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ARRL Propagation Bulletin ARLP016 (2007)

ARLP016 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 16  ARLP016
From Tad Cook, K7RA
Seattle, WA  April 13, 2007
To all radio amateurs 

ARLP016 Propagation de K7RA

So far this month we've observed 9 days in a row with 0 sunspots,
and all of our reporting week for this bulletin (April 5 through 11)
falls within this string of inactive solar days. As noted in last
week's bulletin, comparing that week's average sunspot number (12.9)
to this week's (0) doesn't mean much when activity is so low at the
bottom of this sunspot cycle. Based on predicted smoothed sunspot
numbers, if this month and the next are truly the solar minimum, we
should probably see several weeks in a row with no sunspots.

Those hoping for more activity and a return to worldwide propagation
on 10 and 15 meters should be cheered by this, because recent
observation shows the rise of any solar cycle is faster than its
decline, and most importantly, the decline of the previous cycle.

Around the last solar minimum, between Cycles 22 and 23, Sun
watchers observed several long strings of days showing no sunspots.
For instance, from February 10 to March 5, 1997, the average sunspot
number was 2.1, and there were 20 days during that period with no
sunspots. Prior to that, another long stretch of inactivity occurred
in the Fall of 1996. The average daily sunspot number was 1.7 from
September 5 to November 8, when 57 of 65 days the Sun showed no
spots. The longest continuous stretch with no sunspots at all in
that period was 38 days, following September 12, when a single
sunspot was visible and ending October 21, when another sunspot
appeared. Both those days had an official sunspot number of 11, and
each time, the single sunspot was only visible for a single day.

This period was preceded and followed by three days of no sunspots,
and October 29 through November 8, 1996, also had zero sunspots,
followed by December 24 through January 3, 1997.

Remember that an average sunspot number of 1 or 2 doesn't correspond
to one or two sunspots. Because of the peculiar manner in which
sunspot numbers are derived, the minimum non-zero sunspot number on
any day is 11. This is because the number of sunspot groups is
multiplied by ten, and the resulting number is added to the total
number of visible spots. So two spots in one group is 12, three
spots is 13, but three spots in two groups yields a sunspot number
of 23.

To observe the rate that the end of Cycle 22 fell toward minimum and
the beginning of Cycle 23 began, it is useful to average sunspot
numbers over each quarter. The average sunspot numbers for the years
1993 and 1994 were 79 and 48. The averages for the four quarters of
1995 were 47.4, 25.3, 21 and 21.3. 1996 had 13.1, 13, 12.4 and 14.2.
For 1997, the quarterly averages were 11.3, 25.4, 37.2 and 48.2. The
quarterly averages of sunspot numbers for the next two years, 1998
and 1999 were 62.9, 80.4 , 111.8, 99.1, 97.2, 147.2, 137.9 and
163.1. You can see that the new cycle rose quickly, beginning in the
second quarter of 1997.

In practical terms, what is the difference between a couple of weeks
of zero sunspots, and three months of a sunspot number around 163?

As an example, every year Saad Mahaini, N5FF of Richardson, Texas
spends three weeks in Syria. Currently operating YK1BA, he will
return to Texas on April 28.

Plugging the numbers into a popular propagation prediction program,
this weekend when he talks to someone back home in the Dallas area,
he might find spotty openings on 20 meters around 2100-2230z and
perhaps 0400z. Less likely, 17 meters might be possible around
1700-2000z, and far less likely, 15 meters from 1500-2100z. He could
also expect strong 40-meter signals from 0000-0400z. These are
normative expectations. Your mileage may vary. Saad may experience
fantastic propagation. These are based on probability, and we can
always be surprised.

But if the sunspot number leading up to this weekend had been about
163, he could expect stronger 40 meter signals over the same period,
20 meter propagation from 2100-0700z with really strong signals from
0000-0430z, and a good 15 meter opening from 1300-2300z. He might
even find 10 meters open over a shorter time during the same period.
Or, the Sun could be covered with spots, and a huge solar flare --
more likely during periods of higher solar activity -- could wipe
out propagation in a radio blackout.

Coming soon, the prediction for the next period of unsettled
geomagnetic conditions is for around April 20, with an expected
planetary A index of 20. After that, a planetary A index of 25 is
predicted for April 28. This same forecast (from the U.S. Air Force,
via NOAA) shows solar flux of 70 until April 16, when it rises to
75. This is a small shift, but may signal the period during which we
could see another sunspot, April 16-27.

David Moore tipped us off to an interesting article from the
European Space Agency about the physics of solar wind and auroras.
The link for that article is at, .

Finally, one more report from the top of Cycle 19. John Hardin, W4NU
of Atlanta, Georgia was first licensed around the age of 16 as
KN4JAG in Atlanta in June 1959, and upgraded to General class a
month later. He writes: "The propagation was awesome. By 1960 I
upgraded from a Knight Ocean Hopper receiver and Globe Scout 680-A
with a 40 meter doublet to a Viking Valiant, NC-57B receiver, and a
3 el Hornet Tribander on my parents' roof. 10 meters was open after
midnight, 15 on into the morning and 20 meters all night. I could
break the DX pileups with ease and ended up with DXCC in the early
1960s. I will never forget how crowded the 10 meter phone band was."

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 April 5 through 11 were 0, 0, 0, 0, 0, 0 and 0
with a mean of 0. 10.7 cm flux was 70.9, 71, 71.2, 71.1, 69.9, 69.4,
and 69.1, with a mean of 70.4. Estimated planetary A indices were 4,
4, 3, 3, 9, 7 and 4 with a mean of 4.9. Estimated mid-latitude A
indices were 3, 2, 2, 2, 8, 6 and 3, with a mean of 3.7.


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