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ARRL Propagation Bulletin ARLP019 (2007)

ARLP019 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 19  ARLP019
From Tad Cook, K7RA
Seattle, WA  May 4, 2007
To all radio amateurs 

ARLP019 Propagation de K7RA

The string of 0 sunspot days ended April 25, and relative to the
bottom of the solar cycle, sunspot numbers over the past few days
have been quite an improvement.  Average daily sunspot number jumped
from the previous week by 23 points to 25.1, and average solar flux
increased over 14 points to 84.8.

This modest increase in sunspot activity may noticeably improve
propagation compared to weeks with no sunspots.  It is our
understanding that popular propagation prediction programs are based
on a statistical model using predicted smoothed sunspot numbers.  It
is probably less accurate to vary a path prediction for a particular
date using daily sunspot numbers, but taking an average of several
days may produce more realistic results.

If we do path predictions from California to Japan for May 4 using
the average sunspot number for the past three available readings
(34), then compare to a prediction using 0 for the sunspot number,
we can get some idea of what the average improvement would be.

Calculating for 0 sunspots, W6ELprop shows us a 50-75% chance of a
17 meter opening from 2130-0600z.  Using 34 as a sunspot number, we
see a 75-100% chance of a 17 meter opening from 2000-0630z and for
the 50-75% chance, that opening extends later, to 0800z.

Similarly, for 20 meters with the sunspot number at 34, the W6 to JA
path shows a 25-50% chance of opening at 1230z, 50-75% at 1300z,
75-100% from 1330-1500z, 50-75% at 1530-1600z and 25-50% at 1630z.

With no sunspots from 1230-1330z there is less than a 25% chance,
25-50% at 1400z, 50-75% at 1430z and less than a 25% chance from
1530-1630z.  Actual on the air results would vary, of course.

Let's look at the average daily sunspot numbers and solar flux for
April, compared to previous months, to spot any trends.

Monthly averages of daily sunspot numbers for April 2006 through
April 2007 were 55.2, 39.6, 24.4, 22.6, 22.8, 25.2, 14.7, 31.5,
22.2, 28.2, 17.3, 9.8 and 6.9.  Monthly averages of daily solar flux
for the same period were 88.9, 80.9, 76.5, 75.8, 79, 77.8, 74.3,
86.3, 84.4, 83.5, 77.7, 72.2 and 72.4.

We've also recently been looking at 3-month smoothed sunspot
numbers, so knowing the April values, we can combine them with
February and March to produce the smoothed sunspot number for March,
which turns out to be 11.2 .

Here are the 3-month smoothed sunspot numbers for the past 18

Oct 05 28 
Nov 05 36 
Dec 05 40.6 
Jan 06 32.4 
Feb 06 18.1 
Mar 06 27.7
Apr 06 38.5 
May 06 39.7 
Jun 06 28.9 
Jul 06 23.3 
Aug 06 23.5 
Sep 06 21.2 
Oct 06 24.1 
Nov 06 23.1 
Dec 06 27.3 
Jan 07 22.7 
Feb 07 18.5 
Mar 07 11.2

Both series of sunspot numbers clearly illustrate the decline of
this sunspot cycle.

Barry Roseman, W0LHK of Stillwell, Kansas sent us a tip to check for a variety of information for users of
the 60 meter band.

For the near term, the US Air Force Space Weather Operation predicts
a planetary A index of 5 for May 4-5, 8 for May 6-7 and 5 again for
May 8-18, so the expectation is for quiet geomagnetic conditions.
Based on the last period of higher geomagnetic activity, April
28-29, the 27.5 day solar rotation probably is what led to a
prediction of 25 for the planetary A index on May 25.

Geophysical Institute Prague predicts quiet geomagnetic conditions
for May 4, quiet to unsettled on May 5, unsettled May 6-7, and quiet
to unsettled again on May 8-9.

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 April 26 through May 2 were 17, 18, 20, 18, 38,
32 and 33 with a mean of 25.1. 10.7 cm flux was 80.5, 82.7 84.9,
84.8, 87, 86.3, and 87.4, with a mean of 84.8. Estimated planetary A
indices were 6, 16, 26, 23, 20, 8 and 3 with a mean of 14.6.
Estimated mid-latitude A indices were 4, 10, 20, 16, 13, 5 and 1,
with a mean of 9.9.


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