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ARRL Propagation Bulletin ARLP018 (2003)

ARLP018 Propagation de K7RA

Propagation Forecast Bulletin 18  ARLP018
From Tad Cook, K7RA
Seattle, WA  May 2, 2003
To all radio amateurs

ARLP018 Propagation de K7RA

Even as this solar cycle slowly declines, there will still be periods of
rising activity, and this week was one of those times.  As this bulletin
being written on Thursday, a large sunspot, number 349 is aimed squarely
shows the spot, centered in the solar disk such that earth has maximum
exposure.  Earth is currently in a solar wind stream, as it has been for
weeks, and the appearance of sunspot 349 as well as other new spots
resulted in a rising sunspot number.  After periods in the single digits,
the daily sunspot number reached 224 on April 29.  It hasn't been this
since March 9.

Average daily sunspot number for this week rose nearly 85 points to 185.1.
Average daily solar flux was up nearly 29 points.  Average daily sunspot
number from this week in 2002 was 152.1, 143.6 in 2001, 144.4 in 2000, 74
in 1999 and 39.7 in 1998.  Just sampling the numbers from Propagation
Bulletin number 18 from 1998-2003 makes it appear that we are currently
enjoying the peak of the cycle.  Averaging the sunspot numbers for the
month of April may give us a better feel for our current place in the
cycle.  Average April sunspot numbers for 1998-2003 are 73.6, 92.9, 193.4,
163.6, 194.9 and 114.3.  Still, what matters most to HF operators is the
sunspots over the recent few days, so enjoy conditions when and if
geomagnetic conditions stabilize to a K index of three or less.

The past week saw active geomagnetic conditions, and the activity only
declined to an unsettled level on April 26-27 when the planetary A index
was 15.  The most active days were April 30 to May 1, with a planetary A
index of 40 and planetary K indices as high as six over both days.  This
indicates a strong geomagnetic storm.

Over the next week geomagnetic activity should settle a bit, providing
improved HF conditions.  The predicted planetary A index for Friday
Monday, May 2-5 is 25, 15, 10 and 15.  Predicted solar flux for those same
days is 145, 140, 135 and 130.

MUFs over most paths should be somewhat lower during May than in April,
with May being further from the spring equinox, 10, 12 and 15-meter
openings should be shorter as well.  To compare, run W6ELprop (which you
can download for free from twice, so you can
ALT-TAB to switch between them.  Try different paths using the same
number, but one program for April 15 and the other for May 15.  Switching
back and forth highlights the differences, and makes it easier to compare.
For instance, a path from Salt Lake City to Costa Rica with a sunspot
number of 111 shows MUFs (Maximum Usable Frequencies) between 1830-2230z
24.4-25.9 MHz for April 15, but 20.4-20.6 MHz for May 15.  This means that
given the same sunspot count, most of the time 17, 15 and 12 meters would
be open on April 15, and only 17 meters would be open on that path for May

For more information about propagation and an explanation of the numbers
used in this bulletin see the Propagation page on the ARRL Web site at .

Sunspot numbers for April 24 through 30 were 171, 173, 193, 200, 175, 224,
and 160, with a mean of 185.1. 10.7 cm flux was 128.3, 143.6, 143.7,
152.2, 155.1, and 153.5 with a mean of 147.2.  Estimated planetary A
indices were 24, 32, 15, 15, 20, 20, and 40, with a mean of 23.7.


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