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ARRL Propagation Bulletin ARLP037 (2021)

ARLP037 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 37  ARLP037
From Tad Cook, K7RA
Seattle, WA  September 10, 2021
To all radio amateurs 

ARLP037 Propagation de K7RA

Sunspot activity increased dramatically this week.

Sunspot numbers (when looking at only the activity during our
Thursday through Wednesday reporting week) peaked at 87 on
Wednesday, September 8 and the day before, solar flux peaked at

Average daily sunspot numbers rose 14, to 64.6, while average daily
solar flux increased from 88 to 92.9.  New sunspots appeared on
September 2, again on September 3, and three more new sunspot groups
arrived on September 4.  Another new one appeared on September 8,
and on that day the total sunspot area was 1000 micro-hemispheres.

On September 9 I was shocked to see the daily sunspot number at 124
and total sunspot area hit 1030 micro-hemispheres.  I'm not certain,
but it looks like we have not seen activity like this in nearly six
years, when the daily sunspot number was 125 on September 29, 2015.

We saw similar large total sunspot area numbers last November 25 and
26, 1180 and 1020 micro-hemispheres.  Sunspot numbers were 40 and 43
on those days, but a few days later on November 29 the sunspot
number rose to 84.

Both the daily planetary and middle latitude A index reached a high
of 14 on September 8.  The averages were 7 and 7.7, down from 9.6
and 10.7 in last week's planetary and middle latitude readings.

Predicted solar flux seems quite promising, at 100 on September 10
and 11, 98 on September 12 and 13, 95 on September 14 to 17, 85 on
September 18, 88 on September 19 to 23, 90 on September 24 to 28, 88
on September 29 through October 1, 86 on October 2, 90 on October 3
to 6, 92 and 90 on October 7 and 8, and 85 on October 9 to 15.  Flux
values are expected to rise to 90 again after October 20.

Predicted planetary A index is 5, 8 and 8 on September 10 to 12, 5
on September 13 to 20, 8 on September 21, 5 on September 22 through
October 1, then 8 again on October 2 and 3, and 5 on October 4 to

On Sunday September 5, reported ''For most of the
past three years, the sun has been absolutely blank.  Today the sun
has six sunspot groups.  They're popping up all over the solar

''The sudden profusion of so many sunspots is a sign of strength for
young Solar Cycle 25.  The solar cycle is actually running ahead of
schedule.  NOAA and NASA predicted that it will peak in the year
2025.  Outbreaks like this one support the idea that Solar Max could
come a year early.''

On September 8 reported a shortwave blackout over
the Pacific Rim caused by a CME (Coronal Mass Ejection) on September
8 at 1736 UTC.

Here is Tamitha Skov's recent forecast, although by now it is a bit
out of date:

An interesting article about recent solar activity, but it is
plagued with many popups:

Recently in this bulletin we mentioned the US Postal Service issuing
stamps with solar images.  Here is an article from June which gives
much more detail on the creation of the stamps:

For more information concerning shortwave 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 2 through 8, 2021 were 33, 33, 68, 66,
80, 85, and 87, with a mean of 64.6.  10.7 cm flux was 85.8, 83.8,
86.5, 93.3, 99.5, 101.2, and 100.4, with a mean of 92.9.  Estimated
planetary A indices were 4, 6, 5, 6, 6, 8, and 14, with a mean of 7.
Middle latitude A index was 3, 6, 5, 8, 8, 10, and 14, with a mean
of 7.7.


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