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ARRL Propagation Bulletin ARLP038 (2000)

ARLP038 Propagation de K7VVV

QST de W1AW  
Propagation Forecast Bulletin 38  ARLP038
From Tad Cook, K7VVV
Seattle, WA  September 22, 2000
To all radio amateurs 

ARLP038 Propagation de K7VVV

Last week's bulletin reported that the sun was almost spotless, with
one day's sunspot number the lowest since February of last year.
Now this week we can report the appearance of sunspot 9169, the
largest observed in nine years. The area of this spot is a dozen
times the area of the earth's surface, and what should make it
interesting for HF radio is that it is rotating toward the center of
the sun's earth-facing hemisphere, which aims its effects right
toward us. A large sunspot such as this can produce big solar
flares, but so far this one, although magnetically complex, seems

This week's big one measures 2,140 millionths of the visible solar
disk. The largest sunspot recorded in the twentieth century was in
1947, and it measured 6,132 millionths of the solar disk. You can
see more information about big sunspots at .

Solar flux has been much higher this week than predicted. Last
week's bulletin projected a solar flux around 170, but by Sunday it
was above 180 and the next day over 200.

The projected solar flux for the next five days, Friday through
Tuesday, is 230, 235, 235, 230 and 225. Planetary A index for those
same days is expected to be around 10, 10, 10, 15 and 12. So for
this weekend we not only have the autumnal equinox, which is a great
time for HF propagation, but solar flux and sunspot numbers are
increasing as well. This means good conditions, as long as
geomagnetic activity stays low.

Geomagnetic activity was anything but low on Sunday and Monday, when
the effects of several solar flares were felt. Planetary A index for
both days was 40 and 45, and K indices were as high as 7.  The
College A index, recorded in Alaska, was 59 on September 17, and the
College K index was 8 during one period. This was indicative of a
severe geomagnetic storm as well as an aurora at that time.

Over the next few weeks expect the higher frequencies, especially 10
meters, to improve as we change to fall conditions.

Last week's bulletin mentioned a chart showing solar cycle progress.
Check out another chart showing activity over the past few months on
Jan Alvestad's Solar Activity Report at ,
or the past year of Daily Effective Sunspot Numbers at the NW
Research Associates site at . For an explanation of
Effective Sunspot Numbers, which are based on ionospheric F-layer
observations rather than solar observations, see .

Sunspot numbers for September 14 through 20 were 109, 113, 148, 146,
154, 140 and 171 with a mean of 140.1. 10.7 cm flux was 150.8,
159.4, 174.6, 181.5, 203.8, 207.1 and 211.4, with a mean of 184.1,
and estimated planetary A indices were 6, 10, 21, 40, 45, 27 and 13
with a mean of 23.1.


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