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ARRL Propagation Bulletin ARLP018 (2005)

ARLP018 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 18  ARLP018
From Tad Cook, K7RA
Seattle, WA  May 6, 2005
To all radio amateurs 

ARLP018 Propagation de K7RA

NASA Science News carries a story titled "Solar Myth," about the
fact that there is still solar activity toward the bottom of the
sunspot cycle. The article gives examples of x-ray flares at the
bottom of recent cycles and shows that the sun is anything but
quiet. The peak and the bottom of the sunspot cycle are only really
known after the fact, when we can look back on a graph showing a
moving average over many months. This is when the graph appears
smooth, and you can clearly see tops and bottoms. Otherwise there is
a great deal of daily variation.

Read the article on the Science@NASA site at,

Over the past week sunspot numbers increased, recovering from the
days of no visible sunspots. Average daily sunspot numbers rose 35
points to 60.9, and average daily solar flux rose nearly 24 points
to 107.7

A new month began last weekend, and we can now examine the April
averages and compare them to previous months.

The monthly average of the daily sunspot number for November 2004
through April 2005 was 70.5, 34.7, 52, 45.4, 41 and 41.5. The
monthly average of the daily solar flux over the same months was
113.7, 95, 102.3, 97.2, 89.9 and 85.9. Whenever we examine monthly
or quarterly averages we see the decline of the solar cycle quite

Currently solar flux is expected to remain above 100 for the next
couple of days, then decline to below 90 after May 12. Geomagnetic
conditions should remain quiet this weekend, then become unsettled
to active May 9-11. The predicted planetary A index for May 6-13 is
10, 5, 10, 25, 20, 15, 12 and 8.

This week I received several messages asking "What do these numbers
mean?" In a very basic sense, HF operators would love to see lots of
sunspots (high sunspot numbers and probably high solar flux) without
any geomagnetic disturbance, which would be indicated with a low A
and K index. But definitely check out the advice toward the end of
each bulletin, in the next to last paragraph.

You can also use W6ELprop, free from to
figure your own propagation paths for various bands and times of day
to anywhere else in the world. Instead of solar flux, use an average
of several days sunspot numbers with W6ELprop. You can get those
recent daily numbers from . The daily sunspot
number is in the column marked "SESC Sunspot Number."

If you would like to comment or have a tip, 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 April 28 through May 4 were 71, 46, 53, 61, 55,
79 and 61 with a mean of 60.9. 10.7 cm flux was 98, 105, 106.4,
111.6, 112.2, 112.3 and 108.7, with a mean of 107.7. Estimated
planetary A indices were 4, 12, 21, 26, 7, 10 and 7 with a mean of
12.4. Estimated mid-latitude A indices were 1, 8, 13, 14, 6, 6 and
4, with a mean of 7.4.


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