ARRL

ARRL Propagation Bulletin ARLP005 (2007)

SB PROP @ ARL $ARLP005
ARLP005 Propagation de K7RA

ZCZC AP05
QST de W1AW  
Propagation Forecast Bulletin 5  ARLP005
From Tad Cook, K7RA
Seattle, WA  February 2, 2007
To all radio amateurs 

SB PROP ARL ARLP005
ARLP005 Propagation de K7RA

Might we see a high-bottom minimum at the end of this solar cycle?
January had a higher monthly average sunspot number than nine of the
previous twelve months. Looking at predicted smoothed sunspot
numbers for 2007, they don't really go any lower this year than what
is predicted for this month and the next.

The monthly averages of daily sunspot numbers for all of 2006 and
January 2007 were 26.7, 5.3, 21.3, 55.2, 39.6, 24.4, 22.6, 22.8,
25.2, 14.7, 31.5, 22.2 and 28.2. Average daily solar flux numbers
over the same months were 83.4, 76.5, 75.5, 88.9, 80.9, 76.5, 75.8,
79, 77.8, 74.3, 86.3, 84.4 and 83.5.

As you can see in the data above, there is a great deal of variation
from month-to-month. A year ago, in February 2006, the average daily
sunspot number for the month was only 5.3. That was the lowest value
for the past year. The month just ended had an average daily value
over five times the number for last February. In fact, January's
average at 28.2 would be very close to the value for last February
squared. But the solar minimum is expected this year, not last
February.

A table of predicted smoothed solar values from August 2006 through
December 2007 can be seen on the web at,
http://sec.noaa.gov/ftpdir/weekly/Predict.txt. These are Zurich
sunspot numbers, which tend to be lower than the values we record at
the end of these bulletins.

Why is August of last year's number predicted, instead of a known
value? Because it is a 13 month smoothed number. So it is based on
the actual sunspot numbers from February 2006 through January 2007,
and the predicted values for February 2007. Likewise, the smoothed
number for November 2006 is based on the actual sunspot numbers from
May 2006 through January 2007, averaged with the predicted numbers
for February through May 2007. The predicted smoothed sunspot number
for December 2007, the last value shown on that table, would be
based on the predicted monthly values for June 2007 through June
2008, all averaged together.

The predicted smoothed numbers from that table, for August 2006
through December 2007 are 15.4, 15.2, 14.0, 12.4, 11.5, 11.2, 11.0,
10.9, 11.0, 11.1, 11.3, 12.0, 11.2, 13.3, 15.6, 18.3, and 21.3. As
you can see, the lowest value is March 2007, at 10.9, and notice
that it rises rapidly at the end of this year.

What difference does higher sunspot activity make for HF
propagation? As an example, for the middle of February, if the
average sunspot number was 11, which is about the minimum value if
there are any sunspots at all, we can compare that with a weekly
average shown in February 2003 in this Propagation Forecast
Bulletin: http://www.arrl.org/w1aw/prop/2003-arlp007.html.

If we pick a couple of locations, for example, Ohio and Spain, we
can run some projections using a propagation prediction program to
make some comparisons. The sunspot number in that 2003 bulletin is
over 150.

With the minimum sunspot number of 11, a program such as W6ELprop
(see http://www.qsl.net/w6elprop/) shows a 20-meter path opening
around 1430z, the signal strength jumping higher around 2000z, and
the band probably dying out in another hour or so. But with the
higher numbers, 20-meters is open almost around the clock, with the
least likely period for propagation around 0430-1130z. On higher
frequencies, the differences are more dramatic. 15-meters with low
sunspot activity shows very little chance of opening, with a slight
possibility in the morning on the Ohio end of the path. But with the
higher solar activity, strong signals are predicted for 1230-2200z
on the 15-meter band. Wait a few more years, and we might be there
again.

Recently a strong solar wind caused geomagnetic numbers to jump high
on Monday, January 29 with the planetary A index rising to 36. A
good place to look for short-term predictions is at,
http://sec.noaa.gov/ftpmenu/forecasts/45DF.html. For the February 1
report, we see quiet geomagnetic indices for the next week, with
higher activity centered on February 13 and again on February 25-26.
This is based on activity during the current and previous solar
rotation.

If you would like to make a comment or have a tip for our readers,
email the author at, k7ra@arrl.net.

For more information concerning radio propagation, see the ARRL
Technical Information Service at,
http://www.arrl.org/tis/info/propagation.html. For a detailed
explanation of the numbers used in this bulletin see,
http://www.arrl.org/tis/info/k9la-prop.html. An archive of past
propagation bulletins is at, http://www.arrl.org/w1aw/prop/. Monthly
propagation charts between four USA regions and twelve overseas
locations are at, http://www.arrl.org/qst/propcharts/.

Sunspot numbers for January 25 through 31 were 11, 11, 11, 13, 27,
33 and 32 with a mean of 19.7. 10.7 cm flux was 79.9, 79.7, 80.5,
81.7, 86.7, 87.5, and 89.2, with a mean of 83.6. Estimated planetary
A indices were 1, 2, 3, 5, 36, 21 and 16 with a mean of 12.
Estimated mid-latitude A indices were 1, 3, 3, 2, 19, 17 and 13,
with a mean of 8.3.
NNNN
/EX