ARRL Propagation Bulletin ARLP015 (2008)

ARLP015 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 15  ARLP015
From Tad Cook, K7RA
Seattle, WA  April 4, 2008
To all radio amateurs 

ARLP015 Propagation de K7RA

The past few days have had very stable geomagnetic conditions.
After unsettled to active geomagnetic indicators on March 26-28,
conditions quieted down dramatically.  Combined with the nearly
two-week run of sunspots and the Spring season, this makes for good
HF conditions.

There were actually three sunspots, beginning on March 23 with the
first one and a sunspot number of 14.  March 24 and 25 each brought
one new spot, and the sunspot number rose to 35 and 52 on those
days.  Activity peaked between March 26-29, with daily sunspot
numbers of 63, 57, 63 and 50.

On Thursday April 3, the sunspot number was back to 14 again, as
one-by-one, the three spots drifted from view.

Today's sunspot number may be back to 0, and it may stay that way
until April 18-20.

Conditions were remarkably quiet on April 2-3, with many periods
having a K index of 0, no matter the latitude.  On April 2 near
Fairbanks, Alaska the College K index as measured by the local
magnetometer was 0 during all eight three-hour periods.  Of course,
this resulted in an A index for the day of 0.

You can go to, to
see the K index and A index for the past 30 days, as measured at
Fredericksburg, Virginia and Fairbanks, Alaska, and the Planetary
index, derived from a number of predominately higher latitude
magnetometers.  With eight K index measurements per day, times three
indexes, times 30 days, you will see over 700 readings.

This weekend, April 5-6, will probably see the return of solar wind
and associated geomagnetic activity.  The predicted Planetary A
index for April 4-13 is 10, 15, 15, 12, 12, 12, 12, 10, 5 and 5.
Geophysical Institute Prague predicts quiet to unsettled conditions
for April 4, active conditions for April 5, unsettled to active
April 6, unsettled April 7, unsettled to active April 8, and
unsettled April 9-10.

Because we now have all sunspot numbers for March, we can take
another look at our three-month moving average to try to spot the
sunspot minimum.

The latest number, 8.4 for February 2008, is the average for all the
daily sunspot numbers from January, February and March.  We simply
add up all the daily sunspot numbers (the sum is 763) and divide by
the number of days (91) to get an average of 8.3846, which we
approximate as 8.4.

May 06 39.7
Jun 06 28.9
Jul 06 23.3
Aug 06 23.5
Sep 06 21.2
Oct 06 24.1
Nov 06 23.1
Dec 06 27.3
Jan 07 22.7
Feb 07 18.5
Mar 07 11.2
Apr 07 12.2
May 07 15.8
Jun 07 18.7
Jul 07 15.4
Aug 07 10.2
Sep 07  5.4
Oct 07  3
Nov 07  6.9
Dec 07  8.1
Jan 08  8.5
Feb 08  8.4

Above it still looks like the sunspot minimum must have occurred in
October 2007.

For a less-smoothed look, here are the monthly averages:

Apr 2007  6.9
May 2007 19.8
Jun 2007 20.7
Jul 2007 15.6
Aug 2007  9.9
Sep 2007  4.8
Oct 2007  1.3
Nov 2007  2.9
Dec 2007 16.3
Jan 2008  5.1
Feb 2008  3.9
Mar 2008 15.9

The monthly averages above still point to October 2007 as the
sunspot minimum, but you can see the data is not as smoothed as the
three-month averages.

Michael Best, WD4DUG sent a link to a NASA article about the recent
three sunspots from the last solar cycle.  You can read the article

Bill Balzarini, KL7BB sent the link about
an ionospheric sounder in Alaska at the HAARP facility.

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 web page 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  

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Sunspot numbers for March 27 through April 2 were 57, 63, 50, 41,
45, 25, and 24 with a mean of 43.6.  10.7 cm flux was 84.8, 82.9,
82.6, 80.5, 79.2, 77.8, and 75.9 with a mean of 80.5.  Estimated
planetary A indices were 31, 21, 8, 8, 4, 4 and 1 with a mean of 11.
Estimated mid-latitude A indices were 19, 13, 6, 6, 2, 5 and 1, with
a mean of 7.4.