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ARRL Propagation Bulletin ARLP018 (2001)

ARLP018 Propagation de K7VVV

QST de W1AW  
Propagation Forecast Bulletin 18  ARLP018
From Tad Cook, K7VVV
Seattle, WA  April 27, 2001
To all radio amateurs 

ARLP018 Propagation de K7VVV

After bottoming out on April 16 at 123.4, solar flux is on the rise.
Average flux values for this week were up over 50 points, and
activity is still increasing with the return of sunspot region 9393.
Solar flux is expected to peak this weekend around 210, but a rise
in geomagnetic activity due to a large M7-class solar flare on
Thursday is expected on Sunday. This flare was from sunspot 9393,
which also hurled a full halo coronal mass ejection. This sunspot
covers about half the area that it did when it was on our side of
the sun last month, but it is still quite large. Last month it
produced the largest solar flare ever recorded, which fortunately
was not pointed toward earth.

On Monday an enormous prominence (a filament of cool dense gas
suspended above the sun) extended over the sun's southwestern limb.
You may still be able to view a photo from the Solar and
Heliospheric Observatory at

N4CD took a trip to Costa Rica from April 4-17, and reported that he
had great HF propagation, even as higher latitudes were disrupted.
He worked about 9,300 stations on his trip. Probably the worst days
during that period were April 8 and 11-13, when geomagnetic storms
raged. But N4CD worked many stations on 10 meters who said that his
was the only signal they heard. These were probably stations working
him on a north-south path. This isn't because north-south
propagation is enhanced during geomagnetic storms, but because when
propagation is disrupted these are often the only paths that work
for high frequencies. He said his best DX for QRP was working WD3P,
who was only running 500 milliwatts into a dipole.

A number of emails have arrived lately asking about some of the
numbers reported in this bulletin, so it is probably time to run the
following explanation.

Amateur Radio operators who use HF generally like increased sunspots
because they correlate with better worldwide radio propagation.
When there are more sunspots, the sun puts out radiation that
charges particles in the earth's ionosphere. Radio waves bounce off
of (refract from) these charged particles, and the denser these
clouds of ions, the better the HF propagation.

When the ionosphere is denser, higher frequencies will refract off
it rather than passing through to outer space. This is why every 11
years or so when this activity is higher, 10 meters gets exciting.
10 meters is at a high enough frequency, right near the top of the
HF spectrum, that radio waves propagate very efficiently when the
sunspot count is high. Because of the shorter wavelength, smaller
antennas are very efficient on this band, so mobile stations running
low power on 10 meters can communicate world wide on a daily basis
when the sunspot cycle is at its peak. There are also seasonal
variations, and 10 meters tends to be best near the spring or fall
equinox. If the ionosphere is not so dense, the Maximum Usable
Frequency may be below 10 meters, and perhaps only signals with
frequencies as high as 15 meters or below will propagate.

The sunspot numbers used in this bulletin are calculated by counting
the spots on the visible solar surface and also measuring their

Solar flux is another value reported in this bulletin, and it is
measured at an observatory in Penticton, British Columbia using an
antenna pointed toward the sun hooked to a receiver tuned to 2.8
GHz, which is at a wavelength of 10.7 cm. Energy detected seems to
correlate somewhat with sunspots and with the density of the

Other solar activity of concern to HF operators are solar flares and
coronal holes, which emit protons. Since the charged ions in the
ionosphere are negative, a blast of protons from the sun can
neutralize the charge and make the ionosphere less refractive.
These waves of protons can be so intense that they may trigger an
event called a geomagnetic storm. In addition, energy from a solar
flare may energize the D-layer of the ionosphere, which absorbs
radio waves.

The Planetary A index relates to geomagnetic stability.
Magnetometers around the world are used to generate a number called
the Planetary K index.

A one-point change in the K index is quite significant. K index
readings below 3 generally mean good stable conditions, and above 3
can mean high absorption of radio waves. Each point change reflects
a big change in conditions.

Every 24 hours the K index is summarized in a number called the A
index. A one-point change in the A value is not very significant. A
full day with the K index at 3 will produce an A index of 15, K of 4
means A of 27, K of 5 means A of 48, and K of 6 means A of 80. You
can find an explanation of these numbers on the web at

The geomagnetic number reported here is the Planetary A index, which
is a worldwide average based on the K index readings from a number
of magnetometers. The numbers reported on WWV are the Boulder K and
A index, measured in Colorado. Generally the higher the latitude of
the measuring station, the higher the K and A indices reported.
This is because the effects of geomagnetic instability tend to
concentrate toward the polar regions of the globe. You can hear the
Boulder K index updated every three hours on WWV, or by calling

For an interesting web page on the earth's magnetosphere, check

Sunspot numbers for April 19 through 25 were 85, 103, 156, 164, 140,
175 and 182 with a mean of 143.6. 10.7 cm flux was 144.5, 180.4,
191.1, 192.5, 196.4, 193.5 and 193.9, with a mean of 184.6, and
estimated planetary A indices were 7, 8, 7, 28, 21, 8 and 7 with a
mean of 12.3.


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