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ARRL Propagation Bulletin ARLP017 (2023)

ARLP017 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 17  ARLP017
From Tad Cook, K7RA
Seattle, WA  April 28, 2023
To all radio amateurs 

ARLP017 Propagation de K7RA

At 0134 UTC on April 27, The Australian Space Weather Forecasting
Centre issued a geomagnetic disturbance warning:

"An equatorial coronal hole is currently elevating solar wind
speeds. Combined with the anticipated impact from a recent CME on
April 27, geomagnetic activity is expected to be at G0-G1 levels
over April 27-28, with a slight chance of an isolated period of G2."

Solar and geomagnetic indicators moved in opposite directions this
week. Average daily sunspot numbers over April 20-26 made a dramatic
drop from 146.9 to 91.4, and average daily solar flux from 164.5 to

Average daily planetary A index more than tripled from 8.1 to 26.9,
while average middle latitude A index more than doubled from 7.3 to

Solar wind and explosions caused all this grief. reported that on April 21, a large magnetic
filament on the Sun exploded, hurling debris toward Earth.

Later they reported that on April 23 at 1737 UTC a CME hit Earth,
sparking a severe G4-class geomagnetic storm. Aurora was visible as
far south as southern New Mexico and Texas.

The planetary K index went as high as 8 over April 23-24.

Predicted solar flux over the next month is 135 on April 28-30, 140
on May 1-2, 135 on May 3-4, 140 on May 5-6, then  145, 150, 155, 160
and 165 on May 7-11, 170 on May 12-13, then 165, 160, 155, 150, 145
and 140 on May 14-19, 135 on May 20-21, 130 and 125 on May 22-23,
120 on May 24-25, then 125, 130 and 135 on May 26-28, 140 on May 29
through June 2, then 145, 150, 155, 160, and 165 on June 3-7.

Predicted planetary A index is 25, 16 and 12 on April 28-30, 8 on
May 1-5, 12 and 10 on May 6-7, 8 on May 8-9, then 5, 5 and 12 on May
10-12, 5 on May 13-15, 8 on May 16-17, 5 on May 18-22, then 15 and
18 on May 23-34, and 15 on May 25-27, then 12 and 10 on May 28-29, 8
on May 30-31, then 5, 5 and 12 on June 1-3, and 5 on June 4-6.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - April 27, 2023 from F.K. Janda, OK1HH.

"The most important event of the last seven days was the solar flare
on 21 April with a maximum at 1812 UTC (1744 - 1857 UT). It was a
long duration event (LDE), accompanied by the ejection of a cloud of
coronal plasma into space, at a location on the Sun where there is a
high probability of the cloud hitting the Earth. It is therefore not
surprising that all forecast centres agreed in predicting the
impending disturbance.

"The speed of the solar wind jumped up on 23 April at 1703 UTC,
after which a geomagnetic disturbance began to develop. It was much
stronger than expected (max K=8 and G4 instead of the expected K=6
and G1-2). Auroras were observed with two maxima - in Europe on 23
April mainly between 1900-2100 UTC and in North America on 24 April
between 0300-0400 UTC.

"Thereafter, propagation conditions deteriorated significantly,
especially on 24-25 April, with one interesting variation of the
evolution: the calming of the geomagnetic field on the morning of 25
April UTC was followed by a further development of the disturbance
with an albeit shorter but significant improvement. The return of
the critical frequencies of the F2 layer and the improvement of
shortwave propagation conditions toward the mean continued only
slowly in the following days, as intervals of increased geomagnetic
activity occurred daily. The lowest f0F2 were observed on the night
of 23-24 April. The following night was slightly better."

Rocky Riggs, W6RJK in Truckee, California wrote:

"I was not very active until recently when I was introduced to POTA.
The park I frequent the most would typically give me 40-60 contacts
in a 2 hour period.

"On Monday, the 24th, I went to the same park, and in 30+ minutes
had no contacts and couldn't hear anyone either. I later found out
that the solar storm was causing most of our radio problems. Until
then, I had never considered much about solar flares, or how the Sun
influences radio propagation. Now, finally, I'm trying to learn as
much as I can. The K7RA Solar Update in the ARRL Newsletter is
FANTASTIC, and will be my source going forward to help me learn and

"Here's my question.  Is there a 'real time' place where I can go to
determine if a particular band has good propagation (I typically use
20m and 40m)?

"You know, like before I go out and get all set up and it's a 'goose

I replied:

"I recommend, and look on the map screen for FT8
signals from your grid square and where they are heard. You don't
have to use FT8 to use this.

"You can also check for the 'country of callsign' option with your
own or any callsign.  When I do this for 10 meters, this week it has
been showing no propagation from my area, but lots of 10 meter
propagation in the south and across the east coast.

"I use FT8 a lot to study propagation."

Angel Santana, WP3GW in Trujillo Alto, Puerto Rico wrote:

"Been doing a lot of FT8 these months. More DXpeditions are
including its operation.  Just last week on April 16th at 1939 UTC
worked VU7W and in WARD April 18th T30UN at 0721 on 40m and 0735 on
30m, the two ATNOs."

(I think WARD refers to World Amateur Radio Day, and of course ATNO
refers to All Time New One, something I did not know until a few
years ago. -K7RA)

"But on the 20th, at 0800 UTC, saw stations on 10 meters, normally
you do not hear them on any mode at that time. Then I began to call
them and a few from Europe contacted me. Then at about 0845 UTC,
'poof' they disappeared.

"These are the things that make me say that it is because of the
'crazy prop' (la propa loca)."

Tomas Hood, NW7US has a monthly propagation column in CQ Magazine,
which is a great resource. In the March issue he writes about the
promising progress of Solar Cycle 25.

Another great resource is Chapter 19, the "Propagation of Radio
Signals" in the 2023 100th edition of the ARRL Handbook. It contains
the most comprehensive treatment of radio propagation I have ever
seen and goes on for 38 pages.

Aurora observed in China:

Aurora in Iowa:

An article explaining aurora:

A Science & Tech article about Sun science:

From 2017, a NASA sunspot video:

Send your tips, reports, observations, questions and comments to When reporting observations, don't forget to
tell us which mode you were operating.

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 April 20 through 26, 2023 were 97, 114, 87, 86,
88, 87, and 81, with a mean of 91.4. 10.7 cm flux was 147, 151.2,
141.2, 135.2, 133.9, 130.7, and 136.5, with a mean of 139.4.
Estimated planetary A indices were 5, 9, 7, 66, 76, 10, and 15, with
a mean of 26.9. Middle latitude A index was 5, 8, 6, 32, 39, 7, and
12, with a mean of 15.6.


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