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ARRL Propagation Bulletin ARLP003 (2005)

ARLP003 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 3  ARLP003
From Tad Cook, K7RA
Seattle, WA  January 21, 2005
To all radio amateurs 

ARLP003 Propagation de K7RA

This has been a very active week for solar flares and geomagnetic
storms. Average daily sunspot number rose over 57 points this week
to 89, and average daily solar flux was up nearly 43 points to
132.7, when compared to the previous week. The average daily
planetary A index more than doubled, from 17.9 to 36.6.

Huge sunspot 720 provided lots of excitement over the past week.
Clay Evans from Colorado wrote to say that on January 14 he was
driving to work near Boulder, and through the heavily filtered
clouds he could see a large dark spot on the sun. This was probably
sunspot 720.

Two large flares erupted from this sunspot on Saturday, January 15.
Over the next few days the geomagnetic numbers bumped up
dramatically, with a planetary A index for January 17-19 of 63, 72
and 62. The high latitude collage A index was 114, 136 and 106 for
those same three days.

On Monday, January 17 another big flare blasted in our direction.
It peaked around 0950z. As sunspot 720 moves off the center of the
visible solar disk, more flares erupted, including a huge X7-class
event on Thursday, January 20. The energy could sweep past earth
today (January 21). This flare was the largest of the past year and
triggered the largest radiation storm in the current 11 year solar

This radiation storm hasn't yet resulted in a geomagnetic storm here
as of the writing of this bulletin on Thursday night. Currently the
prediction is for a planetary A index of 25, 20 and 20 for January
21-23, but this could be higher depending on how direct the
radiation hits earth. With sunspot 720 moving beyond view, daily
solar flux values are expected to dip below 100 around January

If you would like to comment or have a tip, email the author at,

For more information concerning propagation and an explanation of
the numbers used in this bulletin see the ARRL Technical Information
Service propagation page at,

Sunspot numbers for January 13 through 19 were 77, 65, 100, 99, 107,
109 and 66 with a mean of 89. 10.7 cm flux was 115.6, 129.8, 144.9,
144.5, 137.5, 124.3 and 132.5, with a mean of 132.7. Estimated
planetary A indices were 13, 12, 22, 12, 63, 72 and 62 with a mean
of 36.6. Estimated mid-latitude A indices were 10, 11, 11, 10, 27,
35 and 31, with a mean of 19.3.


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