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ARRL Propagation Bulletin ARLP030 (1999)

ARLP030 Propagation de K7VVV

QST de W1AW  
Propagation Forecast Bulletin 30  ARLP030
From Tad Cook, K7VVV
Seattle, WA  July 23, 1999
To all radio amateurs 

ARLP030 Propagation de K7VVV

Thanks to KH6BZF for writing a great bulletin last week while I was
out of town.

Solar flux and sunspot numbers were down again this week, with the
average sunspot numbers down nearly 60 points and the average solar
flux down over 10 points.  Conditions are improving though, with the
predicted solar flux rising over the next few days to 160, 165 and
170 for Friday through Sunday.  Planetary A indices are forecast at
10, 9 and 9 over the same days.  Beyond the weekend the solar flux
may rise as high as 180 over the next week, then fall to 165 by
August 6, 155 around August 9, and 140 around August 12.  The
current high K and A index values should settle down this weekend.

W7MIC sent email recently asking about some of the numbers used in
this bulletin.  Here is a very basic explanation.

Amateur Radio operators who use HF generally like increased sunspots
because it correlates with better worldwide radio propagation.  When
there are more sunspots, the sun puts out radiation which charges
particles in the ionosphere.  Radio waves bounce off of these
charged particles, and the more dense these clouds of ions the
better the HF propagation.  When the ionosphere is more dense,
higher frequencies will reflect off of the ionosphere rather than
passing through to space.  This is why every 11 years or so when
this activity is higher 10 meters gets exciting.  It 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 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.

The sunspot numbers used in this bulletin are calculated by counting
the sunspots on the visible solar surface and also measuring their
area.  The solar flux is measured at an observatory in British
Columbia using an antenna pointed toward the sun tuned to 2.8 GHz,
which is a wavelength of 10.7 cm.  Energy detected seems to
correlate with sunspots and with the density of the ionosphere.

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 reflective.
These waves of protons can be so intense that they may trigger an
event called a geomagnetic storm.

The Planetary A index relates to geomagnetic stability.
Magnetometers around the world are used to generate a number called
the Planetary K index.  You can hear the Boulder K index updated
every three hours on WWV, or by calling 303-497-3235.

A one point change in the K index is quite significant.  A K index
below three generally means good stable conditions, and above three
can mean high absorption and poor reflection of radio waves.  Each
point higher than three is a big change in conditions.

Every 24 hours the K index is summarized in something called the A
index.  A one point change in 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 number reported here is the Planetary A index, which is sort of
a worldwide average based on the K 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 get a lot more information by reading the chapter on
propagation in any recent edition of the ARRL Handbook.  Another
good source is a book titled The New Shortwave Propagation Handbook,
by George Jacobs, Ted Cohen and Robert Rose, published by CQ

Note that this week we have two sets of solar statistics, covering
the past two weeks.  Users of the WA4TTK solar graphing software
will want to create two files from this bulletin, each with just one
of the following paragraphs.  This will allow the software to
seamlessly grab the data and enter it into the database in a two
step process.

You can write to the author of this bulletin via

Sunspot numbers for July 8 through 14 were 143, 170, 174, 202, 199,
188 and 120 with a mean of 170.9.  10.7 cm flux was 149.1, 150.8,
156.3, 152.6, 154.1, 143.5 and 129.6, with a mean of 148, and
estimated planetary A indices were 8, 6, 5, 6, 14, 6 and 6, with a
mean of 7.3.

Sunspot numbers for July 15 through 21 were 109, 97, 97, 119, 126,
115 and 124 with a mean of 112.4.  10.7 cm flux was 129.5, 132,
136.7, 138.2, 141.3, 139.5 and 147.3, with a mean of 137.8, and
estimated planetary A indices were 10, 5, 5, 4, 4, 6 and 13, with a
mean of 6.7. 


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