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ARRL Propagation Bulletin ARLP036 (2003)

ARLP036 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 36  ARLP036
From Tad Cook, K7RA
Seattle, WA  September 5, 2003
To all radio amateurs 

ARLP036 Propagation de K7RA

Daily sunspot numbers were lower this week and solar flux was about
the same.  But the average daily planetary A index dropped by more
than half to 14.3, the lowest it has been since the reporting week
of July 3-9, reported in Propagation Forecast Bulletin 28 on July
11, 2003.  The best day this week, in terms of stable geomagnetic
conditions, was Sunday, August 31 when the planetary A index was
only 7, and the mid-latitude A index was 5.  The eight mid-latitude
K indices for that day (reported every three hours) were 1, 2, 1, 1,
2, 1, 2 and 1.  This is rather remarkable compared to recent

The forecast for the next few days is for unsettled to active
geomagnetic conditions, with the predicted planetary A index for
Friday through Monday, September 5-8, at 20, 12, 12 and 15.
Predicted solar flux for Friday and Saturday is 115 and 120, then
125 for Sunday through Friday, September 12.

On August 29 a solar wind passed earth around 1500 UTC, but this and
subsequent wind from a coronal hole did not cause major
disturbances.  Over last weekend were predictions of possible aurora
for September 1 or 2, but the interplanetary magnetic field pointed
north, which protects the earth from disturbances.  It is when the
field points south that we are vulnerable, and despite a continued
solar wind, any geomagnetic upset over the next few days is expected
to be minor because of the north pointing field.  Participants in
this weekend's All Asian DX Phone Contest or the North American CW
Sprint have nothing to fear.

August has ended, and statistics show the average daily sunspot
number dropped from 132.8 to 114.3 from July to August.  The average
daily solar flux declined by a smaller margin, from 127.7 to 122.1.

How does August compare with recent history, considering the solar
cycle is declining?  The average daily sunspot number for August is
identical to April's value, 114.3, and only February and March had
lower sunspot averages for 2003.  Average daily sunspot numbers by
month, from August 2002 through August 2003 were 191.0, 206.4,
153.9, 159.8, 144.8, 150.0, 87.9, 119.7, 114.3, 89.6, 118.4, 132.8
and 114.3.  Average daily solar flux by month, from August 2002
through August 2003 was 183.9, 175.8, 167.0, 168.7, 157.2, 144,
124.5, 133.5, 126.8, 116.6, 129.4, 127.7 and 122.1.  This looks like
a steady decline in numbers.  Especially dramatic are comparisons
between August 2002 and 2003.

Perhaps we are coming to the phase in the cycle following the peak
when geomagnetic disturbances subside.  This would be good for HF
operators.  Although we want plentiful sunspots, we also need stable
geomagnetic conditions, especially in latitudes toward the poles and
for propagation over polar paths.

Currently the prediction for the bottom of the sunspot cycle is for
September 2006 to May 2007.  The forecast for solar flux is about
the same.  Looking further out, a rough estimate shows the flux and
sunspot values on the other side of the minimum for December 2007 at
about the same level as December of next year, 2004.  Between those
dates are very low levels of activity, a good time to explore 160,
80, 60 and 40 meters.

The cycle prediction is from the August 12 edition of the NOAA Space
Environment Center's Weekly Preliminary Report and Forecast of Solar
Geophysical Data.  It is posted weekly at, and the guide to the
Report and Forecast is at

Sunspot numbers for August 28 through September 3 were 146, 132,
120, 101, 59, 90, and 74, with a mean of 103.1.  10.7 cm flux was
118.7, 116.3, 114, 109.7, 108.1, 105.7, and 110.5, with a mean of
118.9.  Estimated planetary A indices were 18, 15, 17, 7, 14, 12,
and 17, with a mean of 14.3.


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