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ARRL Propagation Bulletin ARLP038 (2020)

ARLP038 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 38  ARLP038
From Tad Cook, K7RA
Seattle, WA  September 18, 2020
To all radio amateurs 

ARLP038 Propagation de K7RA

Lots of news lately about the solar cycle, but no sunspots. Today,
September 18 is the 29th consecutive day with zero sunspots.

News was about the cycle minimum being announced recently as
occurring during December 2019, which indicates the beginning of
Solar Cycle 25. The reason for the delay in the announcement is the
nature of moving averages, which in this case is a smoothed sunspot
number, derived from arithmetic averaging of sunspot numbers over a
whole year. That is, half the numbers before December and half after
to derive a mid-point average.

Here is the announcement:

Recent news stories give predictions for the next solar cycle, such
as this one from SpaceRef:

The recent reporting week (September 10-16) gave us an average daily
solar flux of 69.2, no significant difference from the previous
week, which was 69.7. Average daily planetary A index was 5.3, up
only a little from 4.4 the previous week. Average daily mid-latitude
A index went from 4.9 to 5.4.

Predicted solar flux for the next 45 days (September 18 til November
1) is the same as reported in past recent bulletins, 70 on every

Predicted planetary A index is 5 on September 18-22, then 8, 10, 15,
10, 25, 15 and 10 on September 23-29, 5 on September 30 til October
14, then 8 on October 15-16, 5 on October 17-19, then as in the
earlier period, 8, 10, 15, 10, 25, 15 and 10 on October 20-26, then
back to 5 on October 27 til November 1.

Geomagnetic activity forecast for the period September 18 to October
13, 2020 from F.K. Janda, OK1HH.
"Geomagnetic field will be
quiet on: September 21-22, October 6-8
quiet to unsettled on: September 18, October 3-5, 9-10, 13
quiet to active on: September 19-20, 23-24, 29-30, October 1-2
unsettled to active: (September 25-28, October 11-12)
active to disturbed: not expected
"Solar wind will intensify on: September (19,) 22-23, 27-29, October
(3-4,) 11-13.

"- Parenthesis means lower probability of activity enhancement.
- The predictability of changes remains lower because there are few
unambiguous indications."

Steven Rudnick, W1KYB of Santa Fe, New Mexico asked, "How did they
come to the conclusion that we are in a new cycle when the sunspot
number has been 0 for 27 days now?"

They probably looked at a couple of factors. One is the polarity of
sunspots. As the Sun transitions from one cycle to the next, the
polarity of sunspots changes.  It starts with a few sunspots having
opposite polarity from sunspots in the recent cycle, then gradually
it shifts so eventually the majority have the new cycle polarity.

The other factor is the smoothed sunspot number, which is an average
based on an entire year of sunspot numbers. This reduces the noise
in the numbers, so it is easier to see the trends.

The smoothed sunspot number for December 2019 would be based on the
monthly sunspot numbers for approximately July 2019 through July

Data is here:

If you look at the monthly averages, they jump all over the place.

The monthly averages for September 2019 through March 2020 were 1.1,
0.40, 0.50, 1.5, 6.2, 0.20 and 1.5.

But the smoothed numbers for those same months were 3.1, 2.6, 2,
1.8, 2.2, 2.8, and currently undetermined for March, placing the
lowest number in December.

The current dearth of sunspots over the past month may just look
like noise in retrospect some time in the future. Or maybe not,
should it continue. I hope it does not persist.

From the NOAA Space Weather Prediction Center, predicted solar flux
and sunspot numbers through December 2040:

NASA on Solar Cycle 25:

The latest from Dr. Tamitha Skov, WX6SWW:

Love her enthusiasm!

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 and the ARRL Technical Information
Service at For an
explanation of numbers used in this bulletin, see

An archive of past propagation bulletins is at More good
information and tutorials on propagation are at

Instructions for starting or ending email distribution of ARRL
bulletins are at

Sunspot numbers for September 10 through 16, 2020 were 0, 0, 0, 0,
0, 0, and 0, with a mean of 0. 10.7 cm flux was 69.3, 68.7, 69.3,
69.8, 68.9, 68.8, and 69.5, with a mean of 69.2. Estimated planetary
A indices were 2, 3, 5, 6, 11, 7, and 3, with a mean of 5.3. Middle
latitude A index was 2, 2, 6, 7, 10, 8, and 3 with a mean of 5.4.


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