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ARRL Propagation Bulletin ARLP012 (2002)

ARLP012 Propagation de K7VVV

QST de W1AW  
Propagation Forecast Bulletin 12  ARLP012
From Tad Cook, K7VVV
Seattle, WA  March 22, 2002
To all radio amateurs 

ARLP012 Propagation de K7VVV

Average daily solar flux and sunspot numbers for the past week were
almost identical to the previous week. Average sunspot numbers were
less than two points lower, and average solar flux rose less than
one point. Geomagnetic activity was essentially the same, with the
average estimated planetary A index less than a point higher this

On March 19 the interplanetary magnetic field moved south, leaving
the earth vulnerable to solar wind. The planetary K index rose to 5
over two 3-hour periods, but during other periods through the day K
indices were very low. This resulted in a planetary A index for that
day of only 17, which indicates unsettled to active conditions. If
the K index were 5 through all eight 3-hour periods, the A index
would be around 50, indicating a geomagnetic storm. An A index of 17
for the day corresponds to an average K index of a little above 3.

The K index is a quasi-logarithmic index over three hours of
geomagnetic activity from a particular observatory. For instance, on
WWV at 18 minutes after the hour (you can also hear this report
anytime by calling 303-497-3235) you would hear the Boulder K index
from Colorado, although they have switched to a new reporting scale
that uses the mid-latitude K index.

The planetary K-index, or Kp, is a mean of the values taken from
thirteen observatories that are between 44 and 60 degrees northern
or southern latitude. The planetary A index, or Ap, is derived from
the average of the Kp values for the day. If the Kp average is 1,
this corresponds to an Ap for the day of 4. Kp of 2 equals Ap of 7,
Kp of 3 equals Ap of 15, Kp of 4 equals Ap of 27, Kp of 5 equals Ap
of 48, Kp of 6 equals Ap of 80, and the scale continues up to a Kp
of 9. The Boulder, or any other local A index is derived the same
way. For a thorough explanation of the scale see, and .

At 1330z on March 20 a coronal mass ejection that left the sun on
March 18 near sunspot 9866 passed earth. It triggered some
geomagnetic activity, but only for a short period when both the
planetary K index reached 4 and the mid-latitude K index reached 5.
The rest of the day had very low K index numbers.

N4KG sent a note about the equinox and the effect on propagation.
He writes ''To my mind, the major benefit of the equinox period is
improved polar path propagation on the high bands (17-10 meters)
from three weeks before to three weeks after the equinox.'' He said
that 9N7RB is coming in during North America's local morning and
evening hours on 10 meters, shortly after sunrise and after sunset,
wrapping around the poles from both sides.

K0ZN is wondering when the next solar minimum will be. According to
a recent NOAA Preliminary Report and Forecast of Geophysical Data,
their prediction shows it to be some time between September 2006 and
April 2007. Still a ways off, and right now we are enjoying the peak
of the current cycle. You can read these NOAA reports at, , and the one showing the latest
solar cycle projection is the March 5 issue. You can also see some
nice historic charts of this and previous solar cycles on WM7D's web
site at, , and .

KN9P and several others wrote in asking about the new unfamiliar
propagation reports on WWV. NOAA is using some new scales. An
article about this is at, and the scales
are explained at .

The latest forecast has unsettled conditions for Friday, with a
planetary A index around 15. Solar flux for the next week and a half
is expected to be a little lower than the past two weeks, between
165 and 170.

Sunspot numbers for March 14 through 20 were 162, 134, 124, 124,
136, 119 and 141 with a mean of 134.3. 10.7 cm flux was 180.7,
175.9, 184.6, 184.4, 178.1, 174.8 and 187.8, with a mean of 180.9,
and estimated planetary A indices were 4, 6, 5, 4, 12, 17 and 7 with
a mean of 7.9.


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