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07 September, 2007
In little more than two weeks the northern hemisphere will see the Autumnal
Equinox, marking the start of Fall north of the equator and the beginning
of Spring south of the equator. The exact time when both northern and
southern hemispheres are bathed in equal sunlight is 0951z, September
23, 2007. Even with few sunspots, this is the
best time for long distance communications between hemispheres.
One change West Coast stations may notice is the summertime openings
in the evening toward Asia on 20 and 17 meters will be shortening, but
daytime propagation should improve. For instance, in mid-August from California
to Japan, after sunset on the West Coast (around 0300z), the path on 20
meters would open, with steadily increasing
signals until sundown in Japan (around 0930z). But at the start of Fall,
the path will close a few hours earlier, but with stronger signals during
the day.
From Texas to Brazil, with low sunspot numbers 15 meters in
mid-August is not usually a reliable path. But with one or two sunspots
at the equinox, 15 meters becomes very reliable from the middle of North
America to South America.
This year we began looking at a 3-month moving average of daily sunspot
numbers. We wanted to see if this might be a good balance between smoothing
out the daily variations to help spot trends, but three months being a
short enough period (compared to a 12 month moving average) that it better
reacts to changes.
Last month's Propagation Forecast Bulletin ARLP032
(http://www.arrl.org/w1aw/prop/2007-arlp032.html)
said if we saw an average sunspot number greater then 20 for August, this
would support a trend with the sunspot minimum centered in March 2007.
Unfortunately, the average was about half that, at 9.9. So that makes
the 3 month June-July-August average 15.4 centered on July, compared to
18.7 for the end of last month, centered on June.
Here is the latest update on the 3-month moving average of daily sunspot
numbers:
Dec 05 40.6
Jan 06 32.4
Feb 06 18.1
Mar 06 27.7
Apr 06 38.5
May 06 39.7
Jun 06 28.9
Jul 06 23.3
Aug 06 23.5
Sep 06 21.2
Oct 06 24.1
Nov 06 23.1
Dec 06 27.3
Jan 07 22.7
Feb 07 18.5
Mar 07 11.2
Apr 07 12.2
May 07 15.8
Jun 07 18.7
Jul 07 15.4
Monthly averages of daily sunspot numbers for April 2006 through August
2007 were 55.2, 39.6, 24.4, 22.6, 22.8, 25.2, 14.7, 31.5, 22.2, 28.2,
17.3, 9.8, 6.9, 19.8, 20.7 , 15.6 and 9.9.
Monthly averages of daily solar flux for the same period were 88.9, 80.9,
76.5, 75.8, 79, 77.8, 74.3, 86.3, 84.4, 83.5, 77.7, 72.2, 72.4, 74.4,
73.7, 71.6 and 69.2.
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 August 30 through September 5 were 15, 14, 26, 14,
15, 14 and 15 with a mean of 16.1. 10.7 cm flux was 71.6, 70.8, 70.8,
69.4, 68, 68.2, and 67.6 with a mean of 69.5. Estimated planetary A indices
were 6, 7, 11, 23, 12, 6 and 12 with a mean of 11. Estimated mid-latitude
A indices were 4, 7, 9, 17, 8, 4 and 10, with a mean of 8.4.
Source: The
American Radio Relay League
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