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ARRL Propagation Bulletin ARLP035 (2021)

ARLP035 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 35  ARLP035
From Tad Cook, K7RA
Seattle, WA  August 27, 2021
To all radio amateurs 

ARLP035 Propagation de K7RA

Sun watchers saw no days without sunspots this week.  Average daily
sunspot numbers rose from 17.7 last week to 21.7 this reporting
week, August 19 through 25.

Average daily solar flux increased from 73.8 to 78.5.

Geomagnetic indicators were quiet, with average daily planetary A
index declining from 6.1 to 4.7, and average daily middle latitude A
index dropping from 7.6 to 5.7.

We are less than one month away from the Northern Hemisphere
autumnal equinox, on Wednesday, September 22, 2021.  Both
hemispheres will be bathed in equal measures of solar radiation,
always a positive for HF propagation.

Predicted solar flux is 90 on August 27 to September 2, 85 on
September 3, 73 on September 4 to 11, 74 on September 12 to 15, 76
on September 16 to 18, 77 on September 19 and 20, 76 on September
21, 75 on September 22 to 29 and 73 on September 30 through October

Predicted planetary A index is 16, 12, 10 and 8 on August 27 to 30,
5 on August 31 through September 1, 8 and 12 on September 2 and 3, 5
on September 4 to 10, then 10 and 8 on September 11 and 12, 5 on
September 13 to 18, 8 on September 19 and 20, 15 and 10 on September
21 and 22, 7 on September 23 and 24, 5 on September 25 to 28, then 8
and 12 on September 29 and 30 and 5 through the first week in

I find it encouraging that the above solar flux forecast from USAF
and NOAA was revised upward over the past few days, and that the
sunspot number (47) on Thursday, August 26 was much higher than the
average daily sunspot number (21.7) over the previous seven days.

Geomagnetic activity forecast for the period August 27 til September
21, 2021 from F. K. Janda, OK1HH.

"Geomagnetic field will be 
Quiet on September 1, 5, 10, 14 to 18
Quiet to unsettled August 28, 31, September 2 to 4, 6 to 9, 19 and 20 
Quiet to active on August 29 and 30, September 11 to 13, 21
Unsettled to active August (27)
Active to disturbed, Nothing predicted

- Parenthesis means lower probability of activity enhancement."

At 0839 UTC on August 26 the Australian Space Forecast Centre issued
a geomagnetic warning.  "A weak slow moving CME associated with a
disappearing solar filament on 23 August may have a glancing impact
at Earth late on the UT day of the 26th or into 27th August.  Mildly
elevated geomagnetic activity may occur following the impact.
Another weak slow moving CME observed occurring towards the middle
of the UT day on 24 August appears to be primarily eastward,
although there is a chance it may also have a glancing impact at
Earth during the 27th August, potentially resulting in mildly
elevated geomagnetic activity."

Darrell Emerson, AA7FV wrote:

"I have a question about a possible 17m propagation anomaly between
my location (AA7FV in Tucson AZ,  DM42pg, 32.3N 110.7 W) and the
NCDXF beacon W6WX (CM97ae).  According to, W6WX is at a
bearing of 301 degrees, and a distance of 1158 km from AA7FV.
Sunset at W6WX is 02:49 UTC, and sunset at AA7FV is 01:58 UTC.

I have been using the program 'Faros' to look at the propagation
times from various NCDXF beacons.  As I'm sure you know, the NCDXF
beacons are time synchronized using GPS, and so by looking at the
arrival times of transmissions from a given beacon, it is easily
possible to distinguish between short path and long path
propagation.  This is what the program Faros does.

As a check, here is data taken at the same time with the same setup
on the beacon ZL6B, which shows no anomaly.  You can see that I was
receiving the beacon from about 0h UTC until about 04:30 UTC.  The
propagation delays (blue dots) line up precisely with the delay
(about 39 ms) expected for short path propagation, which is marked
by the horizontal dashed line labeled 'SP'.  No surprise here.  The
expected long path delay would be nearly 100 ms, but there are no
signals observed with that delay, during this time period."

Darrell sent graphics which I am unable to reproduce in this
bulletin, but you could email him via his address on if you
want copies.

I referred his quesion to Carl, K9LA, who responded:

"Thanks for those interesting plots from Faros.

ZL6B sunrise is around 1906 UTC, so I would have expected that you'd
see him again later in the day (a bit after 1906 UTC).  But perhaps
the MUF wasn't high enough at that time.  Or something else was going
on.  Dropping out around 0430 UTC is understandable, as your sunet is
around 0200 UTC and the MUF was slowly dropping until ZL6B (still in
daylight) went away around 0430 UTC.

As for the W6WX results, being only 1158 km from you says a
relatively high elevation angle would be required on the true great
circle path.  And that says the MUF over that path would not likely
be high enough for normal refraction at a high elevation angle on 18
MHz (since we're just starting to come out of solar minimum).  So
your comment about an unusual ionized cloud (or an enhanced area in
the ionosphere) is a possible explanation.

The 20 msec or so delay suggests an off-great circle path from an
enhanced area of ionization.  And my guess is that this area was
south of you and W6WX as that puts it closer to the equatorial
ionosphere, where more interesting short-term events happen than
north of your location.  It's too bad that the azimuth arrival angle
isn't measured."

If you're interested in some more Faros results, check out

By the way, Carl says he accidentally deleted an email from someone
named Edgar in Toronto who had questions about VOACAP, and now he
has no way to respond.  If you are Edgar, please contact K9LA.

Ken Brown, N4SO (EM50tk) in Alabama reported hearing the XE1FAS/B
beacon on 28.171 MHz at 0542 UTC (12:42 AM local time) on August 26.
"Just above the noise and then faded out."  The path distance was
1,001 miles.

Reader K6HPX has some fascinating antenna photos on his
profile.  Check it out.

The latest from Dr. Tamitha Skov, WX6SWW:

You can always find her new videos here:

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 August 19 through 25, 2021 were 25, 14, 25, 16,
14, 29, and 29, with a mean of 21.7.  10.7 cm flux was 74.9, 77.7,
77.1, 77.1, 78.1, 80.9, and 83.6, with a mean of 78.5.  Estimated
planetary A indices were 4, 5, 3, 3, 4, 5, and 9, with a mean of 4.7
Middle latitude A index was 5, 6, 4, 4, 8, 5, and 8, with a mean of