ARRL Propagation Bulletin ARLP031 (2007)

ARLP031 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 31  ARLP031
From Tad Cook, K7RA
Seattle, WA  July 27, 2007
To all radio amateurs 

ARLP031 Propagation de K7RA

Average daily sunspot numbers and solar flux are down this week.
Average daily sunspot number declined nearly 28 points to 1.7, and
the daily solar flux average was down nearly 7 points to 67.4.  When
the solar flux is less than 70, it often means no sunspots, and the
sunspot number has been zero for a week.  Predicted solar flux for
the next week is 70, which suggests a sunspot or two, with the
chance for sunspots increasing after August 2.

Currently the IMF (Interplanetary Magnetic Field) points south,
making our planet vulnerable to geomagnetic upset.  The effect from
solar wind should be mild, with the planetary A index for July 27-31
predicted at 15, 5, 5, 8 and 15.  Geophysical Institute Prague
predicts quiet to unsettled conditions for July 27, quiet for July
28, quiet to unsettled July 29, unsettled July 30, quiet to
unsettled July 31 and quiet again on August 1-2.

In response to our links to historical sunspot activity graphs at, Sid Sosin, W7SID of Bellevue, Washington commented, ''What
data was available on sunspot activity in 1761 and the remainder of
the 18th century, and the 19th century, for that matter?''

Galileo was an early observer and recorder of sunspots, and you can
see his wonderful daily sketches from the 18th century at
.  You can even watch animated versions of his drawings, showing the
daily progression of sunspots across the sun's earth side face at
the above-mentioned link, and at

Jim Henderson, KF7E of Queen Creek, Arizona asked what the lowest
recorded solar flux is.  We went over that a couple of months ago in
ARLP022, and the lowest value I've seen is right around 65.

Robert Wood, W5AJ of Midland, Texas sent a link to a picture of the
sun and asked about a dark patch near the northern pole.  I don't
know what it is, but I assume it might be a cooler area, because
sunspots are dark and they are cooler relative to nearby areas.  It
sure is a nice picture though.  See it at

Don Josephs, K5DEJ of Fredericksburg, Texas wrote to ask about the
relationship between solar flux, sunspot numbers and the A index,
and what they mean to propagation.

You can see from this table at that solar flux tends
to rise and fall with sunspot number.  The flux is 10.7 cm
wavelength (about 2.8 GHz) energy measured at a Canadian observatory
in British Columbia.  One advantage of measuring solar flux is that
the measurements are objective, while the sunspot number is somewhat
subjective.  But for our purposes, if we want to predict radio
signals on a certain day over a certain path, an average of recent
sunspot numbers is a better value to use than solar flux.

The A index is derived from eight daily readings from magnetometers.
We generally want to see those numbers low, especially if we are
trying to propagate radio signals over polar paths, because higher
geomagnetic activity signals greater absorption.  But watch over the
next few years as solar activity increases.  Greater solar activity
not only may signal more sunspots, but it also means more solar
flares and solar wind, which often affects earth's geomagnetic
field.  So increased sunspot activity is a kind of double-edged
sword.  We want more sunspots, but along with that can come greater
chances of geomagnetic storms.

A good way to visualize the effects of sunspot activity over a
particular path is to use a couple of free computer programs,
W6Elprop and VOAprop.  W6Elprop will give you likely signal levels
over a path between two locations, while VOAprop is great for
visualizing how signal coverage shifts through the day.  This link
will take you to past bulletins which discuss these programs, and
recommend ways to use them:

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 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

Sunspot numbers for July 19 through 25 were 12, 0, 0, 0, 0, 0 and 0
with a mean of 1.7. 10.7 cm flux was 68.3, 67.3, 66.4, 66.2, 67.2,
68, and 68.6, with a mean of 67.4. Estimated planetary A indices
were 3, 12, 12, 4, 3, 2 and 2 with a mean of 5.4. Estimated
mid-latitude A indices were 3, 9, 10, 3, 4, 2 and 2, with a mean of