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ARRL Propagation Bulletin ARLP018 (2024)

ARLP018 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 18  ARLP018
From Tad Cook, K7RA
Seattle, WA  May 3, 2024
To all radio amateurs 

ARLP018 Propagation de K7RA


"A period of planetary G3 geomagnetic conditions has been observed
on 02-May, associated with two recent CME arrivals and a sustained
period of southward IMF conditions. Further periods of G3 are
possible over 02-03 May.


02 May: G2-G3 
03 May: G2, chance G3, declining to G1"

After the record sunspot numbers reported in last week's Propagation
Forecast Bulletin ARLP017, the April 25 through May 1 reporting week
has more modest numbers. In fact, the average daily sunspot number
(124.6) is less than half the value (265.9) in the previous

Seven new sunspot groups appeared this week.

One new sunspot group emerged on April 25, another on April 27, two
more on April 29 and one each on April 30 and May 1.

Average daily solar flux shifted from 216 to 144.9.

Average daily planetary A index dropped from 13.9 to 9.6.

The solar flux estimate for the next month has values peaking at 205
on May 15-16 and again on June 11-12.

The values are 135 on May 3, 132 on May 4-5, then 134 and 136 on May
6-7, 138 on May 8-9, then 140, 155, 160, 175 and 180 on May 10-14,
205 on May 15-16, then 200, 195, 190, 185, 180, and 165 on May
17-22, 145 on May 23-24, then 140, 135, 130, and 125 on May 25-28,
then 120, 115 and 120 May 29-31, 125 on June 1-2, 130, 145, and 150
June 3-5, 155 on June 6-7, then 160, 175 and 180 on June 8-10, and
205 on June 11-12.

Predicted planetary A index is 18, 20, 18 and 10 on May 3-6, 5 on
May 7-22, 15 on May 23, 12 on May 24-25, then 10, 8, 15, 18 and 10
on May 26-30, then 8 on May 31 through June 3, and 5 on June 4
through the middle of the month.
Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - May 2, 2024 from OK1HH.

"The number of sunspot groups at the present stage of the 11-year
cycle varies between five and twelve. Of these, one to three can be
described as active regions, whereby their size and magnetic
configuration suggest the possibility of energetic flares of
intermediate magnitude. A number of these are accompanied by CMEs,
which, given their position on the Sun, are expected to strike the
Earth. Therefore, predictions of increased geomagnetic activity are
quite often made, but most of them do not come true. Conversely, if
the Earth is affected, a geomagnetic disturbance so strong that it
affects the conditions for shortwave propagation will develop.

"CME collisions with the Earth have mainly caused magnetic storms
and subsequent deterioration of shortwave propagation on 21-22 April
and 27-28 April. Especially in the latter case, the recovery from
the disturbance was very slow, even multi-day, due in part to the
decrease in solar radiation. Added to this was another geomagnetic
disturbance in the late evening hours UTC on 30 April, which caused
a decrease in MUF and a worsening on 1 May."

NOAA article about Solar Cycle 25 progress:

In an email Thursday morning, Dr. Tamitha Skov, WX6SWW, wrote:

"Region 3654 sure has been a point of contention this week. Despite
a lot of big flares, the eruptions that have occurred have been
rather unspectacular-- or so we all thought. Twice now, we have had
stealthy solar storms hit Earth. The first one that hit yesterday,
was not particularly impactful. In fact, for those who have been
following the news in our community chat here on Patreon, you have
seen me discuss the event there, instead of posting an official
'Snapshot.' I did this because that storm was weak and slow, without
much southward-pointing field. In other words, it was so
yawn-worthy, I haven't bothered to update my twitter feed with the

"That all changed when yet another stealthy solar storm hit, just a
few hours ago. This one is much stronger. The top solar disk image
shows the source region for this event. It was an unimpressive event
in coronagraphs, with no clear Earthward directed signature.
However, it has a strong magnetic field, that is pointing southward,
and is fast. This one has given us G3-levels momentarily, but could
keep us at sustained G2-level conditions.

"Both of these events have eluded detection by several (if not all
of the big space weather forecasting agencies) so it is clear,
stealthy solar storms continue to be a problem through solar
maximum. I had been working on a formal forecast, but I am thinking
I will do an impromptu live forecast today since things are
unfolding faster than I can update my current work. Stay tuned. I
will likely go live this afternoon (PDT time), a few hours from now.
Till then, know that we could very easily hit G2-levels within the
next hour at SWPC, if conditions remain as they are."

From Universe Magazine, another Radio Blackout:

There was another blackout on Thursday, when two CMEs caused a G3
geomagnetic storm. According to, another CME is
expected on May 4.

From NDTV, an earlier disturbance:

From reader David Moore, an article on a fluffy corona:

From The Daily Galaxy, Solar fury:

Cosmic rarity, But did they really need to reference astrology?

From, more on Radio Blackouts:

Another from about the Solar max:

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 web page 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 .

Also, check this QST article about Solar Indices:

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

Sunspot numbers for April 25 through May 1 2024 were 196, 154, 126,
119, 88, 85, and 104 with a mean of 124.6. 10.7 cm flux was 166.7,
152.6, 152.6, 140.1, 137.6, 130.2, and 134.8, with a mean of 144.9.
Estimated planetary A indices were 3, 19, 12, 7, 6, 12, and 8, with
a mean of 9.6. Middle latitude A index was 3, 11, 12, 7, 7, 10, and
10, with a mean of 8.6.


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