, 1 and 2 with a mean of 2.  Estimated
mid-latitude A indices were 2, 0, 1, 4, 2, 1 and 1 with a mean of


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