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ARRL Propagation Bulletin ARLP012 (2007)

ARLP012 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 12  ARLP012
From Tad Cook, K7RA
Seattle, WA  March 16, 2007
To all radio amateurs 

ARLP012 Propagation de K7RA

We just saw more 0-sunspot days, followed by the brief emergence of
small spots, then more days with no sunspots. The average daily
sunspot number for the past week was down nearly 70% from the
previous week, to 5.9.

A solar wind hit earth on Tuesday, March 13, and a rise in
geomagnetic indices was the result, with the planetary A index
rising to 26. The A index measured at Fairbanks, Alaska, the college
A index, rose to 50. The next period of higher geomagnetic activity
is predicted for March 27, then April 9 and April 23.

Last week's note about 11 being the smallest sunspot number greater
than 0 brought several puzzled responses, including one from Darrell
Bellerive, VA7TO of Grand Forks, British Columbia, who wondered if
there were other gaps in the range of possible sunspot numbers.

Barry Pfeil, K6RM of Mountain View, California wrote, "I thought the
sunspot number was LITERALLY the number of sunspots and could be
anything from 1 on up."

One might think so, but the number is actually somewhat subjective,
and factors in the number of groups of sunspots. The National
Geophysical Data Center has a link at, which explains the
method. The number of groups of spots is counted, and that number is
multiplied by 10, then added to the number of individual sunspots.
So if you saw 7 sunspots in 4 groups, the sunspot number would be
47. One spot would be 11, two spots in one group would be 12, etc.
By subjective, I mean that someone must look at the spots, decide
what constitutes a group, and count them.

Note that the Vernal Equinox is coming up in a few days. At 0007z on
March 21 (which is Tuesday, March 20 in North America) the sunlight
hitting the northern and southern hemispheres will be equal. All
over the world, the sun will rise in the morning, and then set 12
hours later. Sunrise in Seattle at 47.67 degrees north latitude will
be 1416z, sunset at 0216z. In Matto-Grosso Brazil at 15 degrees
south latitude, sunrise at 0943z, sunset at 2143z. In England at
52.2 degrees north latitude, 1 degree west longitude, sunrise at
0611z, sunset at 1811z. With an equal measure of sunlight striking
both hemispheres, the equinox is a good time for HF propagation,
with the possible exception that we have very low sunspot activity.

A correction from last week: Dave Green, VE3TLY was wrongly
identified as VE5TLY.

If you would like to make a comment or have a tip for our readers,
email the author at,

For more information concerning radio propagation, see the ARRL
Technical Information Service at, For a detailed
explanation of the numbers used in this bulletin see, An archive of past
propagation bulletins is at, Monthly
propagation charts between four USA regions and twelve overseas
locations are at,

Sunspot numbers for March 8 through 14 were 0, 0, 16, 14, 11, 0 and
0 with a mean of 5.9. 10.7 cm flux was 72.5, 71.6, 71.2, 71, 71.2,
71.4, and 70, with a mean of 71.3. Estimated planetary A indices
were 4, 2, 4, 8, 9, 26 and 8 with a mean of 8.7. Estimated
mid-latitude A indices were 3, 1, 2, 6, 7, 18 and 9, with a mean of


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