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ARRL Propagation Bulletin ARLP043 (2005)

ARLP043 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 43  ARLP043
From Tad Cook, K7RA
Seattle, WA  October 14, 2005
To all radio amateurs 

ARLP043 Propagation de K7RA

The fourth quarter of 2005 began nearly two weeks ago on October 1,
but last week's bulletin failed to mention the average sunspot
numbers for the quarter just ended. This is a normal quarterly
exercise, as we hope to spot trends in the solar cycle.

Looking at the past two weeks, the average daily sunspot numbers
over the past week were up over 10 points to 21.7. Average daily
solar flux rose, but also by a slight degree, only two and a half
points to 78.4.

From the first quarter of 2003 through the third quarter of 2005,
the average daily sunspot number was 120.3, 107.3, 110.2, 99.2,
72.9, 71.3, 69.3, 61, 46.1, 55.7 and 58.

The average daily solar flux for the same period was 134.3, 124.2,
120.8, 137.4, 111.1, 99.5, 111, 104.8, 96.4, 93.1 and 93.6.

We could almost convince ourselves that the cycle has already
bottomed out if we look at quarterly sunspot numbers this year.
Last year's quarterly sunspot numbers dropped steadily from 72.9 at
the first quarter of 2004, to 71.3, 69.3 and 61. 2005 began with a
big drop, to 46.1 in the first quarter, then rose to 55.7 and 58.
Could this cycle really have hit bottom during the first quarter of

The NOAA Space Environment Center Preliminary Report and Forecast
for October 4 (on the web in PDF format at, shows the same
smoothed sunspot number history and prediction table on page 10 that
it has shown for quite some time now. In this table, the sunspot
cycle appears to reach minimum around December 2006 to January 2007.

But could it have reached minimum two years earlier? Maybe we
should compare the quarterly numbers from the last sunspot cycle
bottom, which according to the graph above the page 10 table was in
1996. The quarterly average sunspot numbers from the fourth quarter
of 1995 through the third quarter of 1997 were 21.3, 13.1, 13, 12.4,
14.2, 11.3, 25.4 and 37.2.

We can see that the previous sunspot minimum was when the quarterly
average numbers were between 11 and 13. This included some long
periods of no sunspots at all, such as the five weeks in September
and October 1996 when every day the sunspot number was 0. The
average daily sunspot number from September 1 through October 31
1996 was only 2.6.

So far this year the quarterly averages were between 46 and 58, a
rather high bottom if it is one. We should probably just chalk this
up to the normal variations throughout the solar cycle, which never
looks like a smooth, predictable moving line unless data from many
days is averaged. I think we should be very surprised if the fourth
quarter of 2005 has an average daily sunspot number above 60.

For the upcoming week, solar flux and sunspot values should remain
about the same, which is low. The predicted planetary A index for
Friday through Monday, October 14-17 is 10, 12, 10 and 5. According
to Geophysical Institute Prague, October 18, 19 and 20 should be
quiet, October 17 quiet to unsettled, unsettled conditions on
October 14 and 16, and unsettled to active conditions this Saturday,
October 15.

Ever wonder why the planetary geomagnetic numbers are higher than
the mid-latitude numbers? It turns out the planetary numbers are all
measured at fairly high latitudes. Check the latitudes of the
observatories that supply the data for the planetary K index (which
is used to calculate the daily planetary A index) on the web at, The
average latitude for observatories in the Northern Hemisphere is 55

For comparison, note that the United States/Canadian border is at 49
degrees from Manitoba to points west, and 55 degrees latitude is
where Hudson Bay turns into James Bay at the northern edge of
Ontario. Since geomagnetic disturbances are generally higher at high
latitudes, no wonder the planetary geomagnetic indices read so high
when activity is up.

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 and an explanation
of the numbers used in this bulletin see the ARRL Technical
Information Service propagation page at, An archive of past
bulletins is found at,

Sunspot numbers for October 6 through October 12 were 28, 31, 24,
16, 11, 25 and 17 with a mean of 21.7. 10.7 cm flux was 79.5, 78.8,
78.1, 78.9, 79.1, 77.6, and 76.8, with a mean of 78.4. Estimated
planetary A indices were 4, 11, 22, 9, 10, 6 and 1 with a mean of 9.
Estimated mid-latitude A indices were 2, 11, 16, 7, 7, 5 and 1, with
a mean of 7.


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