QST de W1AW
Propagation Forecast Bulletin 32 ARLP032
>From Tad Cook, K7RA
Seattle, WA July 29, 2005
To all radio amateurs
SB PROP ARL ARLP032
ARLP032 Propagation de K7RA
We're coming to the end of a month of large contrasts in terms of
solar activity. Around July 4 there was a huge increase in sunspot
numbers, followed by a very quiet period in which no sunspots were
seen. Although five days following mid-July were devoid of any
sunspots, our sun rotates relative to Earth, so in a little less
than four weeks that very active area of sunspots is back again in
the same position.
The popular figure of 27.5 days is regarded as the period of the
sun's rotation relative to our planet, but the actual figure varies
according to which latitude of the sun we are observing. If the
Earth was stationary and our observations of the sun were from a
fixed point, the rotation near the sun's equator would be about 25.6
days, and 30.9 days at 60 degrees latitude. We are most concerned
with sunspots near the sun's equator, because they are in the most
geoeffective or Earth-affecting position compared to those at higher
latitudes. More about the sun's rotation is found about 2/3 down
the page at
Watch for sunspot and solar flux numbers to rise over the next few
days, peaking from August 1-6 as last month's spotted region returns
to view. Geomagnetic indices should drop and stay low. Predicted
planetary A index for July 29 through August 2 is 15, 12, 10, 5 and
5. Over the long term, the general direction of the cycle is down,
although variations such as the increased activity around July 4 are
occasionally observed. Looking at a smoothed prediction for sunspot
numbers over the next few years at
www.sec.noaa.gov/weekly/pdf/prf1557.pdf, sunspots should
continue to decline overall, and are predicted to rise again to the
current level around December 2007.
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,
www.arrl.org/tis/info/propagation.html. An archive of past
bulletins is found at, www.arrl.org/w1aw/prop/.
Sunspot numbers for July 21 through 27 were 0, 0, 20, 18, 23, 29 and
19 with a mean of 15.6. 10.7 cm flux was 72.8, 73.6, 80.1, 80.2,
83.9, 86.5 and 90.6, with a mean of 81.1. Estimated planetary A
indices were 29, 13, 5, 5, 6, 6 and 17 with a mean of 11.6.
Estimated mid-latitude A indices were 19, 8, 2, 3, 3, 4 and 15, with
a mean of 7.7.
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